1. Definition

 

    Wave soldering refers to the use of a pump to use molten liquid solder to form a solder wave peak of a specific shape on the solder liquid surface, and the PCB loaded with components is soldered at a specific angle and at a certain angle. The immersion depth passes through the solder wave crest to achieve the soldering process of the joint. It is also called group welding or flow welding abroad.

 

2. Development history of solder wave soldering technology

 

1. Development of solder wave soldering power technology

 

     In 1949, Americans S.F. Danko and Abramson invented the dip soldering method for printed circuit boards, heralding the birth of a new process for PCB soft soldering. However, the dip soldering method is only suitable for low-end electronic products and cannot meet the production requirements of modern electronic products that are light, thin, short, and small.

 

     In 1956, the British company Fry's Metal invented the printed circuit board wave soldering method, which meant the beginning of a new era in the field of PCB welding. It moved PCB from manual soldering iron point by point welding to machine automated large-area high-efficiency welding. The PCB welding process has truly entered the era of automation.

 

     Although solder wave soldering technology was invented in the United Kingdom, the fastest technological advancements are in the United States, Switzerland, Italy, the Netherlands, and Germany. Electrovert Company in the United States, EPM Company in Switzerland, IEMME Company in Italy, Slotec Company in the Netherlands, ERSA Company in Germany, etc. Their product technologies basically reflect the latest development results of modern wave soldering technology.

 

2. The role and classification of solder wave crest power technology

 

     1) Function and requirements: Solder wave crest power technology is to study the specific solder wave crest required by the wave soldering process. It is the core that determines the quality of wave soldering and the most characteristic core component of the entire system. It is also an important criterion for measuring the performance of the wave soldering system. Solder wave crest power technology is a comprehensive technology that integrates solder wave crest dynamics theory, fluid mechanics, metal surface theory, metallurgy, thermal engineering, electrical engineering and other multi-disciplinary knowledge. Its technical requirements mainly include:

 

        ● The wave crest is stable and the height is adjustable; it has a certain ability to inhibit the oxidation of high-temperature liquid solder;

 

        ● Under the premise of ensuring the thermal capacity, the solder tank capacity is the smallest; it has good thermal properties, saves energy and electricity, and has complete automatic temperature control measures;

 

        ● All solder materials can be in a circulating state, thereby maintaining a high degree of uniformity in the composition of the solder material and avoiding dead ends of deposited eluted phases in local areas;

 

        ● The design and arrangement of the heater should ensure that the solder is heated evenly without hot spots;

 

        ● The contamination of the solder tank by impurity metal is minimal, and the concentration of impurity metal should be maintained at a small safety level and achieve dynamic balance during long-term operation.

 

     2) Classification of power technology used in solder wave cresting

 

         The classification of solder wave power technology used in wave soldering equipment can be roughly summarized as shown in the figure below.

 

 

 

 

 

3. Mechanical pump type solder wave peak generator

 

      1) The structure and principle of the centrifugal pump type solder wave crest generator:

 

         The typical structure of the mechanical pump type solder wave peak generator is shown in the figure below

 

 

 

     The centrifugal pump type solder wave crest generator uses an electric motor to drive a pump blade, and uses the centrifugal force of the rotating pump blade to drive the liquid solder fluid to flow into the pump chamber. Driven by pressure, the liquid solder flowing into the pump chamber is rectified by the rectification structure and then flows to the nozzle to form a solder wave peak. Most of the solder in the solder tank is sucked into the pump cavity from the lower bottom surface of the center part of the pump impeller shaft.

 

 

 

    In order to reduce the generation of welding slag, lower the surface tension of the liquid solder and reduce tipping and bridging, the American Hollis Company (later merged by Electroce) invented a centrifugal pump with a Z-shaped rectification structure that can inject oil into the solder. The structure of the solder wave peak generator is shown in the figure below.

 

 

 

 

 

2) Structural principle of spiral pump type solder wave peak generator

 

     The typical structure of an axial flow pump is shown in the figure below. The difference between it and the centrifugal pump is that it propels the liquid solder in a different manner. It uses a propeller of a special shape to rotate to generate axial thrust, forcing the fluid to flow along the axial direction. The screw pump type solder wave crest generator is also a structural form that is widely used in industry.

 

 

 

 

 

3) Mechanical pump type solder wave peak generator

 

      The solder wave generator based on the mechanical pump wave soldering method based on the invention of Fry's Metal has a long history, but it has a complex structure, many rotating parts, easy wear of the machine parts, low reliability, and difficult maintenance;

 

4) Liquid metal electromagnetic pump type solder wave peak generator

 

        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 changes in the magnetic field that generates electromagnetic thrust, as shown in the figure below.

 

      

 

 

 

         The working process of a liquid metal electromagnetic pump 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, thereby pumping gas in the pump ditch. The alternating thrust magnetic field of the same frequency and phase is excited in the gap. 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.

 

3. System classification and characteristics of wave soldering equipment

 

1. Classification according to welding process: it can be divided into two categories: primary welding system and secondary welding system.

 

2. Classified by system size: it can be divided into four categories: microcomputer, minicomputer, medium-sized computer and large computer.

 

3. Classification according to the number of wave crests:

 

      1) Single wave crest machine: It has only one wave crest and is widely used in THT PCB welding. Specially designed single wave crest machines (such as Ω wave, O wave, etc.) can be compatible with both THT and SMT PCB welding.

 

      2) Double wave machine: It is mainly designed for the shadow effect and airbag shielding effect that exist in SMT wave soldering. With the popularity of SMT, this type of model has become the first choice for users.

 

4. Flux coating system

 

1. The role of flux coating system in wave soldering process

 

      Since the surface of the metal to be welded is generally contaminated by oxides, which will affect the welding between the wave solder and the base metal, flux needs to be used to remove the oxide layer.

 

      Therefore, the purpose of setting up a flux coating system in a wave soldering system is to automatically and efficiently apply flux to the soldered surface of the PCB.

 

2. Technical requirements for flux coating system

 

      To evaluate the working condition of a flux coating system, it is usually considered from the perspective of the thickness and uniformity of the applied flux layer, that is,

 

     ● The coating layer should be uniform and have good coverage of the welded surface;

 

     ● The coating thickness is appropriate and there is no excess flux flowing;

 

     ● High coating efficiency, minimum flux consumption while ensuring wave soldering requirements;

 

3. Commonly used coating methods and structural analysis

 

    1) The classification of flux coating methods mainly includes the categories shown in the figure below.

 

 

 

    2) Foam wave coating method: The device generally consists of a flux tank, a nozzle and a porous foam tube immersed in the flux tank. The structural diagram is as follows.

 

 

 

    3) Spray coating method

 

      Spray methods can be roughly divided into three categories: direct spray, rotary screen spray and ultrasonic spray.

 

① Direct spray coating method.

 

(a) The applicable scope and characteristics of the direct spray coating method: This method is only suitable for coating liquid flux with low solid content. It uses the negative pressure effect generated by the high-speed air flow of filtered clean compressed air to transfer the liquid flux. The flux droplets sucked out from the needle-shaped holes of the spray head and mixed in the high-speed air flow are crushed and split into scattered and extremely small mist-like droplets due to the action of air resistance to form directional high-speed mist. Apply the flux to the soldering surface of the PCB using a circular airflow. Since the solvent in the flux is usually composed of highly volatile materials, under the action of high-speed airflow, the solution will evaporate on its own after the flux is sprayed on the workpiece, or it will evaporate quickly in the form of liquid particles as soon as it is sprayed out of the nozzle. , the flux falling on the surface of the workpiece is very sticky.

 

  (b) Direct spray coating system structure: The direct spray coating system usually consists of a flux storage tank, a spray head, an air flow regulator, etc. The specific structural diagram is shown in the figure below.

 

 

 

 

 

② Rotary screen spray coating method.

 

(a) The scope and characteristics of the rotary screen spray coating method: This method mainly involves immersing a part of the rotary screen (mesh) made of stainless steel or other flux-resistant materials into a flux container, and the immersed part of the mesh is The eye is filled with flux; the rotary screen rotates around its axis, and the rotation speed is variable; the air nozzle is located on the axis of the rotary screen and is directed towards the PCB located above the flux container; the air flow can be a continuous air flow or when the PCB is on When above the nozzle, a position sensor controls the intermittent air flow that starts and stops; the rotation speed and air pressure of the screen determine the amount of flux deposited on the PCB, and changing the mesh size of the rotary screen can control the size of the mist particles. This method is most suitable when the PCB adopts long insertion method, and the height of the component leads extending out of the PCB board can reach 5cm. When foam wave coating is used, the height of the lead exposed to the PCB board is usually limited to less than 10mm.

 

(b) Rotary screen spray coating system structure: The structural diagram of the rotary screen spray system is shown in the figure below.

 

 

 

(3) Ultrasonic spray coating method

 

      The ultrasonic spray coating method uses the cavitation effect of ultrasonic energy to turn the liquid flux into an atomized form and apply it to the soldering surface of the PCB. According to the different methods of the atomization process, it can be roughly divided into the following two forms.

 

     ① Jet ultrasonic atomization method. The basic structure of this method is similar to the direct spray method. The difference is that an ultrasonic vibrator is introduced at the nozzle of the spray gun to form an ultrasonic suspension oscillation atomization zone. The high-speed jet flow of flux induced by direct pressure or negative pressure usually has relatively coarse particles and is very uneven. However, after passing through the ultrasonic suspension oscillation atomization zone and further refinement, a mist flow with finer and more uniform mist particles can be formed.

 

    ② Ultrasonic atomization spray method. The fundamental difference between the ultrasonic atomization spray method and the jet ultrasonic atomization method is that the latter first generates a high-speed jet flow, and then atomizes it into a mist flow through the ultrasonic suspension oscillation zone; while the former directly sprays the liquid flux Atomization, and then supplemented by appropriate technical means to form a directional mist flow. The order in which the atomization process occurs between the two is different, and obviously the effects will also be different. The ultrasonic atomization spray method forms a pure mist flow, the mist particles are fine and uniform, never mixed with liquid particles, and it is a truly pressure-free coating method; while the jet ultrasonic atomization method, due to the high-speed jet flow flowing through the ultrasonic When the oscillation zone is suspended, affected by the differences in the amplitude and phase distribution of vibration in the ultrasonic action zone, the uniformity of the thickness of the formed mist particles will certainly be affected, and may even be mixed with liquid particles.

 

       Since the mist droplets formed by ultrasonic atomization are much finer than direct atomization and air atomization, the coating effect is good, the flux consumption is small, and the residue on the PCB board after wave soldering is minimal. So far, it is generally believed that the ultrasonic atomization spray coating method is the most advanced design currently on the market.

 

(4) The main characteristics of various spray methods are compared in the following table

 

 

 

5. Preheating system

 

1. The role of preheating system

 

As a station in the wave soldering equipment system, the preheating system has the following functions in practical applications:

 

① Promote the flux activity to be fully exerted. The PCB coated with flux needs to be heated to the activation temperature so that the chemical components can interact with the base metal oxide and the metal surface oxide can be removed (for example, rosin-based flux needs to be heated to about 104°C), and it needs to be at this temperature Only by staying there for a sufficient time can the flux be able to fully purify the soldered surface of the PCB. Therefore, sufficient preheating time is required before welding.

 

② Remove excess volatiles from the flux to improve the welding quality. Since before wave soldering, the volatile materials in most fluxes are still mixed with rosin. Some organic acid fluxes also contain moisture. If wave soldering is performed directly in this state, the heat of the solder tank will quickly vaporize the solvent, which will not only cause the solder itself to splash, but also the vapor will be trapped in the filled solder to form pores. The latent heat of vaporization consumed by the volatilization of a large amount of solvent will cause the PCB welding surface temperature to drop sharply, leading to the occurrence of welding defects such as virtual soldering, bridging, and tipping. Therefore, it needs to be preheated and evaporated in advance.

 

③ Reduce thermal shock during wave soldering. Preheating can gradually and evenly heat the PCB temperature, thereby minimizing the thermal shock during wave soldering, easing the thermal stress, minimizing the warpage and deformation of the PCB, and improving the mechanical flatness of the PCB.

 

④ Reduce thermal degradation of components. Due to preheating, the thermal shock during wave soldering can be reduced to a minimum, thereby minimizing the risk of damage to heat-sensitive components.

 

⑤ Improve production efficiency. The preheating treatment also shortens the time required to heat the PCB to the wetting temperature during the wave soldering process, thus speeding up the wave soldering process and improving production efficiency.

 

2. Basic technical requirements for preheating systems

 

The basic technical requirements for the preheating system are as follows:

 

① Wide temperature adjustment range to cover the activation temperature requirements of various types of fluxes.

 

② There should be a certain preheating length to ensure that the PCB remains at the activation temperature for a sufficient time. The residence time and temperature are important parameters to ensure that the flux is suitable for purifying the surface to be soldered.

 

③ There should be no visible open flame to prevent flux droplets from burning on the heating element and causing a fire.

 

④ Minimal interference to the normal operation of the flux coating system and minimal thermal impact.

 

⑤ Resistant to impact and vibration, high reliability and easy maintenance.

 

3. Commonly used preheating methods and their characteristics

 

1) Commonly used preheating methods

 

According to the different heat source transfer methods, it can be divided into two types: radiation type and volumetric (hot air) type. In practical applications, the respective advantages of these two basic preheating methods are usually combined to develop a volume (hot air)-radiation combination method. The knot used so farThe structural form is mainly as shown in the figure below.

 

 

 

(1) Radiant preheater: The heat transfer of the radiant preheater relies almost entirely on thermal radiation. The temperature of the lower side of the PCB is controlled by the temperature of the heater and the distance between the heater and the lower side of the PCB. To increase thermal efficiency, tubular heater systems are often equipped with reflectors to concentrate the heat energy on the underside of the PCB. The flat heater is located under the conveyor and very close to the PCB surface, which greatly improves the heat transfer efficiency between them.

 

(2) Volumetric preheater: The volumetric preheater allows controllable heated air (hot air) to pass through the lower surface of the PCB to complete preheating. The method mainly relies on conduction and convection. The pressure airflow can help quickly blow away the solvent vapor on the lower surface and holes of the PCB. For PCBs with metallized holes, volume heaters should be preferred because the inside of the hole is often blocked by radiant heat propagating in a straight line. This is especially important for multi-layer PCBs.

 

(3) Volume-radiation combined type: This combined preheating method combines the advantages of volumetric and radiation preheating methods, so it is commonly used on some new models. Especially for surface mount components (SMA) with a large number of micro-slits on the board surface, using this kind of preheating structure design can improve the preheating effect even more obviously.

 

2) Comparison of volumetric and radiant heating efficiency

 

    The figure below compares the relationship between heating time and temperature rise of the two preheaters.

 

 

 

3) Development trend of preheater

 

(1) Increase preheating temperature and preheating time

 

  Due to the popularization and application of no-clean flux and water-soluble flux, new requirements have been put forward for the preheating temperature in wave soldering. Usually, the activation temperature of chemical activation substances used in clean-free flux is higher than that of the activators used in rosin flux. It can only be used at a higher temperature and maintained for sufficient heating time. The chemically active substances in it are fully activated to achieve the purpose of purification. When using water-soluble flux, the required preheating temperature must reach around the boiling point of water, and sufficient preheating time is required so that the moisture in the flux can be fully vaporized during the preheating process and the active material can be fully activated. .

 

There are usually two measures to increase the preheating time:

 

① Reduce the transmission speed of PCB. Reducing the PCB transmission speed will lead to a decrease in production efficiency, affecting production capacity and economic benefits, which is undesirable.

 

② Increase the length of the preheating zone. In order to meet the application needs of no-clean flux and water-soluble flux, newly designed models almost universally adopt more than two temperature zones.

 

(2) Improve preheating temperature control accuracy

 

Since the active material used in no-clean flux has a very narrow activation temperature range, either too high or too low, the preheating temperature must be strictly controlled within its activation temperature range in order to achieve the ideal effect of no-clean flux.

 

(3) Add an upper auxiliary heating device

 

With the increase in the installation density of SMA and the large-scale use of some components (such as connectors) and structural parts (such as radiators, electromagnetic compatibility components) that absorb large amounts of heat, the problems caused by large amounts of heat absorption in the wave soldering process are intensified. Welding defects (such as bridging, cold welding, pulling points, etc.). Therefore, a heater should be added to the upper part of the preheating area, as shown in the figure below. This is very beneficial for increasing the pre-welding temperature of the upper surface of SMA, minimizing the temperature difference with the solder wave peak, suppressing a large amount of heat absorption during wave soldering, and reducing welding defects.

 

 

 

6. Pinch-feeding system

 

1. Function

 

In wave soldering, the function of the PCB pinching system is to enable the PCB to enter and exit the solder wave peak at a certain optimal inclination angle and speed to ensure that each solder joint on the PCB spends 5 to 5 seconds in the solder wave peak. 8s to obtain better welding results.

 

2. Technical requirements

 

① The transmission is smooth, without jitter and vibration, and the noise is small.

 

② The transmission speed should be continuously adjustable within a given range (such as 0~3m/min), the speed fluctuation should be less than ±10%, and the mechanical properties should be good.

 

③ It is best to have a certain inclination angle for pinch feeding, and the range can be selected between 4° and 8°.

 

④ The clamping claw has good chemical stability. It will not corrode, stick to tin, or react chemically with the flux under the repeated action of flux and high-temperature liquid solder. It has good elasticity and stable clamping force.

 

⑤ Easy to install and unload, easy to maintain.

 

⑥ Compact structure, little impact on overall machine dimensions.

 

⑦ Good thermal stability and not easy to deform.

 

⑧ The clamping width can be easily adjusted according to the different widths of the PCB.

 

3. Structural classification of common pinch feeding systems

 

The structural classification of currently common pinch feeding systems is shown in the figure below:

 

 

 

1) Frame type clamping system

 

The frame type is also called the bracket type. It uses a special frame as a carrier to fix and clamp the PCB. Under the drag of the drag mechanism, the PCB wave soldering process is completed through each process area configured by the wave soldering equipment. . This pinch-feeding method is the only optional method in the secondary welding system. Most of the assembly lines of some domestic mass-produced products (such as color TVs) also use this pinch-feeding method. Even the primary welding system uses this pinch-feeding method. There are also many pinch-feeding methods. In this dragging method, the frame (bracket) as the PCB carrier is essential. It is usually made of aluminum alloy and needs to be strong and not deformed. The clamping width can be adjusted according to the size of the PCB. It has many uses on the assembly line, such as it can also be used as a carrier for component insertion, welding, leg cutting, cleaning and transportation. It can not only easily adjust the clamping width, but also adjust the angle of PCB entering the wave peak to suppress the shadow effect in SMA wave soldering.

 

The following two forms of dragging methods used in frame structures are currently popular.

 

① Chain type clamp feeder. It is composed of two transmission chains, running on two parallel special aluminum alloy guide rails, connected by a shaft with sprockets and driven synchronously to ensure that the two chains can run synchronously. It is currently the most widely used dragging method on all models. The advantages of this drag method are large drag torque, strong load-bearing capacity, and no sliding phenomenon; its disadvantages are that the transmission has noise (especially at high speed) and jitter, the structure is complex, and the friction between the chain and the guide rail during the drag process There is friction between them. This frictional resistance will become more obvious as the transmission speed increases, resulting in poor uniformity of transmission speed changes.

 

② Steel belt type clip feeder. The typical structure of this pinch feeding method is shown in Figure 1.18. The frame with the PCB clamped is supported on the guide rail by four rollers (usually replaced by bearings) installed on both sides of the frame, and the steel strip is dragged by the magnetic attraction of the magnet to the steel strip. The steel belt is usually made of magnetizable spring steel belt (such as 65Mn). The motor drives the steel pulley through the reducer, chain, sprocket, etc., and drives the steel belt to circulate on the guide rail. Since in this pinching structure, the steel belt moving linearly on the guide rail only plays a dragging role and does not carry any weight, so when the frame performs pinching motion on the upper surface of the guide rail, there is only a gap between the roller and the upper surface of the guide rail. The rolling friction and friction resistance are very small, the transmission is smooth and uniform, without noise, jitter and other phenomena. The linearity of speed change within the speed regulation range is relatively good, and the structure is simple, making maintenance easier.

 

 

 

2) Claw type clamping system

 

  The claw-type clamping system eliminates the frame (bracket), and directly installs the clamping claws of the required shape on the driven chain to form the PCB carrier.

 

   Due to the design and its own elasticity, the claws should have a certain amount of support for the PCB to prevent the PCB from falling in the hot zone. A claw cleaning device at the machine entrance removes residue from the claws before new PCBs are fed into the clamping system. This claw-type clamping system uses a handwheel to adjust the clamping width. The sprocket shaft is driven by a DC motor. The output shaft of the motor usually also uses an adjustable slipping clutch to prevent stalling and overloading, and also Protects the load and power supply. Once the pinch speed is set, the machine runs at that speed and can be reset within the limits specified by the design. The load range of this pinching system is required to be 0~10kg, and the speed change error range is not greater than ±5%.

 

   The claw-type clamping system has high production efficiency, easy operation for loading and unloading PCBs, and has a wide range of applications. Therefore, in the existing wave soldering equipment system, the proportion of this pinching method exceeds that of the frame-type pinching system. However, the structure of the claw-type clamping system is more complicated than that of the frame-type clamping system, and maintenance is also more complicated.

 

  3) Robot clamping and feeding system

 

It is a pinch feeding method developed by the Swiss EPM company, and was first used in the EPM Elever robot-type wave soldering machine developed by the company. When this clamping method works, the robot hand grasps the PCB and makes the PCB move in the four-axis directions of X, Z, around the Z axis, and around the Y axis. The expansion and contraction of the claw fingers is the fifth axis movement.

 

7. Cooling system

 

1. Function and technical requirements

 

1) Function: It is to quickly dissipate the waste heat accumulated on the PCB after passing through the solder wave peak area.

 

 

2) Technical requirements

 

① The air flow should be directional and should not cause severe heat dissipation on the surface of the solder tank.

 

② The wind pressure should be appropriate. If it is too strong, it will easily disturb the solder joints.

 

     The melting point of lead-free solder is high, and the temperature range of the mushy zone that achieves complete phase change is also wide. During wave soldering, the adhesion between the copper conductor and the substrate is greatly reduced. In addition, the solidification process takes a long time in the mushy zone, which can easily lead to Copper conductors peel away from substrate. And due to the long time it takes to solidify the mushy zone, any mechanical vibration of the system will cause the surface of the solder joint to wrinkle and become uneven. Based on this point alone, some companies have proposed using a freezer for rapid cooling, which is beneficial to suppressing weld edge finning and refining grains. The author believes that it is equally important to reduce the stability of the equipment and the absence of mechanical vibration during operation.

 

2. Common structural methods

 

   The common structural forms of cooling systems in wave soldering equipment are fan type, air curtain type, compressed air type, etc. The structures are generally very simple, so they will not be introduced in detail.

 

8. Electrical control system

 

1. The role of the control system

 

The function of the control system is to comprehensively process the information flow between various workstations and components of the entire machine, and to coordinate and control the process of the system. It is the control center of the system and one of the important criteria for measuring the function and advancement of the whole machine. It is an important factor affecting the system reliability and welding effect.

 

2. Basic requirements for control systems

 

The requirements for control system performance can be summarized as follows:

 

● Control actions are accurate and reliable;

 

● Can fully embody and reflect the requirements of wave soldering process specifications;

 

● Friendly human-machine interface and easy operation;

 

● Good operability and maintainability;

 

● Complete security measures and strong fault tolerance;

 

● Low cost and wide supply of maintenance spare parts;

 

● Can fully reflect the progress and development of modern control technology.

 

3. Main control parameters and index requirements

 

1) Solder tank temperature

 

  The solder tank temperature is a key parameter that affects the wave soldering effect, and its index requirements can be summarized as follows.

 

Temperature adjustment range: room temperature to 300℃

 

Temperature control range: 200～280℃

 

Temperature control accuracy: ±1℃

 

2) Preheating temperature

 

Temperature adjustment range: room temperature to 300℃

 

Temperature control range: 80～270℃

 

Temperature control accuracy: ±3℃

 

3) Pinch feeding speed

 

Speed adjustment range: 0～3m/min

 

Speed control range: 0.5～2.5m/min

 

Speed control accuracy: ±5%

 

4) Flux density (for rosin-based flux)

 

Density adjustment range: 0.8～0.9g/cm3

 

Density control range: 0.82～0.88g/cm3

 

Density control accuracy: ±0.004 (20℃)

 

9. Commonly used solder wave rectification structures

 

1. The purpose of setting up the solder wave peak rectification structure

 

Usually, the solder flow generated in the pump channel by a pump (especially a mechanical pump) is in a turbulent flow state, which is mixed with a large amount of vortex and tumbling motion. Such a flow pattern not only results in large resistance losses along the way, but also if it is ejected directly from the nozzle, the waveform will be severely undulating, often with unevenness along the width direction. Such solder wave crests cannot be used at all. The purpose of setting up a fluid rectification structure in the pump channel is to level the turbulent flow state of the fluid in the pump channel into a laminar flow state, so as to obtain a smooth solder wave peak shape that can meet the needs of welding, and at the same time, it can also greatly reduce the flow of the pump channel. Losses caused by flow resistance along the way.

 

2. Common rectification structures Common rectification structures

 

According to its working principle, it can be divided into two types: damping type and shunt type.

 

1) Damping type

 

We know that the energy loss of a fluid in a turbulent flow state in a pipeline is proportional to the square of the fluid flow rate. Using this principle, some damping structures can be set up in the pump channel, so that the fluid with a fast flow rate in the pump channel will have a large energy loss, so the stall will be larger, and the deceleration effect will be more obvious; while the flow rate is small, because the energy loss is small, the speed loss is also small. Small. In this way, the velocity difference between each part of the fluid is reduced, and the flow pattern develops towards laminar flow, thereby achieving the purpose of leveling the flow velocity. The most commonly used damping rectification structures are plate type, screen type, plate-screen composite type and labyrinth type rectification structure.

 

① Plate structure: Two rectifier plates are installed on the opposite two-phase walls in the pump channel cavity, as shown in the figure below. The advantage of the plate structure is that it is easy to process and has a better effect on eliminating vortices in the pump channel cavity.

 

 

 

② Screen structure: that is, 1 to 2 layers of sieve plates with uniform small holes (φ2 ~ φ3mm) are set under the nozzle mouth, as shown in the figure below. The screen structure has better rectification effect.

 

③ Plate screen composite type: Screen processing (drilling) is very labor-intensive, especially when using multi-layer screens, while plate processing is particularly simple. Therefore, in order to simplify the manufacturing process and ensure the leveling effect, a composite rectification method in which a layer of screen is added at an appropriate position above the rectifier plate can be adopted.

 

 

 

④ Labyrinth rectification structure: This is a relatively complex structure that flattens the turbulent solder fluid into a laminar flow state, as shown in the figure below. The advantage of this structure is that the rectification effect is very ideal, but the disadvantage is that the structure is complex and the hydraulic loss is large.

 

 

 

2) Diversion type

 

Split-flow flow rate leveling technology is widely used in axial flow pump-type solder wave peak generators. This structure is mostly installed in the main pump channel and near the nozzle. There are three common structural forms: guide plate, square grille and guide plate-square grille composite type.

 

① Guide plate: The guide plate is mainly used in the transition section between the main pump channel (main flow channel) and the pump cavity. Its main function is to adjust the flow of the fluid in the main pump channel when it enters the transition section, so that it enters the pump cavity. The fluid flow along the entire cross-section was previously made more uniform to suppress the unevenness of the solder wave peak, as shown in the figure.

 

 

 

② Square grid: This kind of square grid like a harmonica grid is set at the bottom of the nozzle mouth. It divides the pipe at the bottom of the nozzle mouth into countless small squares, that is, a large-section pipe is turned into countless small-section pipes. The combination is shown in Figure 1.22. The cross-sectional shape of this small grid can be square, rectangular, honeycomb or other shapes. The side wall area of each small pipe is equal, that is, the friction resistance is equal. In this way, when the fluid flows through these small pipes, due to the different flow speeds, the losses along the way are also different. A fluid with a high speed has a large energy loss and a large velocity attenuation; while a fluid with a small flow speed has a small energy loss, so