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Outgassing is the main culprit, but does not explain them all.

Many articles written about post-wave-soldering solder ball formation cite as the culprit outgassing, or vapor escaping. Generally this is true, but it does not explain the small solder balls often found between leads of wave-soldered connectors or SO-component leads. These are often positioned with a curious regularity. In fact, the physical mechanism behind the solder separation process is responsible for the formation of this specific solder ball pattern.

In wave soldering, solder balls cannot be completely avoided since they are part of the process of solder separation. The liquid solder bridges all joints when the board is in the solder wave; however, when the board separates from the wave, all solder between the joints and between the wave and leads must separate.

This separation process is similar to the behavior of water flowing from a tap that is closing. At first a large, steady liquid column flows from the tap. While closing the tap, this column becomes thinner and thinner. At a certain point during closing, the lower part of the column separates into a string of single droplets. That is how a thinning, freely-moving liquid flow behaves. It ultimately finds its optimal physical equilibrium by separating into single droplets.

During separation of the solder, surface tension reduces the liquid surface area for a given liquid volume. This causes the solder to withdraw from the space between joints, both horizontally and vertically. Although this appears to be an uncontrolled process, it still obeys the physical laws.

The liquid solder behaves like water. When the separation is almost complete, the thin solder column that exists between the joints on the board, and also between the wave and the lead, will end up in one or more single droplets. Normally, one will not find traces of that process unless the solder has a tendency to adhere to the soldermask. In that case, a specific pattern of solder balls can be found between the joints. This is especially true with closely spaced solder joints, such as between SO-component leads, where these small balls sometimes form with a mathematical regularity.

Figure 1 illustrates the formation of water (and by extension solder) globules and thus the formation of solder balls in wave soldering. The thin water flow from the tap was made with a professional camera using a short shutter time (1/1600 sec.). Single droplets are formed as the flow drops further from the tap. This cannot be seen with our eyes because the droplets are moving too fast to be seen as distinctly separate. However, when a single droplet hits a metal sink, one can hear the impact.

The soldered connector joints in Figure 2 show solder balls between the joints. The origin of these solder balls lies in the separation mechanism from the solder as it withdraws to the single joints. Like the water stream, tiny droplets can form, and their adhesive behavior causes the tiny balls to stick to the soldermask.

Now the solder ball formation is explained, but why are they found on the board?

Solder balls end up on the board surface due to the adhesion of the solder to the soldermask, in combination with flux residues. Refraining from using soldermask will usually eliminate the problem of solder ball adhesion, but is not always a viable option. A good combination of soldermask and flux can prevent this adhesion.

The weakening of the soldermask during soldering might have an effect on solder ball adhesion. It is possible to reduce this effect by optimizing machine settings. In general, however, this problem can be solved only through proper material selection. Prevention is always better than trying to find a solution in the solder process settings. For more in-depth information about the physics that affect solder separation during solder drainage, see Chapter 2.2 of Soldering in Electronics, second edition, by R. J. Klein Wassink.

Gert Schouten is a senior engineer at Vitronics Soltec (vitronicssoltec.com); gschouten@vsww.com.