Bridging seems random, yet always in the same place. Why?

Post-wave solder bridging has been discussed at great length, but it might be interesting to look at it from another point of view. Most process engineers focus on the soldering process and how it could be improved to reduce solder bridging. However, in their efforts to optimize the process, they often are confronted with the maddening fact that a stable process sometimes seems impossible. Solder bridging seems to occur more or less randomly, yet always at the same position. Although nothing was changed in the process, the outcome is, in fact, unpredictable.

If we look at a soldered board from another perspective, we have a stable process for almost all joints that were soldered. Whatever was changed in the machine settings in order to optimize the process did not affect the majority of joints. They were soldered with quality and without solder bridges.

For what reason do the majority of joints never cause problems, yet some other ones often do? The answer is not so difficult, but it is important to recognize. The fact that most joints solder well without leaving solder bridges after they separate from the wave, even under less-than-optimal process conditions, is that these joints are well-designed. There is no other reason for the stable process quality on these joints than the joint design that fits the process. In fact, it is the only way to build a repeatable, robust process.

It is common practice to try to fit the existing process to the design. Although this might provide a solution, it limits the process window. From that point of view, it is not always the best way to solve the problem. Optimizing the process might reduce bridging frequency for certain designs. However, this if often a critical process for such joints, one that for some unknown reason always seems to be unstable on such joints. There is, however, an example that demonstrates unstable behavior related to design; this might explain the so-called bi-stable behavior of certain joint design configurations during soldering.

Bi-Stable Behavior Model

The behavior of liquids depends on wetting properties and capillary forces. Both also depend on surface tension. The result of these forces on a joint to be formed depends on the design. Although the joint itself is important, the space in between joints is also important because small spaces between joints will act as capillaries.

How these liquid forces act on different structures can be demonstrated using a cubic metal frame. If one dips such a frame into a soap solution, then removes it, an interesting phenomenon occurs. One expects a full symmetrical model because the wire frame is symmetrical. However, we find two symmetries, depending on which side is viewed. Most interesting, we can change this symmetry by adding a light breeze to the side of the squared soap film in the middle of the structure. Like a flip-flop, the configuration changes with 90°, yet maintains the structure.

This model teaches that there are two stable situations; whichever one will prevail depends on trivial factors that are hard to influence (Figure 1).


Analogous to this model, we can look at critical solder joints as they separate from the wave. Sometimes that creates solder bridges and sometimes nicely separated joints, using the same process settings during wave soldering.

The same basic physical mechanisms as applied to liquid soap based on surface tension are involved in liquid solder and can be projected onto the soldering of such joints. This might explain occasionally bi-stable behavior on specific joint configurations.

If, instead of a cubic frame, another frame were dipped into the liquid soap, such as a tetrahedron, the same stable soap film structure would always come from the soap solution. All soap surfaces are, in this case, finally connected at one point. This model shows a clear mono-stable behavior. This, again, is analogous with most soldered joints on a PCB. They will, under normal process conditions, always provide sound joints that never bridge.

Gerjan Diepstraten is a senior process engineer with Vitronics Soltec BV (vitronics-soltec.com); gdiepstraten@nl.vitronics-soltec.com. His column appears monthly.

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