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With the continuous development of light, thin, power saving, miniaturisation and planarisation of electronic and information products,it is necessary to use surface mount SMT technology for different electronic products. And tin beads have serious harm to electronic products, so how to reduce tin beads is one of the key control contents of SMT manufacturers.
According to the relevant cases,SMTproduction,reflow soldering, due to the solder paste metal particles spattering,easy to form tiny spherical solder beads or irregularly shaped solder particles, which is tin beads see the following figure. Tin bead is one of the main defects of SMT production, the diameter of about 0.2 ~ 0.4mm,mainly in the side of the patch components or IC pins, not only affect the appearance of the PCBA product, but also in use may cause a short-circuit phenomenon, which seriously affect the quality of electronic products and life, and may even cause personal injury.
What is tin bead?
1.Tin beads are found on boards that have been reflow soldered, and you can see at a glance that it is a large ball of tin, embedded in a bank of flux positioned close to discrete components that have very low heights above the ground, such as chip resistors and capacitors, thin small form factor packages (TSOPs), small form factor transistors (SOTs), D-PAK transistors, and resistive assemblies. Because of their location in relation to these components, solder balls are often called ‘satellites’. For obvious reasons, solder balls are sometimes called ‘balls extruded from the middle of a sheet’ or something similar. Compared to solder balls, solder spheres are characterised by tiny balls clustered along the perimeter of the flux residue, or these balls stick to the perimeter of close-pitch pads and soldermasks.
2.Basically,solder balls can form a solder ‘bridge’ from one component terminal to another, causing an electrical connection that is not designed for. This creates a risk of short circuits, which can occur where the beads were originally formed,or anywhere on the assembly, if vibration causes the beads to loosen and move. Although a short circuit does not necessarily occur even if the bead appears as described above, the bead is still a defect that should be minimised or eliminated as much as possible.
What are the causes of tin beads?
1.Tin beads can be caused by various reasons such as collapse, extrusion beyond the printed solder paste, failure of the excess paste during the soldering process, and the soldering process where the solder paste plates melt and become independent of each other,forming in the component body or near the pads.
2.If the pad is designed as a square chip component,if there is more solder paste,it is easy to produce solder beads, and most of the beads appear on both sides of the chip component. For example, if the pad is designed as a square wafer component and more solder paste is present after printing,beads will be easily generated.The solder paste that is fused with the part of the pad will not form beads.
3.However, when the amount of solder is increased, the element exerts pressure on the body insulator, the solder paste in the lower part of the assembly,and hot melting occurs during reflow soldering because the surface can melt the solder paste into a ball, which has a tendency to rise up in the assembly.However,this tiny force is formed during the bead's cooling period, and there is a gravitational force between each element on both sides and separates the soldering plates. If the component gravity is high and more solder paste is squeezed out,it can even form multiple beads.
What are the main factors for the formation of tin beads in the PCBA manufacturing process?
1.Improperly set reflow temperature profile.
First of all, if the preheating is not sufficient, did not reach the temperature or time requirements,the flux is not only less active, and volatilisation is very little, not only can not remove the oxide film on the surface of the solder pad and solder particles, and can not solder paste powder up to the surface of the solder, can not improve the liquid wetting,easy to produce tin beads. The solution is: to make the preheating temperature at 120 ~ 150 ℃ for an appropriately long period of time.
Secondly, if the preheating area temperature rises too quickly, to reach the flat top temperature time is too short,resulting in the solder paste inside the water,the solvent is not completely evaporated, to reach the reflow soldering temperature area,that may cause moisture,solvent boiling, spattering out of tin beads. In this regard,attention should be paid to the rate of heating, preheated solder wetting is affected, easy to produce tin beads. As the temperature rises, the wetting of liquid solder will be significantly improved, thus reducing the production of tin beads.But the reflow soldering temperature is too high, it will damage components, printed circuit boards and pads, so we must choose the appropriate soldering temperature,so that the solder has a better wetting.
2.Flux fails to play a role.The role of the solder flux is to remove the solder pad and the oxide film on the surface of the solder particles,thereby improving the liquid solder and solder pads,component pins, between the end of the solder, the wetting. If the coated solder paste, placed too long,the flux is easy to volatilise, the loss of flux deoxygenation, liquid solder wetting becomes poor,and then the soldering will inevitably produce tin beads. The solution is: select the working life of more than 4h solder paste, or as much as possible to shorten the placement time.
3.The opening of the template is too large or badly deformed.If the beads always appear in the same position, it is necessary to check the design structure of the metal plate.Template opening size accuracy does not meet the requirements for the pad is large, as well as the surface material is softer,such as copper templates,will result in leakage of solder paste profile is not clear, and each other bridge,this situation is more often found in fine-pitch device pad leakage in the print, and then after the soldering will inevitably result in the production of a large number of tin beads between the pins.The solution is: should focus on the different shapes of the pad pattern and the centre distance, select the appropriate template materials and template production process to ensure that the quality of the solder paste printing, reduce the size of the template holes, strict control of the template production process, or switch to laser cutting and electropolishing method of making the template.
4.Excessive placement pressure. Excessive placement pressure can squeeze the solder paste outside the pad, if the solder paste is coated thicker, excessive placement pressure is more likely to squeeze the solder paste outside the pad, and then soldering will inevitably produce tin beads.The solution is control the thickness of solder paste, while reducing the placement pressure of the patch head.
5.Solder paste contains water. If you take out the solder paste from the freezer and use it directly with the lid open, water vapour condensation will occur due to the large temperature difference, which will easily cause water to boil and splash during reflow soldering, resulting in the formation of tin beads.The solution is solder paste from the freezer after taking out,usually should be placed at room temperature for more than 4h, to be sealed cylinder within the pad temperature to the ambient temperature, and then open the lid to use.
6.Printed board cleaning is not clean,so that the solder paste remains on the surface of the printed board and through-hole.The solution is to strengthen the operator and craftsmen in the production process of the sense of responsibility, in strict accordance with the requirements of the process and operating procedures for the production, to strengthen the quality control of the SMT process.
7.Non-contact printing or printing pressure is too large.Non-contact printing in the specimen and PCB leave a certain gap between, if the squeegee pressure control is not good, it is easy to make the specimen underneath the solder paste agent to the surface of the PCB non-solder pad area,and then after the flow of soldering will inevitably produce tin beads. The solution is: if no special requirements, it is appropriate to use contact printing or reduce the printing pressure.
8.Flux failure. If the patch to the reflow soldering time is too long, then due to the oxidation of solder particles in the solder paste, flux deterioration, activity is reduced, which will lead to the solder paste is no longer flowing, the solder ball will be generated. The solution is: choose a longer life of the solder paste at least 4h.
The above is the PCBA manufacturer to play to you about the tin beads of some common situations, I hope you understand in the SMT playing parts can be avoided as far as possible.