There are often many small solder beads on the wave soldering plate, as shown in Figure 1.The tin beads will be washed away during cleaning, t. However, with the application of the no-clean process, the solder balls are no longer cleaned after soldering, causing the tin bead phenomenon.
What are the causes of tin beads during PCB manufacturing and assembly?
Some people use a high-speed camera to study the generation mechanism of tin beads. It is observed that when the liquid solder in the solder wave and the pad are peeled off (this process is called peel-off), the tin beads will form as show in fig 2. The high surface tension of the liquid solder leads to the formation of tin beads. This small tin bead has a high kinetic energy and bounces up and down between the PCB surface and the tin wave 1 or 2 times. If attached to the surface of the PCB will become a solder ball. The main influencing factors include:
1) Before the introduction of low solid content flux, rosin type high solid content flux was commonly used, and the solid content was often as high as 20% ~ 30%. At that time, tin bead bouncing also existed, but it was never a problem. Because the flux containing rosin forms a liquid flux layer on the surface of the PCB during soldering, if a solder ball hits this liquid layer, the tin beads will drift away and will not adhere to the surface of the PCB. The disadvantage is that a lot of flux residue is left after soldering.
When the low solid content flux was introduced, the environment on the PCB surface changed fundamentally, and the solid content of no-clean flux rarely exceeded 3%. When soldering, it is impossible for such a low solid content to form a liquid layer on the PCB surface. Moreover, the activators currently used are often based on organic acids and rarely synthetic rosin, which also makes the above phenomenon more obvious. The problem arises when the solder balls directly hit the PCB surface covered by a blank, clean solder mask. The problem of solder ball adhesion depends not only on the flux but also on the characteristics of the solder mask.
2) Solder mask
PCB board factory production lines often use two-component solder mask. A survey of a solder mask manufacturer found that only a small change (less than 1%) in the ratio of resin and catalyst is sufficient to change the process behavior of the solder mask, and this change will have a significant impact on subsequent soldering processes Impact. The parameter that produces this change is the degree of curing or cross-linking of the polymer chain. If there is a slight mismatch between the resin and the catalyst ratio, the solder mask cannot be completely cured.In the subsequent welding process, the solder heat will cause the solder mask to soften, and the tin beads will easily adhere to it. The roughness of the solder mask also has a great effect on the adhesion. The smooth solder mask has a very low surface roughness, which provides a large area of adhesion to the solder beads, as shown in Figure 3.
3) Equipment parameters
The impact of Pcb's soldering system on the formation of tin beads is very limited. However, sometimes people still try to reduce the tin bead phenomenon from this aspect. In fact, the flux composition used in soldering has a relatively greater impact on this aspect. For example, no solder beads will stick to the PCB when using a flux containing rosin. Therefore, some PCB manufacturers use a separating agent in the synthetic flux to replace the rosin, so that the necessary flow layer is formed during soldering to prevent the adhesion of tin beads. However, this type of flux brings another problem-the aesthetics of the PCB surface.
Suggestions for Improvement
(1) Select the appropriate flux.
(2) Use a smooth solder mask.
(3) Bake the pcb board at 125 ° C for more than 4h.