I. Overview
1. The formation of solder wave crest dynamics theory
Theoretically, wave soldering is a logical extension of the dip soldering method. From dip soldering to wave soldering, the fundamental difference lies in that: the dip soldering method is implemented by the static liquid level dipping method of liquid solder, while the wave soldering method is implemented by the wave peak dipping method of liquid solder. The solder wave generator forms the core of the wave soldering equipment system and has also become a key technology in the design of the wave soldering equipment system. It is well known that the performance of the solder wave peak generator determines the function of the entire system.
The solder wave peak generator is a power system that generates liquid metal solder wave peaks. The generation of liquid metal solder wave peaks, the stability of the wave peaks, the design of the working wave peak shape, the thermal characteristics of the solder wave peaks, the ability to inhibit the oxidation of high-temperature molten solder, the wettability of the base metal to be soldered, and the contour conformation of the solder joints There are certain regularities in the influence of . Describing these regularities involves knowledge of fluid mechanics, electromagnetic fluid mechanics (for electromagnetic pumps), metallurgy, metal surface theory, thermal engineering and other aspects. Often the solution to a problem involves the intersection and penetration of knowledge from the above disciplines. Therefore, they together constitute the basic content of the solder wave crest dynamics theory.
2. The guiding significance of solder wave crest dynamics theory to the development of wave soldering technology
The research results of solder wave crest dynamics not only enable people to obtain high-quality solder joints that are smooth, bright, conformable, and free of defects such as sharp edges and burrs, but also reveal a series of constraints and applications for designing solder wave crest generators. basic principles to be followed.
①Due to the high temperature, the PCB should not be immersed in the solder for too long. However, from a metallurgical point of view, in order to obtain a good solder joint, the PCB and the solder must be kept in contact for sufficient time so that the welding part can obtain enough heat and reach a good wetting temperature.
② When the molten liquid solder flows from the bottom of the PCB, it will cause scrubbing on the welding parts, which has a good fluxing effect. For this reason, the solder wave peak should have a long contact time with the PCB, and the generation of solder slag should be minimized, which means that the contact between the wave peak and the air should be minimized.
③In order to reduce tipping and bridging, when the PCB exits the wave peak, the moving speed of the PCB relative to the solder should be close to zero.
④ When the PCB passes through the wave peak, the solder wave peak should be flat and smooth and can fully contact the entire soldered surface of the PCB. The uneven waves will cause the molten solder to overflow to the component surface of the PCB, or cause partial solder leakage.
⑤ The generation of solder slag must be minimized as it can accelerate the wear of the solder pump system (for mechanical pumps) and cause uneven wave crests. Excessive solder slag will also accelerate the consumption of major components such as tin in the solder, which not only increases the production cost of the product, but also seriously pollutes the environment.
If you want to obtain the best results from wave soldering, you must properly handle the above issues to meet the above contradictory conditions as much as possible.
These contents constitute the research content of solder wave crest dynamics. It is also the research results and application of solder wave crest dynamics that make the wave soldering process still one of the main soldering methods for PCB, especially in mass production. Some foreign researchers believe: "The continuous development of electronic manufacturing technology has prompted many companies to gradually adopt reflow soldering technology. However, wave soldering will still be used in production for many years." And this will be even more true in our country.
2. Dynamic phenomena of solder wave peaks in wave soldering
For the sake of discussion, we might as well think of the liquid solder flow sandwiched between the PCB and the wave nozzle wall during wave soldering as being approximately similar to the flow of viscous fluid in a rectangular pipe, as shown in the figure below
The theory of fluid mechanics tells us that when fluid flows in a pipe, the cohesion (viscosity) of the fluid itself and the adhesion between the fluid and the solid wall cause differences in the speed of the fluid everywhere. The fluid close to the pipe wall must adhere to the On the wall, the relative velocity is zero, that is, the fluid has no slip on the wall. Near the wall, as the normal distance from the wall increases, the influence of the wall on the fluid weakens, and the flow velocity of the fluid will increase rapidly. At a certain distance, it will approach the original velocity that is not disturbed by the solid wall. Therefore, the velocity change phenomenon only occurs The thin layer of fluid immediately adjacent to the wall is called the boundary layer or boundary layer.
For the convenience of analysis, first cut two sections on both sides of the nozzle along the direction perpendicular to the flow rate of the solder, namely the O-O section in the above figure (shown in Figure 1 below) and the O′-O' section (shown in Figure 2 below) ) to analyze its flow velocity distribution pattern. Suppose N-N and M-M are the center lines of the O-O and O′-O′ sections respectively. The following two situations will be discussed.
(1) When the PCB is stationary, that is, v0=0
As mentioned earlier, due to the influence of the fluid wall effect and cohesion, the velocity distribution of the solder fluid sandwiched between the PCB and the nozzle wall is parabolic, and the flow velocity close to the PCB surface and the nozzle wall is zero, while in The flow velocity at the center line is maximum.
(2) When the PCB is moving, and v0 = vx
When the PCB moves in the direction indicated by the counterflow arrow with v0=vx, the distribution of fluid velocity in the O-O and O′-O′ sections changes, as shown in the following two figures.
Flow velocity distribution in the pipeline when PCB moves in reverse direction
Flow velocity distribution in the pipe when PCB moves forward
According to the laws of fluid mechanics, the tangential velocity v0 of a viscous fluid particle in the tangential direction of the wall is equal to the tangential velocity vC of the corresponding point on the rigid wall, that is, vC = v0
That is to say, the fluid particles close to the boundary wall have the same velocity as the corresponding points on the boundary wall. Obviously, in the case of the O-O section as shown in Figure 2.4, the zero point of the fluid velocity will no longer appear on the boundary wall, but on the A-A surface inside the fluid, and the maximum velocity line in the pipe will also move from N-N to N On the ′-N′ surface. We call the fluid layer between the zero velocity line and the underside of the PCB the boundary layer. At this time, all the vortex motion of the incoming flow is concentrated in the boundary layer. In this layer, the velocity changes greatly along the tangential direction of the PCB surface, so the velocity gradient in the normal direction of the PCB surface is very large. In other words, there is considerable vortex motion in the fluid, which will intensify the adhesion of viscous fluid particles to the rigid wall. According to this theory, when the PCB and the liquid solder move relative to each other during wave soldering, they must carry a large amount of liquid solder that is adhered to the surface of the base metal to be soldered. This just constitutes the inevitable condition for pulling points and bridging. Moreover, the greater the movement speed (v0) of the PCB relative to the reverse flow speed (v1) of the fluid in the wave crest, the more solder is carried, and the more serious the tack and bridging will be. Therefore, slowing down the movement speed of the PCB (v0) or speeding up the reverse flow of the fluid (v1) can compress the thickness of the boundary layer, thus effectively suppressing the vortex motion in the boundary layer. The excess solder that adheres to the PCB wall and moves along with the PCB is greatly reduced, which effectively suppresses the occurrence probability of tipping and bridging. This is a fact that has been proven by a large number of production practices.
However, the situation of the O′-O′ cross-section shown in Figure 2.5 is different from the O-O cross-section shown in Figure 2.4. Since the movement direction of the PCB (v0) is the same as the fluid flow direction (v2) at this time, there is no boundary layer problem, so there is no vortex motion caused by the solder reflow. By adjusting the forward flow velocity (v2) of the fluid, you can obtain the best detachment conditions at the point where the PCB detaches from the wave peak.
3. The influence of wave speed of wave solder on wave soldering effect
It has been discussed in the solder wave peak dynamic phenomenon that when the PCB enters the wave peak working range, since the movement direction of the PCB is opposite to the flow direction of the liquid solder, there is a boundary layer close to the lower surface of the PCB. The thickness of the boundary layer is related to the PCB pinching speed and the fluid flow velocity in the opposite direction of PCB movement. For example, when the PCB pinching speed is constant and the reverse fluid flow speed is increased, the thickness of the boundary layer will become thinner, the reflow phenomenon will be significantly weakened, and the reverse scrubbing effect of the solder fluid on the PCB will be significantly enhanced. Obviously It is not easy to produce peaking and bridging phenomena, but it is very likely that the amount of solder required to form the normal contour of the solder joint has been scrubbed away excessively, resulting in insufficient solder, dryness, and asymmetrical contours of the solder joint. and other defects. On the contrary, if the fluid velocity is too low, the scrubbing effect will be reduced and the solder joints will be plump, but the probability of peaking and bridging will also increase. Therefore, there is an optimal liquid solder fluid velocity for a specific PCB and its pinching speed. The figure below shows a typical solder wave peak shape and its flow rate adjustment structure.
In the picture above, a large pressurized chamber is used to press the molten solder into the nozzle, and a smooth laminar bidirectional liquid flow is obtained through the nozzle. The liquid solder flow passes through the nozzle flange to form a solder wave crest. The shape of the nozzle controls the shape of the solder wave crest and therefore the effect of the wave crest dynamics. A buffer net is installed in the pressure chamber to ensure the formation of laminar flow to ensure the smoothness of the wave crest. In order to reduce the generation of solder slag, the solder pouring down from the wave crest must not have excessive turbulence when it returns to the solder tank. The adjustable side plates placed on the front and rear outer sides of the nozzle form a solder return channel and help form the optimal wave peak, which forces the wave peak solder material to return to the solder tank from a position far below the liquid level. Thus, the liquid level in the solder tank remains undisturbed. Adjusting the inclination angle of the side plate can make the solder gradually return to the solder tank along the inclined surface and continue to decelerate, thereby achieving the purpose of controlling the flow rate of the solder and minimizing the turbulence on the surface. Adjusting the side plate in front of the nozzle can control the shape of the wave peak entering the working zone, thereby controlling the velocity characteristics of the fluid in this zone. Similarly, in the PCB exit section, there is also a problem of matching the PCB conveyance speed and the fluid speed to obtain the best detachment conditions. This can be achieved by adjusting the side plate behind the nozzle.
4. Types and characteristics of solder wave crests
There are various wave soldering equipment currently operating in industrial production. From the perspective of waveform types, these devices can be roughly divided into the following two categories.
(1) One-way crest type
This type of structure in which the crest of the nozzle solder flows out from one direction is more common in early equipment. Nowadays, unidirectional waveforms other than hollow waves are less common on newer machines.
(2) Two-way crest type
The characteristic of this two-way wave peak system is that the liquid solder coming out of the nozzle flows in both forward and backward directions at the same time after reaching the top of the nozzle. Depending on the needs of the application, this shunt can be symmetrical or asymmetrical, and even extenders are added along the rear direction of the transfer to make the wave crests wider and flattened in the PCB drag direction. Currently, the most commonly used wave soldering equipment is the two-way wave soldering type. Due to the distribution characteristics of the wave crest surface velocity, the bidirectional crest system can minimize the problem of solder joint sharpening. Since the solder in the wave peak flows in both forward and backward directions, there must be a region with zero velocity on the surface of the solder wave peak. The PCB exits near the zero speed area (as shown in the figure below), which is extremely important for the formation of tip-free solder joints.
5. Surface tension of molten solder after bidirectional wave peak
To understand how bidirectional wave nozzles minimize solder joint tipping, it is important to understand the phenomenon of surface tension and its relationship to wetting. When surface tension fails to wet a surface, the molten solder will form into small balls. Surface tension can control the condition of the liquid wetting the surface, and can also control the wetting of the solder on the base metal surface that has been coated with flux. In the image below, you can see the PCB passing through a bidirectional wave crest. The solder has wetted the copper surface of the PCB and is being pulled out of the wave crests to form a thin layer. The size of this thin layer is controlled by several factors, such as the surface tension of the solder, the velocity characteristics of the point where the wave peak contacts the solder thin layer, and the quality of the molten solder thin layer at that point.
The larger the thin layer area, the more difficult it is for the surface tension of the molten solder peak to drag the excess solder back to the peak. When the size of the solder thin layer reaches a certain limit, the surface tension will separate them. At this time, if the excess solder is not dragged back to the wave crest, a solder joint tip will be formed. It can be seen from this extremely rough model that our goal is to minimize the solder thin layer area. There are two methods adopted:
● Change the surface tension of the solder;
● Change the peak velocity characteristics of the point where a thin solder layer is produced.
There are many ways to achieve the above goals. The surface tension of the solder is affected by the temperature of the solder. High temperature will reduce the surface tension, but the increase in temperature will intensify the oxidation of the surface of the molten solder, which will also increase the surface tension of the molten solder. big. Therefore, raising the temperature cannot significantly improve the surface tension of the solder. Injecting oil onto the solder wave peak can reduce the surface tension.
The size of the solder thin layer can also be reduced by using the inclined conveying method. Therefore, tilting the conveyor at a certain angle will help peel the solder faster and return it to the wave crest. Another way is to make the wave crest very wide. When using an inclined conveyor, the wide wave peak can make the PCB leave from the vicinity of the wave peak where the relative speed is zero, which gives the surface tension enough time to completely drag the excess solder in the solder thin layer back to the wave peak.
6. Physical and chemical processes in wave soldering
The contact process between the PCB and the wave peak during wave soldering can be roughly divided into three wave zones according to their different working principles, as shown in the figure below.
1. Cut into the wave crest point (A)
The starting point of the PCB and the wave peak is shown in the partial enlargement of point A in the figure below. Since the PCB and solder fluids move in opposite directions, the speed difference at this point is the largest. Therefore, the flushing effect of turbulent flow is greatest at this point. This function is used to remove the preheated flux and rust film residue mixture from the base metal surface so that the solder is in direct contact with the base metal on the PCB. When the wetting temperature is reached, wetting occurs immediately. If the wire surface has been coated with a flux protective layer before wave soldering, the turbulence of the solder fluid will help wash away these surface layers. When the surface is plated with fusible alloys such as tin-lead solder or pure tin, this principle manifests as a combined melting-rinsing effect. For soluble coatings like Ag or Au, the principle manifests itself as a combined dissolution-rinsing action.
2. Heat exchange area (A-B)
The area where the PCB is immersed in the solder wave peak, such as the area between the entry point and the exit point as shown in the figure below, is the heat exchange area. It applies heat and solder to PCB pads, holes and component pins. The greater the heat absorption of the welded area, the longer the time required to be immersed in the molten solder to reach the wetting temperature. Therefore, the PCB must be immersed in the solder wave peak long enough so that the surface energy at the wetting temperature can adsorb the molten solder alloy to the base metal surface to form a well-filled weld.
3. Peel off the peak point (B)
The point where the PCB exits from the solder wave peak, as shown in the right partial enlarged view (B) in the figure below, is usually called the peeling area. In order to understand the role of this wave zone, let’s first briefly analyze the role ofVarious forces on the molten filler metal of the filler weld, as shown in the figure. Surface energy in the form of wetting will keep the filler metal in the weld, while gravity FG (or the mass of the filler metal) will try to pull the filler metal downward. The balance of these forces, which also depends on solderability, demonstrates the need for an appropriate aperture/wire diameter ratio. In order for these forces to reach equilibrium in the exit zone, the exit point must be located at a location where the solder wave and the PCB are relatively stationary. This can be ensured by making the exit speed of the PCB match the retreat wave speed as accurately as possible.
PCB passes through the three wave areas of the solder wave peak
The force on the liquid filler metal in the filling weld
FG—gravity; f1, f2—surface tension in holes a and b; R1, R2—radii of curvature of the solder-wetting concave surface in holes a and b
7. The physical nature of the role of protective oil in wave soldering
Oil injection in the solder wave has a great influence on the mechanical state of the exit point. Usually the protective oil is injected at the pump impeller and into the solder wave peak (such as the early Z-shaped solder wave peak system of the American Hollis Company), and oil is also sprayed on the solder wave peak surface (such as the 6TF series produced by the Swiss KIRSTN Company). Hollow wave injection system). The entire wave crest is covered with an oil layer. The real purpose of mixing the molten solder with the protective oil is to reduce the surface tension of the molten solder at the point where the PCB leaves the wave crest, minimizing the thin layer of solder at the peeling point to eliminate soldering. Click on the tip and bridge. When the solder joint leaves the wave peak, it is coated with a layer of oil. This coating prevents oxidation and makes the solder joint particularly bright, which facilitates the inspection of solder joint defects (such as micro-cracks, etc.). Since there is a tendency to reduce solder joint sharpening, the speed of the transmission device can be accelerated, thereby improving production efficiency. Reducing the surface tension can increase the ability of the liquid solder to wet the PCB copper pad, so the temperature of the solder wave peak can be appropriately reduced (about 10°C) without affecting the welding effect.
The oil injection process has been eliminated in new equipment in recent years for the following reasons:
● When using protective oil for wave soldering, the oil will be wrapped in the solder joints, which will affect the mechanical and electrical properties of the solder joints and may lead to the formation of corrosive acidic residues;
● The soldered PCB components will be very dirty and have an oily coating layer, which is difficult to clean;
● When using protective oil for wave soldering, it must be cleaned after welding, which is not conducive to environmental protection.
8. Sufficient and necessary conditions for obtaining tip-free solder joints
The following is an analysis of the sufficient and necessary conditions for obtaining tip-free solder joints based on the velocity distribution of droplets at the breakaway peak of the PCB, as shown in the figure below.
Peeling status of PCB pad and wave solder
Assume that the PCB pinching speed is v0, the peeling flow rate of the solder on the wave peak surface against the PCB direction at point A is v1, and the sagging speed of the solder on the solder joint caused by gravity is vg. Analysis based on the flow pattern and stress of the droplet at point A.
The sufficient and necessary conditions to obtain tip-free solder joints are as follows.
① Sufficient condition: vg=0, where vg——the sagging speed of the solder droplet, which is affected by the angle α of the PCB exiting from the solder wave crest surface, the PCB pinching speed v0, the solder flow rate in the opposite direction of the PCB v1, The comprehensive influence of factors such as the surface tension of the solder and the wetting force of the component pins.
② Necessary conditions: v1>v, 0v1>0,
v0——PCB exit speed, that is, pinch feeding speed;
v1——The flow speed of the solder in the opposite direction of PCB movement, which is affected by factors such as the temperature of the solder, the surface state of the PCB, the state of the component leads, and the performance of the flux.
The tilted pinching method combined with the width of the wave peak can make the PCB leave from (or near) the wave peak where the relative speed is zero, which allows the surface tension of the solder to have enough time to completely drag the excess solder back. crest.
9. Determination of the optimal entry angle (inclination angle) range
The size of the boundary layer can also be reduced by entering at an angle, and its core can still be integrated into changing the flow rate of the fluid. Figure 2.12 shows the change in fluid flow rate entering the working section. The length of the line segments with arrows in the figure below indicates the flow velocity at each point, and the direction of the arrow indicates the direction of the flow velocity. As the fluid slides down the nozzle, the flow velocity gradually increases, and the angle between the direction and the horizontal becomes larger and larger. When the PCB enters and the inclination angle increases from α1 to α2, the working section where the PCB cuts into the wave peak moves from the low-speed A-A section to the high-speed A′-A′ section. Obviously, the effect of changing the inclination angle is completely consistent with the effect of changing the fluid flow rate, and the analysis will not be repeated here.
Effect of inclination angle on wave soldering effect
Relevant research reports state that the friction characteristics of the solder during wave soldering are about 3°, so setting the inclination angle of the conveyor to 4° to 9° can make the solder better peel off from the PCB surface and return to the wave peak. Through comprehensive test effect analysis, it is generally believed that an inclination angle of 6° to 7° is optimal.
10. The relationship between wave crest height and wave crest pressure and its influence on wave soldering effect
The height of the wave crest depends on the pressure the pump can produce. In other words, the higher the wave peak, the greater the pressure required from the pump. The pressure is generally proportional to the square of the fluid velocity in the pump, so as the pressure increases, the velocity increases faster. An increase in Reynolds number may cause the fluid to enter a turbulent flow (turbulent flow) state, resulting in instability of the wave crest and an increase in the pressure on the wave crest. When the wave crest is low, the flow rate of the fluid in the pump is low and in a laminar flow state, so the wave crest formed is stable and has small beating, and the pressure on the wave crest is naturally smaller.
The relationship between wave peak height, solder flow rate and pressure is shown in the figure below.
For example, when the wave peak height is 10mm, the jet pressure of the wave peak on the PCB is approximately 4.78mmHg, and the flow rate is 26.6cm/s. When the wave crest height is 8mm, the jet pressure drops to 2.45mmHg, and the flow rate is 19.0cm/s. Some people think that the higher the peak jet pressure, the better. This is wrong. Too much pressure can easily cause the solder to flow onto the non-soldering surface of the PCB. At the same time, according to the welding principle, it can be known that the welding process mainly relies on the wetting effect of the solder on the base metal and the capillary phenomenon to form a joint. The joint formed through such a metallurgical process is the most reliable. It is not ruled out that appropriate jet pressure has the effect of accelerating wetting, but attempts to rely on increasing jet pressure to fill welds may instead cover up those welds that are not wetted or have incomplete wetting and are not easily discovered, leaving behind consequences. . According to the analysis of the dynamics of the solder wave peak, the working height of the solder wave peak is 6 to 7 mm for the best effect.
11. Basis for selecting the optimal volume of the solder tank
There are currently different opinions on the selection of the volume of the solder tank. Some say that the larger the volume, the better, while others say that the smaller the volume, the better. These are all one-sided. In fact, for a certain wave peak width and width, there is an optimal volume problem, which is determined by the dynamic characteristics of the solder wave peak. When designing the volume of the solder tank, the circulation coefficient of the solder tank must first be determined based on the analysis of the fluid dynamics characteristics. The so-called circulation coefficient of solder can be defined as
Solder circulation coefficient = mass of solder above the liquid level/total mass of solder in the solder tank
The size of this coefficient varies with different model designs, and this coefficient is confidential in design calculations. The value of the solder circulation coefficient is directly related to the overall performance of the solder peak generator. For example, when the solder wave peak shape structure has been set, that is, when the mass of the solder above the liquid level during operation is a known quantity, the size of the solder circulation coefficient determines the volume of the solder tank.
① The smaller the coefficient, the larger the volume. The advantages of large volume are: large heat capacity, small temperature fluctuations during operation, small liquid level fluctuations, and easy stabilization of wave peaks. However, this will cause high consumption of the solder material, high power consumption, and it is difficult for the impurity metal concentration to reach a dynamic balance during operation, so it is easy to cause the accumulation of metal impurities, and it is easy to cause a dead corner of the deposited eluted phase, resulting in the solder material in the solder tank. Ingredients are unevenly distributed. When the accumulation of metal impurities reaches the allowable maximum, the entire slot of solder must be renewed, causing even greater losses.
② On the contrary, when the coefficient is too large, the volume of the solder tank will be very small, and the situation will be exactly opposite to the above. Therefore, in design calculations, a compromise is generally made between the above two extreme situations, and the value is determined according to the overall design and economic requirements, so that the final comprehensive effect and performance can be optimized. Therefore, we cannot say one-sidedly that large volume is better or small volume is better.
12. Design of solder wave peak shape and its impact on wave soldering effect
The greatest contribution of solder wave crest dynamics is that it provides theoretical guidance and basis for designing the solder wave crest shape. The shape of the nozzle controls the shape of the solder wave crest and therefore controls the dynamics of the solder wave crest. There are various solder wave shapes popular in wave soldering systems around the world (as shown in the figure below), and each design claims to have the best solder wave dynamics. Now only some of the most representative waveforms are selected, and their characteristics are preliminarily analyzed from the perspective of solder wave peak dynamics.
Arc wave Narrow arc wave is shown in Figure (a) below. This is a solder waveform often used in the early days of the development of wave soldering technology. The nozzle structure that forms this kind of wave crest is relatively simple. Because the wave crest is relatively narrow and the heat supply is insufficient, the production efficiency is low, and peaking and bridging are easy to occur during welding. A typical application case of this waveform is the world's first industrial wave soldering machine developed by the British company Fry's Metal. Its main operating parameters are: the maximum wave peak height is 12.7mm, and the maximum value of the nozzle opening above the liquid level is 19.1mm. , the depth of the PCB substrate pressed into the wave peak by the welding surface is 0.8mm, and the maximum conveying speed of the PCB substrate during welding is 120cm/min. Obviously its production efficiency is very low. To improve the thermal characteristics of the solder wave peak and increase production efficiency, the only way is to increase the wave peak width, so the wide arc wave emerges at the historic moment.
A wide arc wave is shown in (b) below. For the arc wave nozzle structure, increasing the wave crest width can only be achieved by increasing the thrust of the pump. If the speed of the pump drive system is too high or the driving torque is too large, it will not only cause increased wear of the rotating parts and complication of the structure, but more seriously, it will intensify the conversion of the solder fluid into turbulent flow, worsen the smoothness of the wave crest, and cause the solder to become turbulent. Oxidation increases, which is undesirable.
Two-way flat wave
In order to achieve the purpose of increasing the width of the wave crest without increasing the speed and torque of the wave crest drive system, people have designed the nozzle structure as shown in the following figures (c) and (d). The two-way wide flat wave overcomes the shortcomings of the arc wave. There is an obvious area of zero velocity at the top of this waveform, which is very beneficial to eliminating cusps and bridges. Moreover, through the function of the expansion board, the wave crest width can be made very wide, with the width reaching 80mm on some models, and the production efficiency has been greatly improved. Therefore, two-way wide flat wave is still widely used in many current models.
Asymmetrical two-way wide flat waves are symmetrical two-way wide flat waves as shown in (c) and (d) above. Because the two-way symmetrical flow is divided and the flow speed is equal, when the PCB enters the wave peak, due to the blocking effect of the substrate, the reverse flow will The surface flow velocity in the substrate conveying direction slows down, the boundary layer thickens, the zero speed zone becomes narrow and blurred, and the PCB exit peak point is difficult to control. Macroscopically, the scrubbing effect on the PCB is not strong, all of which are detrimental to the elimination of Pulling points and bridging are both disadvantages. In order to enhance its reverse scrubbing effect and further suppress peaking and bridging, the waveform needs to be designed into an asymmetrical two-way wide flat wave, as shown in Figure 2.14 (g), (h), (i) below. This increases the flow and velocity of the fluid in the direction opposite to the PCB, which not only significantly enhances the scrubbing effect on the PCB, but also helps to accurately adjust the position of the speed zero zone so that it coincides with the exit point of the PCB, thereby ensuring that the welding process is optimal conditions required. The most representative waveforms are:
(1) Z-shaped wave Z-shaped wave is an improved design made by the American Hollis Company to reduce defects such as solder bridging and sharpening (Hollis patent). The specific methods are to increase the width of the wave surface, use an inclined conveyor belt, and use a refueling mixed solder wave. However, due to problems such as oil odor and smoke exhaust treatment, it was later improved in the direction of dry Z-shaped wave (without using oil), as shown in the figure (i) below.
(2) λ-shaped wave
The standard λ-shaped wave is shown in the figure below. It is a patented technology of Electrovert in the United States. It has obvious effects in reducing fins and bridges and does not require oil addition. The waveform is designed by applying the interrelationship of thermodynamics and fluid mechanics. As can be seen from the figure below, the PCB begins to receive force and comes into contact with the wave peak at the high-speed point. Therefore, the scrubbing effect of the solder is also optimal. Since the baffle is placed in front of the nozzle to control the shape of the wave crest, it also controls the velocity characteristics of the wave crest, thus forming a large area in front of the nozzle where the relative velocity is zero. Therefore, when a conveyor with a wide adjustable range of tilt angle is used, it is more convenient to control the PCB to detach at the point on the wave peak where the relative speed is zero. The post-heating effect produced by the molten liquid solder immediately behind it can allow the surface tension of the solder at the separation point to be maintained at a minimum level for enough time, which helps to reduce solder joint sharpening. It is said that after using this nozzle, even when the conveyor speed exceeds 6m/min. It can still successfully achieve peak-free wave soldering, and the maximum wave peak height can reach 12.7mm. 13. Waveforms suitable for surface assembly (SMA) wave soldering
1. Analysis of SMA wave soldering method
In order to meet and solve some special requirements in wave soldering of surface mount components (hereinafter referred to as SMA), some special measures must be taken in the design of the solder wave shape and nozzle structure to effectively remove the problems existing in SMA wave soldering. The gas shielding effect and shadow effect accelerate the welding process and prevent parts from being damaged, falling off, electrode corrosion and other problems caused by being heated for a long time. The following introduces several commonly used SMA wave soldering methods.
2. Welding method
1) Jet wave soldering method
When the liquid solder is beaten into a wave that is higher than the general laminar flow and has a hollow center, it is customary to call it a hollow wave. The liquid solder forms a large upward pressure on the PCB solder joint, forcing the liquid solder to flow to the solder pad to form a solder joint, as shown in the figure below.
Hollow wave, also known as chip wave, is a narrow-band high-energy wave that can surround the solder around SMD pins and pads. This eddy current ensures that the solder penetrates into all pins and pads of the device, even for dense arrays of SMDs, and this wave can remove some of the oxide and flux residue on the PCB. After the PCB passes through the sheet wave and then passes through the lambda wave, a good welding effect can be obtained.
From the perspective of solder wave crest dynamics, this kind of wave can obtain the following benefits:
● Large wave pressure, strong penetration into fine welds;
● Strong forward tilting force and significant scrubbing effect;
● The wave column has a thin cross-section, a hollow center, and a small contact surface between the wave crest solder and PCB, which is conducive to gas emission.
The problems with hollow core waves are as follows:
● The solder material splashes down at high speed, the liquid surface rolls too much, and the solder material oxidizes violently;
● The wave column section is thin and small, and the heatThe source supply is limited. When welding solder joints with large heat capacity, heat opacity will occur and the solder joints cannot reach the required wetting temperature;
● The wave peak impulse is large and the scrubbing effect is excessive, which can easily cause defects such as dry solder joints and asymmetrical contour conformation;
● The width of the nozzle outlet is very narrow, the liquid solder flow velocity is very high, and the wave crest is not easy to stabilize.
2) Double wave soldering method
The guiding ideology of the design of the solder double wave peak generator is: first use a narrower and larger turbulence wave to spray the liquid solder onto the pads required to be welded and the electrodes (or pins) of SMC/SMD ; Then use a wide and flat laminar flow wave to trim the solder on the solder joints to remove defects left by the spoiler wave, such as bridges and pull points. Therefore, the functions of the two wave peaks before and after are complementary to each other but each performs different tasks.
The design of solder double wave peak generator has single tank double pump, double tank double pump and other structural forms. The single-tank double-pump type has a simple structure, but has poor flexibility in adjusting the height and angle of the front and rear wave crests; while the advantages and disadvantages of the double-tank double-pump type are exactly opposite to the single-tank double-pump type, such as when the inclination angle of the pinching system needs to be changed. , the operation of double tank and double pump is more flexible.
The distance between the two wave peaks should not be too large to prevent the SMC/SMD from passing through the first wave peak and not immediately entering the second wave peak. The discontinuous temperature change will cause the SMC/SMD to increase the number of thermal shocks and cause damage to the SMC/SMD ( The possibility of microcracks, etc.) increases. However, the distance cannot be too small. If it is too small, two waves will interfere with each other, and they will collide during the splashing process to produce solder beads.
At present, the more mature double-peak waveform combinations include the following.
(1) The waveform combination of turbulent wave-wide flat wave is shown in the figure below. The first solder wave peak nozzle is narrow, the nozzle orifice plane is porous, and it keeps reciprocating along the wave peak width direction driven by the driving mechanism. The wave column-shaped wavelet array extruded from the hole on the nozzle mouth plane has a large upward force and constant reciprocating displacement, which is not only beneficial to eliminating a large number of bubbles formed during the wave soldering process, but also beneficial to The penetrating effect of the liquid solder on the weld seam of the welding part ensures the penetration of the welding part. The second wave peak set at the back is usually a wide flat wave, the wave peak is relatively stable, and the wave pressure is uniform, which can modify the solder joints, such as eliminating pull points, solder tumors, bridges, etc.
The biggest problem with turbulent waves is that the molten solder on the wave crest rolls and the exposed surface is extremely large, so the solder is oxidized extremely severely.
(2) Hollow wave-wide flat wave
This waveform combination is shown in the figure below. The first wave peak adopts a jet hollow wave with a wave speed larger than that of a general laminar wave, a thin wave column section and a hollow center to eliminate the shadow effect and bubble shielding effect. The second wave uses a traditional laminar flow wave for solder joint trimming.
(3) Double peaks with nozzles tilted at 45°. This waveform structure is to set the peak nozzles in any of the waveform combinations introduced above at an angle of 45° to the PCB pinching direction, as shown in (b) below.
Compare the two forms of solder flow traces shown in the figure below with 90° and 45° nozzle arrangements: In wave soldering, the flow traces of the wave solder along the PCB surface are usually along the wave peak direction, so in the figure ( In the situation shown in a), there are a large number of backflow shadow areas on the PCB surface, and the soldering skip phenomenon is serious. In Figure (b), the wave solder first flows into the PCB surface along the upper right corner of the PCB at an angle of 45°. The flow path of the liquid solder surrounds each SMC/SMT on all four sides, so there is no liquid on the PCB surface. In the shadow area where the solder cannot flow, the soldering skip phenomenon is effectively suppressed; and the flow channel is unobstructed, so the bridging phenomenon can be greatly reduced.
(4) Bubble solder wave
The bubble-type solder wave was designed by the Japanese and is used on some Japanese models. It injects inert gas (such as nitrogen) into the liquid solder from the bottom of the solder tank, and generates solder waves containing countless small bubbles inside the solder tank, as shown in the figure below. The greater kinetic energy of the thermal expansion of these gases during the floating process is used to break up and capture other bubbles surrounding the solder joints, making it easier for the liquid solder to enter the welding area.
(5) Oscillating wave
In a similar traditional wide flat wave (laminar flow wave), adding an oscillation source to the solder tank will cause a slight mechanical oscillation on the surface of the solder wave peak. This oscillation can break the gas surrounding the pad and eliminate bubbles. The shielding effect can also overcome the negative surface tension existing in the non-wetted parts of the part, allowing the liquid solder to enter the area that needs to be welded, thus overcoming the shadow effect problem. In addition, the slight mechanical oscillation present in the solder also has the function of wiping the metal surface, increasing its wettability and accelerating the welding process. The crest structure of the oscillating wave is mostly in the form of a single crest, so compared with the double crest method, it also has the advantage of a simple structure.
The oscillating waves currently used in industry mainly have the following two forms according to the different ways of oscillation generation.
①Ω wave. The Ω wave structure is shown in Figure 1 below. It is a patented technology of the American Electrovert Company. Its technical key points are: several oscillators are placed in front of the nozzle opening of the wave crest, causing a slight mechanical oscillation to be generated in the solder at the front end of the wave crest. , its amplitude characteristics are shown in Figure 2 below.
As shown in the figure below, the small vibration amplitude of the vibrator can cause the liquid solder to vibrate greatly on the wave crest surface. This vibration increases the kinetic energy of the liquid solder and also enhances the wiping effect on the base metal surface. , thus improving the wetting performance of the base metal surface, thus achieving the purpose of eliminating the bubble shielding effect and shadow effect that appear in SMA wave soldering. After passing through the trembling wave area at the front end, there is a wide flat wave similar to the lambda wave. It is wide and flat and has a good trimming effect on the solder joints.
The Ω wave has similar characteristics to the double wave crest. It has both a "disturbed" entrance part and an almost stationary exit part, so it has good adaptability to SMT. However, according to relevant information, Ω waves are unable to cope with SMC/SMD installation density when the installation density is very high and the height difference between components is too large.
② Electromagnetic micro-amplitude vibration waves. The benefits of the micro-amplitude mechanical oscillation formed on the Ω wave to SMA wave soldering have been discussed previously. However, in the Ω wave, the generation of this mechanical micro-amplitude oscillation is formed by an external oscillation generator in the wave crest nozzle channel, so it increases the complexity of the structure. The electromagnetic micro-amplitude vibration wave is a unique phenomenon of the single-phase AC electromagnetic pump solder wave peak generator. It is formed by using the change of the magnetic field that generates electromagnetic thrust, as shown in the figure below.
Taking a single-phase electromagnetic pump as an example, the generation process is roughly as follows: when an AC voltage of a certain amplitude (such as 50Hz) of a certain amplitude is connected to the electromagnet excitation coil, a corresponding excitation current will be generated in the coil. The alternating thrust magnetic field of the same frequency and phase is excited in the air gap of the pump groove. This magnetic field will generate an electromagnetic force F in the liquid metal in the pump groove that forces the liquid metal to move. Its changing pattern is as shown in the figure below. Since the electromagnetic force F appears twice in a cycle from zero to a positive maximum value, as the electromagnetic force F changes periodically, the pressure of the liquid metal in the pump cavity will also change periodically. The change in pressure in the pump chamber is transmitted to the crest surface of the nozzle, causing the solder material to vibrate in the vertical direction on the crest surface. Due to the influence of the mass, kinetic energy and inertia of the solder fluid, its vibration amplitude is significantly weakened compared with the pulse amplitude of the electromagnetic force, so the amplitude on the wave surface is slight.
The 100Hz micro-amplitude mechanical vibration present in the single-phase electromagnetic pump solder peak generator will not affect the laminar flow state of the peak solder during the circulation process. Moreover, the vibration direction of the wave crest solder is vertical, which is extremely beneficial for squeezing air bags, filling micro gaps, eliminating shadow areas, etc. It has all the characteristics of Ω wave, but compared with Ω wave, the structure is greatly simplified.
(6) Rotating wave Orbital Wave (O wave for short)
It is a patented technology designed by Italian IEMME company in the late 1990s. It also uses a single wave crest structure to achieve the purpose of SMA wave soldering, so it also has a simple structure compared with the double wave crest structure.
The technical key points of the formation and function of O wave are: a series of "S" shaped stainless steel blades are installed in the standard single wave crest, as shown in the figure below. A DC motor is used to drive a cam externally, so that the connecting rod pushes the "S" shaped blade to move back and forth. The vortex generated by the constantly reciprocating blades can cause the liquid solder to enter all directions of the PCB welding area. It not only pushes away various bubbles, but also pushes the liquid solder to immerse into all dead corners of the PCB, thereby eliminating the problem of SMA wave soldering. The bubbles exist for the purpose of masking effect and shadow effect. After the PCB passes through the rotating wave zone, it then enters the later plane wave zone for final solder joint trimming.
14. The relationship between the accumulation of impurity metal in the solder tank and the dynamics of the solder wave peak
When discussing the volume of the solder tank earlier, it was mentioned that the design of the optimal volume is guided by the theory of solder wave peak dynamics. If the volume of the solder tank is optimal, then when the PCB passes through the solder wave peak, the solder in the solder tank will be continuously taken out by each solder joint, and the liquid level of the molten solder in the solder tank will continue to decrease. , so the solder tank must be constantly replenished with new solder. Since the new solder continuously added to maintain the liquid level contains less impurities, its dilution effect will continuously reduce the impurity content of the solder in the solder tank. When the speed at which metal impurities dissolve into the solder tank offsets the dilution effect of the metal impurities in the solder brought out by the soldered PCB and the newly added solder, the impurity concentration in the solder tank reaches a dynamic balance. This dynamic balancing process cannot be achieved if the volume of the solder tank is too large.
15. Main ways to stabilize wave crests
In the analysis process of the previous sections, some technical measures to stabilize the solder wave peak have been involved, which are now briefly summarized as follows:
● The volume of the solder tank should be appropriate;
● The fluid pressure generated by the pump must be appropriate. If the pressure is too high, the fluid flow rate increases, and when the fluid flow pattern develops into a turbulent (turbulent flow) state, it will cause serious instability in the wave crest;
● It is necessary to follow the principle of solder wave peak dynamics and add necessary rectification and steady-flow structures;
● Wave crest height and wave pressure must be appropriate;
● On the premise of meeting the optimal peak height of 6 to 7mm for normal operation, try to increase the width of the peak nozzle;
● Try to maintain the required normal liquid level of the liquid solder in the solder tank. If it is consumed, it should be replenished in time.
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