From the point of view of “reducing process, saving cost and reducing pollution “, more and more electronic welding adopt” no cleaning “process after welding. However, if “tin beads” appear on the surface after welding, it is impossible to meet the requirement of “no cleaning “, so the prevention and control of” tin beads “is particularly important in the implementation of” no cleaning “process. The appearance of “tin bead” not only affects the appearance of board grade products, but also affects the reliability of products because of the dense components on printed boards.

    According to the whole electronic welding situation, the process of “tin bead” may appear, including “SMT surface mount” welding process ,” wave soldering “process and” manual welding “process. We discuss the causes of” tin bead “and the methods of prevention and control from these three aspects. Because “wave peak welding” and “manual welding” have been carried out for many years, many aspects have been relatively mature, therefore, this paper uses more space to introduce the “SMT surface mount” welding process of “tin beads” reasons and prevention and control measures.

1 About the Form and Standard of Tin Beads

Some industry standards explain the problem of tin beads. There are mainly MIL-STD-2000 standards in which “tin beads are not allowed “, while IPC-A-610C standards in which” less than 5 per square inch “. in the IPC-A-610C standard, a minimum insulation gap of 0.13 mm is specified, and tin beads with diameter within it are considered qualified; while tin beads with diameter greater than or equal to 0.13 mm are not qualified, the manufacturer must take corrective action to avoid this phenomenon. A new version of the IPCA-610D standard for lead-free welding does not make the tin bead phenomenon clearer, and the requirement of less than 5 tin beads per square inch has been deleted. The standards for automotive and military products do not allow any “tin beads “. The circuit boards used must be cleaned after welding or removed manually.

Photographs of the shapes and sizes of tin beads:

2 Causes and Preventive Control Methods of “Tin Beads” in SMT Surface Mounting Process

During the welding process of “SMT surface mount “, the factors of” temperature, time, quality of solder paste, printing thickness, making of steel mesh (formwork) and mounting pressure “of reflow welding may lead to the production of” tin produced. Therefore, finding the possible cause of “tin beads” and preventing and controlling it is the key to achieve no “tin beads” on the plate.

1）“Tin bead” condition caused by the quality of solder paste itself

1. Metal content in solder paste. The mass ratio of metal content in solder paste is about 89-91 and the volume ratio is about 50%. Usually, the more metal content, the closer the metal powder arrangement in the solder paste, the more chance to combine the tin powder particles and not easy to be blown away during gasification, so it is not easy to form “tin beads “; if the metal content is reduced, The probability of tin beads increases.
2. Oxide content in solder paste. The content of oxide in solder paste also affects the welding effect. The higher the oxide content, the greater the surface tension of metal powder after melting with pad, and the higher the oxide content of metal powder in “reflux welding section “. This is not conducive to the complete wetting of molten solder, resulting in fine tin beads.C.Particle size of metal powder in solder paste. Metal powder in solder paste is a very small near-circular sphere. The commonly used ball diameter of welding powder is about 25-45μm, and the oxide content in finer powder is low, so the phenomenon of “tin bead” will be alleviated.D、Welding paste anti-heat collapse effect. In the hot section of reflow welding, if the solder paste has a poor thermal collapse resistance, the printed solder paste begins to collapse before the welding temperature (before the solder begins to melt), and some solder paste flows outside the pad. When it enters the welding zone, the solder begins to melt.It can be seen that the quality and selection of solder paste also affect the production of tin beads, the content of metal and its oxides in the solder paste, the particle size of metal powder, the anti-heat collapse effect of solder paste and so on, all affect the formation of “tin beads” to varying degrees. 

   2）Analysis on the Causes of Improper Use of Tin Beads

    

1. Tin beads are produced when passing through a reflow welding furnace. We can roughly divide the reflow welding process into four stages: preheating, heat preservation, welding and cooling. “Preheating section” is to make the printed board and watch components slowly heating up to 120-150℃, so that solder paste can remove volatile solvents, reduce the thermal impact on the components. In this process, gasification will occur inside the solder paste. If the adhesion between the metal powder in the solder paste is less than the force produced by the flux gasification, a small amount of “solder powder” will flow down or fly out of the pad. In the “welding” stage, this part of “solder powder” will also melt and form “tin beads “. It can be concluded that “the higher the preheating temperature, the faster the preheating speed, it will aggravate the gasification of flux and cause collapse or spatter, forming tin beads “. Therefore, we can take moderate preheating temperature and preheating speed to control the formation of tin beads.
2. Printing thickness and amount of solder paste on printed board. solder paste printing thickness is a major parameter in production. printing thickness is usually between 0.15-0.20 mm. too thick or too much can easily lead to “collapse” and form “tin beads “. When making steel mesh (formwork), the size of the pad determines the size of the opening of the template. Usually, in order to avoid excessive printing of solder paste, the size of the printing hole is controlled at about 10% less than the contact area of the corresponding pad. The results show that the phenomenon of tin beads is reduced to a certain extent.
3. If the mounting pressure is too high during the patch process, when the component is pressed on the solder paste, a part of the solder paste may be squeezed under the component or a small amount of tin powder will fly out, and the solder powder will melt in the welding section to form a “tin bead “. Therefore, the appropriate mounting pressure should be selected.The solder paste usually needs to be refrigerated and must be restored to room temperature before it can be opened for use. If the solder paste temperature is too low, it will open the package, which will cause moisture on the paste surface. This moisture will cause welding powder to fly out after preheating, in the welding section will let hot melt solder spatter to form “tin beads “. The air humidity in the general area of our country is high in summer. When the solder paste is removed from cold storage, the bottle cap should be opened at room temperature for 4-5 hours.E.The production or working environment also affects the formation of “tin beads “. When printed boards are stored for too long in a damp warehouse, small drops of water are found in the packaging bags containing printed boards, which are the same as moisture absorbed by solder paste. Will affect the welding effect and form a” tin bead “. Therefore, if there are conditions, the PCB or components are dried before mounting, then printed and welded, which can effectively inhibit the formation of tin beads.F. the shorter the contact time between solder paste and air, the better, this is also a principle of using solder paste. After taking out part of the solder paste, close the lid immediately, especially the inner lid must be pressed down to extrude the air between the lid and the solder paste, otherwise it will have a certain impact on the life of the solder paste. At the same time, it will cause drying faster or moisture absorption next time, thus forming “tin beads “. 

   It can be seen that there are many reasons for the emergence of “tin beads “, only from a certain aspect of prevention and control is far from enough. We need to study how to prevent all kinds of unfavorable factors and potential hidden dangers in the production process, so that the welding can achieve the best effect and avoid the production of tin beads.

   3 Prevention and Control of Tin Beads in the SMT Surface Mounting Process

1. Selection of solder pasteIn the selection of solder paste, we should insist on the trial under the existing process conditions, so as to verify the applicability of the supplier solder paste to its own products and processes, and to understand the concrete performance of the solder paste in practical use. In the evaluation of solder paste, we should pay attention to various common parameters, such as the ratio of welding oil to welding powder, the particle size of tin ball, etc.Correct selection of solder paste is not necessarily the best parameters, more often, for the SMT process and product characteristics, suitable is the best. Therefore, select the solder paste suitable for its own process and products, and set all the parameters, make the basis of quality control acceptance and quality inspection in the later supplier delivery process, check the written information provided by the supplier on the one hand, On the other hand, take a small number of different batches of products for trial.High quality suppliers will put forward corresponding process suggestions in the process of cooperation, and carry out the upgrading and defect improvement of solder paste products according to the specific requirements of customers; therefore, relatively stable and high integrity suppliers, It can provide great help for customers to prevent and control tin beads in solder paste quality. 

   2）SMT Surface Mounting Process Control and Improvement

   In all the process control process, there are strict documents for the preservation, removal and use of solder paste, return temperature and stirring. The main points are as follows:

1. Storage in strict accordance with the storage conditions and temperature provided by the supplier, generally solder paste should be stored in 0-10℃ refrigerated conditions;
2. After the solder paste takes out, before use, should carry on under the normal temperature return temperature, before the solder paste does not return the temperature completely, may not open;
3. During stirring, the stirring shall be carried out in accordance with the stirring method and stirring time provided by the supplier；
4. During printing, we should pay attention to the strength of printing and the cleanliness of the steel mesh surface, wipe the excess solder paste residue on the steel mesh surface in time, and prevent PCB contamination of the plate surface in this process, resulting in the production of tin beads in the welding process.
5. In the process of reflow welding, the operation should be carried out strictly according to the reflow welding curve that has been set, and should not be adjusted at will;
6. During the process of “SMT surface mount “, the” opening mode “and” opening rate “of steel mesh (formwork) may lead to some defects in” printing characteristics “and” welding characteristics “of solder paste, thus causing” tin beads “. In the related experiments, we improved the steel mesh, changed the opening of 1:1 steel mesh which was easy to produce “tin beads” to the wedge of 1:0.75, and the probability of “tin beads” produced by the better test effect decreased obviously until it was basically eliminated.By modifying the opening mode of steel mesh and batch printing test, it can be seen that the opening method of steel mesh can effectively prevent and control the production of “tin beads” after modification. The printing effect and welding effect of the modified “anti-tin bead” steel mesh are shown in figure 2:According to many comparative experiments and combined with “Fig .2″, it can be seen that the second modified steel mesh has no obvious tin beads, and the solder amount of tin paste is not on the low side. From this, it shows that through the opening change of steel mesh, the prevention and control of “tin beads” in the process of “SMT surface mount” still has certain effect. Meanwhile, we sent the changed welding products to “Saibao Laboratory” for testing (report number “FX03-2081691″), and tested the 0603 components on the circuit board. The shear force at the R124、R125、R126、C16、C57″ five component points is “58.14 N、56.53N、51.87N、50.90N、52.35N”, and the welding strength can meet our requirements.Although the process of “SMT surface mount” is more complicated to prevent and control “tin beads “, it is believed that it can be achieved without” tin beads “or effectively reduce the probability of” tin beads “after long-term efforts and experience. 

1.  Causes and preventive control methods of “tin bead” in “wave soldering” processIn the process of wave soldering, there are two conditions for the production of tin beads: one is that when the board is exposed to tin liquid, because the flux or the plate itself is too much water or the high boiling point solvent is not fully volatilized, when the temperature is high, the liquid solder spatter out and form small tin beads; Another case is that when the circuit board leaves the liquid solder, when the circuit board is separated from the tin wave, the circuit board will pull out the tin column along the direction of the extension of the pin, and the excess solder will fall back into the tin cylinder under the wetting effect of the flux and the fluidity of the tin liquid itself. As a result, the spilled solder sometimes falls on the circuit board to form a “tin bead “.Therefore, we can see that in the aspect of “wave soldering” prevention and control of “tin beads “, we should start from two big aspects, on the one hand, the selection of raw materials such as flux, on the other hand, the process control of wave soldering.1）Cause analysis and preventive control methods of fluxA.The content of water in flux is larger or exceeds the standard, and it can not volatilize fully after preheating;

   B. there are high boiling point substances or non-volatile substances in flux, which can not be fully volatilized after preheating; these two reasons are caused by the “quality” problem of flux itself, which can be solved by “increasing preheating temperature or slowing down the speed of plate walking “. In addition, prior to the selection of flux, the actual process should be confirmed for the samples provided by the supplier, and the standard process should be recorded during the trial. In the absence of “tin beads “, audit the other instructions provided by the supplier. Check the supplier’s initial instructions during subsequent receipt and acceptance.

   2）Process Cause Analysis and Preventive Control

   A. preheating temperature is low, the solvent in flux is not completely volatile;

   B. the speed of walking plate is too fast to achieve preheating effect;

   C.Chain (or PCB plate) angle is too small, tin liquid and welding surface contact with the middle of the bubble, bubble burst after the production of tin beads;

   D. flux coating is too large, the excess flux can not flow completely or the wind knife does not blow off the excess flux.

    

   The emergence of these four adverse causes is related to the determination of standardized processes. In the actual production process, the parameters should be corrected strictly in accordance with the established operation guidance documents, and the parameters that have been set can not be changed at will. Relevant parameters and the technical level involved mainly have the following points:

   A. about preheating: generally set at 90℃-110℃, here “temperature” refers to the actual heating temperature of the welded surface of the PCB plate after preheating, not the “surface” temperature; if the preheating temperature does not meet the requirements, it is easy to produce tin beads after welding.

   B. on plate speed: in general, it is recommended that customers set the plate speed at 1.1-1.4 m / min, but this is not an absolute value; if you want to change the plate speed, you should usually change the preheating temperature; for example, to speed up the plate, the preheating temperature should be raised properly to ensure that the preheating temperature of the PCB welding surface can reach the predetermined value; if the preheating temperature is constant and the plate speed is too fast, the flux may volatilize incompletely, resulting in “tin beads “.

   C. with regard to the inclination of the chain (or PCB plate): this inclination refers to the angle between the chain (or PCB plate) and the tin liquid plane. When the PCB plate passes through the tin liquid plane, there should be only one tangent point between the part surface and the tin liquid plane.

   D. the main function of “wind knife” is to blow off the excess flux on the PCB surface and make the flux evenly coated on the surface of the PCB parts. In general, the inclination angle of the wind knife should be about 10 degrees.

    

   In the actual production, combined with the actual situation of its own wave peak welding, the selection of related materials, at the same time, the strict “wave peak welding operation rules “, and strictly in accordance with the relevant rules for production. It is proved by experiments that under the condition of strict implementation of technology, the “tin beads” produced by “wave soldering welding process problems” can be overcome completely “.

    

   5 Causes and Preventive Control of “Tin Beads” in “Manual Welding” Process

   In the process of “manual welding “, the probability of” tin beads “is not high, the common is rosin flying

   Splash, occasionally “tin beads” spatter or residual tin slag on the surface of the pad, compared with rosin spatter, the existence of “tin beads” or tin slag is more potential harm to product safety.

   The main reason for the appearance of tin slag and “tin beads” may be that the flux has completely evaporated before the heat source is removed, so the solder fluidity is extremely poor, and the soldering iron head forms a sharp, column or short welding situation with the soldering iron drawing out, or accidentally causes the solder liquid to splash away from the soldering iron head, and after cooling, it is attached to the plate or element. Another possibility is not to preheat the soldering iron head first in the welded part, but to iron the solder wire first, then put it in the welded position, because of the large temperature difference caused by the solder spatter, thus forming a “tin bead”。

   No matter which of the above reasons, it is more important to teach the operator to grasp the correct welding time and position, appropriate amount of solder and pay attention to timely and correct cleaning of soldering iron head. In the actual production, the “manual welding” staff are often specially trained in welding technology, and the “manual welding process requirements” are strictly compiled, and the “manual welding” is standardized and controllable. Through long time observation, the production of tin beads can be effectively avoided in the process of manual welding.

   Conclusion: in view of the problem of “tin beads “, we have spent more than half a year together with the relevant customers to do a large number of experiments, and the different welding processes have been carefully analyzed. Practice has proved that through material selection, process control and so on, in the current electronic welding process, it is possible to eliminate or reduce the probability of “tin beads” to a lower level.

       Looking forward to the future, a series of in-depth research will be carried out on the solder paste formulation and production process, including “solvent, rosin resin, activator, thixotropic agent, surfactant and other types of additives selection, compatibility, proportioning, and solder paste production process involved in temperature, time and other aspects “. It is hoped that the production of “tin beads” will be solved or prevented from the technical point of view of the product, so as to ensure that the “tin beads” will be eliminated or less in the welding process, so as to cooperate with more customers to achieve the “no cleaning” process after welding.