1. Bridging phenomenon during peak soldering

 

 

 

1 Overview

 

 

 

If welding is the most harmful hidden defect in automated welding (wave soldering, reflow soldering, etc.), then bridging is the welding defect with the highest probability of occurrence and the most complex causes in automated welding processes. It involves many factors, such as the design, manufacturing, custody and storage of PCB, the type, length, surface condition and heat capacity of component pins, the brand, quality and chemical composition of the auxiliary materials used (solder, flux, etc.) Impurity tolerance, correct selection of wave soldering process parameters, reasonable selection and adjustment of solder wave peak shape, type of welding equipment, and even climate environment (sunny days, rainy days, temperature and humidity in the air)... these are all factors that cause bridging potential factors. Eradicating bridge defects is a systematic project. If you don't pay attention to one link, all your efforts may be wasted. Therefore, it is necessary for us to conduct in-depth research on the nature of the bridging phenomenon to find the key to solving the problem.

 

 

 

2. Bridge connection

 

 

 

1) Definition

 

The phenomenon of excessive solder connecting adjacent conductors of equal or unequal potential is collectively called bridging.

 

2) Bridging phenomenon

 

(1) Phenomenon A: There is a bridge between the pad and the wire, as shown in the figure below.

 

 

 

In order to avoid this phenomenon, in the through-hole mounting (THT) method, usually the safe distance between adjacent pads or wires should be not less than 0.8mm as much as possible, and the pads of dense solder joint groups (such as dense multi-core sockets) The minimum distance between them must be no less than 1.0mm.

 

 

 

(2) Phenomenon B: Bridge connection occurs in the space between two adjacent pins, as shown in the figure below.

 

 

 

This phenomenon is usually caused by the adjacent leads extending too far out of the pad and the spacing between pins being too short. In addition, too fast welding speed and too small inclination angle also have a certain impact on the occurrence of this phenomenon.

 

 

 

(3) Phenomenon C: Horizontal bridging and longitudinal bridging between multi-core contacts, as shown in the figure below.

 

 

 

The main influencing factors of longitudinal bridging are improper selection of solder waveform, excessive lead extension length and too fast pinching speed. The occurrence of transverse bridging phenomenon is mainly caused by the lateral flow of solder in the solder wave peak.

 

 

 

(4) Phenomenon D: Compound bridging phenomenon between multi-core contacts, as shown in the figure below.

 

 

 

The reason for the composite bridging phenomenon is mainly due to the poor flatness of the wave peak, which leads to obvious lateral flow of the wave peak solder. The existence of vortex motion in the wave crest also has a certain influence on the occurrence of composite bridging phenomenon.

 

 

 

(5) Phenomenon E: In SMT wave soldering, bridges are formed due to the reflow of liquid solder caused by the obstruction of tall components, as shown in the figure below.

 

 

 

The main reason for this phenomenon is poor PCB design.

 

 

 

(6) Phenomenon F: The bridging phenomenon that occurs along the upper surface of SMC during SMT wave soldering, as shown in the figure below.

 

 

 

This phenomenon is most likely caused by contamination of the SMC surface and improper pinching direction during wave soldering.

 

 

 

(7) Phenomenon G: The bridging phenomenon occurs in the gap along the lower surface of SMC during SMT wave soldering, as shown in the figure below.

 

 

 

This phenomenon may occur because the distance between the two pads connected to the two electrodes of the SMC is too small. In addition, the narrow gap between the component body and the PCB surface forms a stagnant dead corner and retains the liquid solder, causing the problem. Bridging occurs in narrow gaps.

 

 

 

An effective way to avoid this phenomenon is to block the bridging channels along the narrow gaps, as shown in the figure below.

 

 

 

 

 

(8) Phenomenon H: The bridging phenomenon caused by improper design of SMT pad spacing, as shown in the figure below.

 

In the figure, although the distance between the body electrodes of two adjacent components is appropriate, the pad spacing is too small, causing the solder filling between the two adjacent electrodes to cause bridging. In SMT wave soldering, the safety distance between adjacent SMC/SMD component bodies and related pads should be as shown in the figure below.

 

 

 

bridging phenomenon

 

 

 

 

 

Design of pad distance between connected SMCs

 

 

 

 

 

Distance design between SMC and SOIC

 

 

 

 

 

  The distance between SMC and transistor

 

 

 

(9) Phenomenon I: In wave soldering, sometimes a large area of non-wetting bridges occurs in a linear area accompanied by a large number of sharp points, as shown in the figure below.

 

 

 

The root cause of this phenomenon is that the flux spray system is not normal and there is a missed coating area.

 

 

 

3. Principle of bridge formation

 

 

 

1) The principle of bridging

 

In order to describe the reasons for bridging between adjacent wires and pads during wave soldering, the following is only a simple model of bridging between adjacent pads or conductors as shown in the figure below. .

 

In the figure below, R1, R2, R3, and R4 are the radii of curvature formed by the molten solder. The curvature radii such as R3 and R4 located outside the solder are set to negative values, while those located inside the solder such as R1 and R2 are set to Positive value, f is the surface tension of liquid solder.

 

 

 

 

 

The internal pressure P3 and P4 of the molten solder forming a bridge between adjacent wires or pads can be expressed as

 

 

 

in the formula

 

P3, P4——Additional internal pressure of molten solder at the bridge connection;

 

f——surface tension of molten solder;

 

R3, R4——radius of curvature at the bridge connection.

 

The force direction of internal pressure P3 and P4 points to the outside of the molten solder in the bridge area, as shown by the arrows in the figure below. The effect of this internal pressure is to continuously expand the molten solder at the bridge and increase the cross-sectional area of the bridge. The internal pressures P1 and P2 on the pads or wires shown in (b) below can be expressed as

 

 

 

 

 

The internal pressures P1 and P2 on the pad or wire are positive, and their direction of action is toward the inside of the liquid solder on the pad, as shown in Figure (b). Therefore, once bridging occurs, since the internal pressure of the liquid solder located in the bridge part is low and the internal pressure of the liquid solder located on the pad or wire is high, the pressure difference between them can be expressed as follows:

 

 

 

It can be seen that the molten solder flows from the pad or wire part to the bridging area, causing the solder to continuously increase and accumulate in the bridging area, thereby aggravating the occurrence and development of the bridging phenomenon.

 

Since the value of P4 is also negative, it also constitutes a driving force for absorbing molten solder in high internal pressure areas.

 

 

 

It can be seen from the above formulas that as the spacing and width between adjacent wires and pads continue to narrow, the curvature radii R1, R2, R3, and R4 will continue to become smaller. It can be seen from the above equations that the additional internal pressure of the molten solder located in the bridge area will also become lower, while the additional internal pressure of the liquid solder on the pad or wire will become higher, and the molten solder will The solder continuously flows from the high internal pressure area to the low internal pressure area, causing more liquid solder to accumulate in the bridging area, thus greatly increasing the probability of bridging. The internal pressure difference ΔP is proportional to the surface tension of the liquid solder, so reducing the surface tension f can reduce the internal pressure difference between them, thereby reducing the possibility of bridging.

 

The bent part of the IC pin also constitutes the negative pressure area in the solder during wave soldering, and its value is

 

 

 

 

 

Therefore, this area forms a high-risk area for bridging of such devices during wave soldering, as shown in the figure.

 

 

 

 

 

2) The area where bridges are formed during wave soldering

 

From the above analysis, it can be seen that the essence of the bridging phenomenon is the pressure difference inside the thin solder layer in the peeling area formed when the PCB exits the wave peak. Ignoring the factors affecting the flow characteristics of the solder, it can be considered that the existence and size of this pressure difference are determined by the surface tension of the molten solder and the tiny curvatures that exist on the final peeling line in the wave peak peeling thin layer zone. The radius of the line segment, as shown in the figure below.

 

 

 

Obviously, the peeling thin layer area formed during the peeling process of PCB and liquid solder during wave soldering is the area where the bridging phenomenon occurs. In other words, all bridges in wave soldering are formed in this area.

 

 

 

4. Influence of bridging phenomenon during wave soldering

 

 

 

Factors that occur In industrial production operations, there are many direct or indirect factors that influence the formation of bridging phenomena, such as the waveform of the solder wave peak, the flatness of the solder wave peak, the temperature of the liquid solder, the distance between adjacent conductors The spacing, the cleanliness of the base metal surface, the purity of the solder, the activity of the flux, the height of the pins protruding from the board, the pinching angle during operation, the depth of the PCB pressed into the wave solder... Let’s introduce each of the above factors and their specific physical and chemical processes one by one.

 

 

 

1) Influence of solder peak shape

 

The wave soldering effect is closely related to the correct design of the solder wave peak shape. The correct selection of the solder working waveform is an important step in reducing welding defects (bridging, pull points, pores, board surface dirt, etc.), as shown in the figure below.

 

 

 

 

 

The shortcomings of the waveform shown in (a) above are as follows:

 

① The front end of the wave is too steep, which not only has no filling effect on the longitudinal dents, grooves, etc. that appear on the wave surface, but also has an amplifying effect.

 

② The rear part of the wave crest is horizontal, which tends to retain oxidized scum.

 

③ The fluid flow rate (v2) is very small (v2＜＜v0). When the PCB and solder wave peaks are separated, the speed is not easy to match, and the separation conditions are poor.

 

④ The wave crest width is too small (k1＜k1＇), the production efficiency is not high, and the penetration performance is poor.

 

The advantages of the waveform shown in (b) above are:

 

① The front part of the wave is wide, the falling process is relatively slow, and it has a good filling effect on the longitudinal dents and grooves appearing on the wave surface.

 

② The liquid flow in the first half of the wave has a long interaction time against the PCB pinching direction, and the wiping and infiltration effects are good.

 

③ The wave surface is curved, making it difficult to retain oxidized scum.

 

④ v2≈v0, the speed matching is good when leaving.

 

⑤ Adjusting the adjustment plate 1 can easily adjust the waveform characteristics of the first half and the size and direction of the wave speed v1, while adjusting the adjustment plate 2 can change the waveform characteristics of the second half and the size and direction of the wave speed v2 to achieve the best breakaway conditions. :

 

v1>v0

 

v2≈v0 (the relative speed of the breakaway point (B) is zero)

 

in the formula

 

v0——PCB speed at breakaway point (B);

v1——Solder fluid velocity at entry point (A);

v2——The velocity of the solder fluid at the breakaway point (B).

⑥ The width of the wave: k1′>k1, so the production efficiency of the waveform shown in (b) above is higher than that of the waveform shown in (a).

 

 

 

2) Influence of solder wave peak flatness

 

The poor flatness of the solder wave peak is the root cause of lateral flow after the wave solder is immersed in the PCB. During the wave soldering process, the lateral flow and vortex motion in the wave peak are responsible for the generation of dense solder joint groups (such as multi-core sockets, etc.) The root cause of the horizontal bridging and composite bridging phenomena is shown in the figure below.

 

 

 

In the figure above, when v_ is too large, it is the main cause of lateral bridging, and the synthetic speed va is often the most likely to cause composite bridging.

 

 

 

3) Effect of temperature The factors that affect the surface tension f are mainly the temperature and viscosity μ of the liquid solder for molten solder. The changing relationship between them can be described by the following formula:

 

 

 

in the formula

 

f——Surface tension;

T——absolute temperature;

μ——viscosity;

μ0 - viscosity at room temperature;

E——activation energy; k——Boltzmann constant.

It can be seen from the formula that as the temperature increases, the surface tension and viscosity of the molten solder will decrease, which is beneficial to the peeling of the molten solder from the PCB. However, if the temperature is too high, the oxidation of the molten solder will be intensified, and the surface of the liquid solder will be wrapped by an oxide film. The overall effect is equivalent to an increase in surface tension and a deterioration in the diffuse flow of the molten solder. At this time, the excess solder in the thin layer is not easily pulled back to the wave crest, and a "bridge" is formed. Therefore, choosing the appropriate welding temperature has a great impact on eliminating "bridging". It will not work if it is too high, and it will not work if it is too low. Since the surface tension of the solder is affected by the temperature of the solder, high temperature will reduce the surface tension, but it will also intensify the oxidation of the surface of the liquid solder. Therefore, excessive heating cannot significantly improve the surface tension of the solder.

 

 

 

4) The influence of spacing between adjacent wires or pads

 

Affects some curve segments located on the peel line of the thin layer area. The size of the radius of curvature is a function of the spacing between adjacent conductors. As the spacing increases, the radius of curvature increases and the additional internal pressure decreases, making it less likely to form bridging conditions. On the contrary, the spacing between adjacent conductors becomes narrower, the radius of curvature of the curve becomes smaller, and the internal pressure of the molten solder across the two adjacent conductors of the stripping line becomes larger, resulting in contact with the adjacent area (pad The internal pressure difference between the molten solder in the adjacent area (pad or wire) increases, and the molten solder in the adjacent area (pad or wire) flows to the crossover area, causing the solder to accumulate in the crossover area and form a bridge.

 

 

 

5) Influence of base metal surface cleanliness

 

The surface cleanliness of the base metal reflects the wetting ability of the molten solder on the surface of the base metal, which is usually described by the wetting force (adsorption force). We know that there is a force field composed of atomic gravity on the surface of pure metal. Only when the surface of the base metal is very clean, can the solder and base metal atoms be close to the distance that can attract and combine with each other, that is, close to the distance caused by the atomic gravity. The distance of action. The wetting force (adhesion) of the solder to the base metal is expressed as the sum of the attraction between atoms and the force that exists when the molten solder wets and spreads through the capillary straws formed along the convexities and scars on the surface. . When oxides or dirt adhere to the surface of the base metal or the surface of the molten solder, it will prevent the atoms of the molten solder from freely approaching the atoms of the base metal, so that wetting will not occur.

 

 

 

The wetting force of the molten solder on the base metal has a great influence on the bridging phenomenon. As shown in the figure below, the influence of the surface cleanliness of the base metal on the formation of bridging phenomenon (ignoring the influence of gravity), the direction of the wetting force Fw of the molten solder between two adjacent pads on the base metal is toward the welding It has a tendency to attract the middle solder toward the pad. Obviously, this force is good for eliminating bridging. However, the additional negative pressure in the liquid solder located in the bridging area will suck the solder on the pad toward the bridging area, and its effect is exactly opposite to the effect of the wetting force. If the pad surface is very clean, the wetting force will be large and the cancellation effect will be great, so it is not easy to bridge; if the surface is contaminated, the wetting force will be very small or even zero, so the cancellation effect of the wetting force will If it is very weak or even lost, the possibility of bridging is very high.

 

 

 

 

 

6) The influence of solder purity

 

We know that the surface tension of a liquid is related to the purity of the liquid. If impurities are mixed into a pure liquid, although there is sometimes a tendency to reduce the surface tension, the viscosity of the liquid solder will increase significantly. The increase in viscosity will lead to a significant deterioration in the fluidity of the liquid solder. The phenomenon is similar to that of increased surface tension. In wave soldering, the solder in the solder tank will be continuously contaminated by impurity metals (such as copper, etc.). When the impurity metal accumulates to a certain concentration, it will change the physical and chemical properties of the alloy, resulting in significant changes in the viscosity of the solder. (increasing), so bridging will occur frequently.

 

 

 

7) The influence of flux activity and preheating temperature. Usually the surface of the base metal to be soldered is wrapped by a thin oxide film. Therefore, before wave soldering, the active substances in the flux must be removed to obtain a pure metal surface. . If the activity of the flux fails to achieve this purpose, bridging will occur due to poor wettability. The full development of flux activity is closely related to the preheating temperature. If the preheating temperature is low, the chemical activity of the flux will not be fully exerted, the surface cleanliness of the base metal will not meet the requirements, and it will easily cause bridging. Preheating temperature that is too high is not conducive to eliminating the bridging phenomenon. There are two reasons for this:

 

① in fluxThe solvent evaporates excessively, the active agent decomposes rapidly, and the purified surface is not protected and secondary oxidation occurs.

 

② As the flux dries up and the PCB substrate enters the wave peak, the flux has lost its function of reducing the surface tension of the liquid solder and accelerating the overflow of the solder.

 

 

 

8) The installation design of PCB components is unreasonable and the heat capacity distribution difference on the board is too large

 

In practice, the following phenomenon often occurs: There are no problems with the setting of wave soldering process parameters, the solderability of the base metal surface, the composition and physical and chemical properties of solder and flux, but there are always problems in some areas with dense solder joints with large heat capacity. A bridging phenomenon occurs. After analyzing this phenomenon, we found that it was purely caused by unreasonable installation design. These PCBs densely distribute components with large heat capacity in a certain area, thus forming a large heat capacity area. Under the same operating conditions during wave soldering, this part of the area cannot absorb enough heat from the wave solder to reach the wetting temperature, resulting in a lower temperature in this part and a higher surface tension and viscosity of the solder, thus Causes poor wettability and forms bridges. Increasing the temperature of the solder tank or reducing the pinching speed can inhibit the occurrence of bridging in this area, but most components in areas with small heat capacity will be damaged due to overheating. Theoretically speaking, this problem can be solved by using a small inertia variable speed automatic dragging and pinching system. However, existing wave soldering equipment does not have such a function.

 

 

 

9) The influence of PCB soldering depth on bridging phenomenon

 

The depth of soldering of the PCB during wave soldering reflects the amount of liquid pressure the PCB experiences on the wave crest. Although the upward pressure generated by immersing the solder to a certain depth is a thrust during the formation of the through-hole solder joint, its main function is to gradually supply the solder and transfer heat, rather than causing the solder to climb up. The depth of immersion should be compatible with the PCB thickness and type. Excessive size not only easily causes the molten solder to overflow to the component surface of the PCB, but is also a factor leading to bridging. The main reasons are as follows:

 

① The flow-blocking effect of PCB thickness is obvious. The pipe cross section becomes narrower and the flow channel is not smooth, which destroys the laminar flow state in the pipe;

 

② When the PCB leaves the solder wave peak, the peeled thin layer area will be widened and thickened, and more liquid solder will be carried out by the PCB and accumulated in the negative pressure area to form bridges and points.

 

 

 

10) The impact of the height of the component pins protruding from the PCB on the bridging phenomenon

 

The height at which the component leads protrude from the soldering pad is an important factor causing bridging between adjacent solder joints, especially for dense solder joint groups (such as multi-core sockets).

 

In the figure (a) below, the pins protrude too far from the board. Due to the shadow effect of the front pins, the peeling thin layer area is elongated when detaching, and the rear solder joints and pins are fully integrated into the thin layer area. Therefore, Created bridges.

 

 

 

 

 

The picture above (b) shows the standard protrusion height. Since the shadow effect of the front pin is not obvious and the peeling thin layer area is very narrow, it is impossible to span the two pads, so it is not easy to form a bridge.

 

 

 

The safe height of the pin extension is related to the shape of the solder wave crest and the tilt angle of the pinch. Based on preliminary calculations based on practical experience, the critical value of the safe height of the multi-core socket (the height where bridging does not occur) is approximately 0.7mm. .

 

 

 

Other influencing factors such as: improper pinching angle and direction when the substrate passes through the wave peak, poor PCB wiring and installation design, contamination of the substrate surface by dirty solder, unreasonable lead bending direction, etc. will not be analyzed in detail.

 

 

 

5. Prevention of bridging phenomenon

 

① Improve the surface tension of solder.

 

② Change the wave speed characteristics of the solder wave peak peeling thin layer area.

 

③ Adjust the welding time and pinch speed.

 

④ Adjust the welding temperature and preheating temperature.

 

⑤ Adjust the pinch angle and pressure wave depth.

 

⑥ Test the effectiveness of flux.

 

⑦ Improve the coating method and amount of flux.

 

⑧ Correct poor design.

 

⑨ Correctly handle the bending direction and extension height of the lead.

 

⑩ Strictly monitor the degree of contamination (impurity metal content) of the solder in the solder tank.

 

 

 

2. The occurrence and prevention of poor filling of metallized holes

 

 

 

1. Phenomenon

 

 

 

Poor filling of metallized holes is a common defect that is difficult to deal with in lead-free wave soldering. The actual case is shown in the figure below.

 

 

 

 

 

2. Basic requirements for filler metal fillability of metallized holes in wave soldering

 

 

 

1) U.S. military standard MIL-S-45743E

 

It is stipulated that metallized via holes and metallized holes with component leads are allowed to be dented on the PCB component surface, and the total dent amount shall not exceed 25% of the hole depth (including the thickness of the pads on both sides of the PCB), and It is required that the wetting performance around the hole is good, as shown in the figure below.

 

 

 

 

 

2) IPC-A-610D Level 2 and 3 requirements

 

① SMT via hole optimization (level 1, 2, and 3 requirements): The hole is completely filled with solder, and the top connection pad is well moistened, as shown in the figure (a) below. Acceptable (level 1, 2, and 3 requirements): The solder wets the hole wall, as shown in (b) below.

 

 

 

 

 

② Minimum acceptable conditions for metallized hole lead welding quality requirements

 

The minimum acceptable conditions of IPC-A-610D for the welding quality of metallized hole leads are shown in the figure below, and the specific data requirements are shown in the table below.

 

 

 

IPC-A-610D Fillability Requirements for Metalized Holes with Leads

 

 

 

Minimum salvageable conditions for metallized hole lead soldering

 

 

 

 

 

3. The main manifestations of poor filling performance

 

 

 

1) Common phenomenon of poor filling of mounting holes

 

(1) Poor through-holes caused by poor solderability

 

① Phenomenon A: The phenomenon of poor penetration caused by good wetting of the pad and hole wall and poor lead solderability is characterized by a small wetting angle of the solder on the pad and hole wall, but a large wetting angle of the lead. The solder liquid level forms an inverted "eight" shape, as shown in Figure (a) below.

 

② Phenomenon B: The phenomenon of poor penetration caused by good lead wetting and poor solderability of the pad and hole wall is characterized by a very small wetting angle of the solder on the lead and a very small wetting angle on the pad and hole wall. Large, the solder liquid level in the pores forms a positive "eight" shape, as shown in the figure (b) below.

 

③ Phenomenon C: The phenomenon of poor through-hole caused by poor solderability of leads, pads and hole walls is characterized by a large wetting angle of the solder on the leads, pads and hole walls, and the solder fillet in the solder corners and pores The liquid surface of the material is all convex, as shown in (c) below.

 

 

 

 

 

(2) Poor penetration caused by improper selection of process parameters

 

① Phenomenon D: Poor penetration caused by insufficient heat supply during the wave soldering process (pinch speed is too fast, solder tank temperature is low) or the flux does not penetrate into the hole in the spray. It is characterized by the presence of holes in the pores. The liquid level is convex, as shown in the figure below.

 

 

 

 

 

② Phenomenon E: The heat supply during wave soldering is appropriate, but the PCB is immersed in the solder material and the wave crest is shallow, resulting in poor through holes, as shown in the figure below.

 

 

 

 

 

4. Analysis of defective factors of through holes in wave soldering

 

 

 

1) Mathematical description of the physical process of solder penetration in wave soldering

 

In "The Occurrence and Prevention of Virtual Soldering Phenomenon" we have already discussed the principle of capillarity. In wave soldering, the penetrability of metallized holes has a great relationship with capillary action. What factors determine the penetration height of liquid solder in the hole?

 

The figure below shows the physical model of the capillary action process of liquid solder in metalized holes during wave soldering.

 

 

 

 

 

Due to the surface tension and surface energy of the liquid solder, the liquid surface in the hole gap presents a meniscus-shaped surface. There is a pressure difference on this surface, which can be described by the Laplace equation as follows:

 

 

 

 

 

θ (contact angle or wetting angle) (0＜θ＜180°)

 

in the formula

 

R1 ——radius of curvature of the meniscus within the gap;

R2——Inner diameter (radius) of the hole;

ΔP——interface pressure;

γ——interface energy.

From a physical perspective, the gap between the inner wall of the PCB metallized hole and the lead surface is a cylindrical structure. Since the thickness of the gap is very small compared to the circumferential length of the hole, the wetting process of the liquid solder in the gap between the hole and the lead can be equivalent to the wetting process of a simple infinitely long flat plate, that is

 

 

 

 

 

set up:

 

D2——The diameter of the large cylinder (metal-plated hole);

D1——The diameter of the small cylinder (lead).

As shown in the figure above, it can be obtained

 

 

 

Thus it can be obtained

 

 

 

last available

 

 

 

 

 

The interface pressure expressed by the pressure corresponding to the height (h) of the liquid column in the gap is equivalent, that is

 

 

 

 

 

in the formula

 

g——gravitational acceleration;

h——Height of liquid column;

ρ——liquid density.

Substituting the above two equations into Eq., the height h can be obtained as

 

 

 

 

 

It should be pointed out that the above laws are obtained under the condition that there is no interaction between liquid and solid. In fact, during the wave soldering process, there will be more or less mutual diffusion movement between the liquid solder and the base metal, which will cause some changes in the composition, density, viscosity and melting point of the liquid solder, thus causing the capillary The caulking process is complicated. It may even happen that the liquid solder that spreads well on the surface of the base metal cannot flow into the gap. The reason for this situation is often because the solder has already interacted with the base metal outside the capillary gap, and has been saturated with the base metal and lost its flow ability.

 

 

 

In the wave soldering process, the process of liquid solder filling the gap is not a simple physical process. It is a comprehensive process of physical, chemical, metallurgical and other factors that occur in the surface state of the hole wall, the spreading ability of the liquid solder, the interaction between the liquid solder and the hole wall matrix material, and the flux. .

 

 

 

In wave soldering, the above-mentioned porosity is further improved due to the effect of flux. After adding flux, the physical process of solder penetration in wave soldering can be described as shown in the figure below.

 

 

 

 

 

2. Analysis of factors affecting poor penetration

 

In wave soldering, the functional relationship between the height (h) of the wave solder penetration hole and each influencing factor can be described as follows.

 

(1) Proportional to the interface energy (γ)

 

Since liquids have a spontaneous tendency to reduce the surface area, if you want to increase the surface area of a liquid, you need to transfer some of the liquid atoms from the interior of the liquid to the surface. A certain amount of work needs to be done to overcome the attraction of the internal atoms to it. This part of the work is converted into The energy of this part of the atoms newly transferred to the surface. We express the work done to increase the unit surface area of the liquid as γ, which is called the liquid surface free energy. Usually, the surface free energy is specifically regarded as the force acting on the unit length of the liquid surface along the tangential direction of the liquid surface to reduce the surface. The work done to increase the surface area of a liquid can also be seen as the force that overcomes the shrinkage of the liquid surface per unit length, so γ can also be called surface tension. Therefore, a small surface tension means that the resistance that atoms overcome when moving from the inside to the surface is small; a large surface tension means that the resistance that atoms overcome when moving from the inside to the surface is large.

 

 

 

However, in wave soldering, when the liquid solder contacts the solid hole wall, it is not simply affected by the surface tension on the solid interface. In fact, it is under the combined action of three phases. The liquid flux covers the liquid solder and the base metal copper, forming a three-phase interaction of liquid flux, liquid solder and solid base metal. The liquid solder atoms at the three-phase interface are affected by three surface tensions at the same time (point A in Figure 10.30), that is:

 

● Surface tension γLS of liquid solder on the solid matrix metal interface;

 

● Surface tension γLF at the interface between liquid solder and liquid flux;

 

● Surface tension γSF at the interface between solid base metal and liquid flux.

 

 

 

The action directions of the above forces are as shown in the figure below. Their combined force determines the wetting of the liquid solder in the pores. When γSF>γLS, the liquid solder in the pores continues to climb and wet along the inner wall of the pores under the action of the resultant force of (γSF-γLS) until it reaches equilibrium. The state at this time can be described by Young’s formula, that is

 

 

 

 

 

Taking Sn-Pb solder as an example, the surface tension γ of liquid solder has the following relationship with temperature T:

 

 

 

in the formula

 

ρ——density of liquid;

K——constant, equal to 2.12;

T0——critical temperature when surface tension is zero;

τ——Constant, value 6~9.

It can be seen from the above formula that as the temperature increases, the surface tension of the liquid continues to decrease. The figure below shows the relationship between surface tension and temperature of tin-lead solder. As the temperature increases, the surface tension of the solder decreases, which helps to improve the wettability of the solder. In order to improve the porosity of the solder, it is very important to choose the appropriate welding temperature.

 

 

 

 

 

(2) Proportional to the wetting coefficient (cosθ)

 

According to Young’s formula, the wetting coefficient (cosθ) can be obtained as

 

 

 

 

 

Obviously, the wetting coefficient cosθ describes the synthesis of surface tension effects on the three-phase interface. Therefore, the quality of the porosity of liquid solder in the pores is not a unilateral behavior of the liquid solder, but should be the comprehensive behavior of the ternary system of liquid solder, base metal and flux. The wetting angle θ when liquid Sn and Sn-37Pb are wetted on Cu is as shown in the table below.

 

 

 

Therefore, during wave soldering, the solder can only fill the pores under the condition that the liquid solder can fully wet the base metal.

 

 

 

(3) Inversely proportional to the density of the solder (ρ)

 

The greater the density (ρ) of the solder, the greater the resistance it encounters when climbing in the pores. Therefore, the height of climb will be limited.

 

 

 

(4) Inversely proportional to the size of the gap (D2-D1).

 

As the gap decreases, the rising height of the liquid solder in the pores increases. As shown in the figure below, the relationship between the rising height of the liquid solder (h) and the size of the gap (D2-D1) between the two copper plates.

 

 

 

 

 

From the perspective of the rising height (h) of the solder, a small gap is better. Therefore, in order for the solder to fill the pores during wave soldering, a small gap must be ensured in the joint and installation design.

 

 

 

(5) It is related to the presence or absence of flux and the strength of activity

 

The presence or absence of flux directly affects the interface energy γ, and the size of the interface energy γSF is also affected by the activity of the flux. Therefore, the stronger the flux activity, the greater the interface energy γSF, and the wettability is greatly improved, so the pore filling ability becomes stronger.

 

 

 

(6) Related to the immersion depth in the wave crest

 

The PCB is immersed into the solder wave peak to a certain depth to obtain a certain upward pressure in the pores, which is beneficial to the filling process of the pores with the solder.

 

 

 

(7) It is related to the oxides retained on the wave peak surface of the solder.

 

Since the oxides retained on the wave crest surface hinder the wetting of the base metal by the liquid solder, it also hinders the filling of the pores by the liquid solder, thus constituting the factor of poor filling of the pores by the solder.