Keep an eye on your pot: your solder pot may be trying to tell you something.

I've said it before, and I'll say it again: Our eyes are probably the most useful diagnostic tool we have in wave soldering. The more we learn about Pb-free soldering, the more we realize what is important in general soldering is even more important in Pb-free. Visual observations are no exception.

I am often asked how frequently to sample a Pb-free solder pot to analyze its alloy composition. Unfortunately, there is no hard and fast rule, since each solder pot experiences a different combination of boards, components and process settings. Some assemblers sample solder pots on a regular basis; others do it only when investigating soldering problems. As a proponent of process control, I obviously prefer active sampling as a means to prevent soldering problems, as opposed to reactive sampling--after defects impact the bottom line. But, as a realist, I understand that's not always the case. I admit it: I've occasionally been guilty of slacking on pot analysis, especially with stable SnPb processes. For assemblers who aren't in the habit of regular sampling, the solder itself can provide a few visual cues to let you know it's time.

Dross formation changes. Solder alloys often contain small inclusions of elements to slow oxide formation or to prevent the oxides from binding with good solder. If dross production increases or the appearance of the dross changes--it seems to get "slushier" due to more included metal--it is likely the antioxidant materials have been used up and require replenishment. This is more commonly seen in processes that run one or two shifts per day than in machines that run around the clock, because machines that run continuously usually get replenished with fresh bars containing antioxidant materials on a more frequent basis. If the dross formation does change--either in quantity or appearance--it's time for an analysis.

Solder bridging rates go up. If a process that has been running steadily suddenly starts producing more solder bridges, it could be because of alloy contamination. It's not really contaminated in the sense that it is spoiled with elements that don't belong and must be discarded. It's likely just out of balance, often with excess copper. A pot analysis will quickly reveal if the alloy is out of balance, and if so, how to bring it back in spec. Raising the pot temperature by 5[degrees]C (~10[degrees]F) increments may help to temporarily alleviate the bridging, but raising the solder pot temperature can be risky business. It can hide--or even worsen--the condition rather than fix it, and temperatures above 265[degrees]C (~510[degrees]F) shouldn't be used without first making sure the PWB laminate and components can take the additional heat.

Sampling is easy. An analytical laboratory set up for this testing will provide a mold, a ladle, an envelope and a procedure. An operator or technician casts an ingot in the mold, lets it cool, pops it out of the mold and into the envelope, and sends it off. A couple days later, the analysis is forwarded to the requestor. The hardest part about it is following the sampling procedure; i.e., stirring the pot properly to get a representative sample of the alloy. Most bar solder suppliers offer solder analysis services to customers for a moderate fee. Speaking of moderate fees, analysis is also cost-effective. Both defects and excess dross production incur costs that immediately detract from the bottom line. Although Pb-free wave solder bridges do not necessarily cost more to rework than SnPb ones, they are still rework and they still cost money. When considering dross generation, Pb-free dross is much more expensive than SnPb dross because of the higher cost of the metals. Avoiding the financial loss caused by both the solder defects and the excess dross is a sound upfront investment.

I have to reiterate that preventive actions are better than remedial ones, but even if there is no regular sampling schedule for the solder pot, the visual cues that it may be going out of balance should not be ignored. Who's the best person in the factory to pick up on those cues? The person who runs the machine every day. The operator can tell the process owner rather quickly if anything has changed, because it will be the operator who spends more time over the hot solder pot scooping the extra dross or straining the liquid metal out with a slotted spoon. It's also the operator who hears the complaints from the inspection and touchup crew about increased bridges. If the operator is on the lookout for these symptoms, they are more likely to help avoid the problems and associated costs before they run out of control.

What's this month's lesson learned? Watch your solder pot. Literally and figuratively. Watch the appearance of the solder and the output. Watch the alloy balance with regular laboratory analysis. Why? Because a watched pot never spoils.

Chrys Shea is an R & D applications engineering manager of Cookson Electronics (cooksonelectronics.com); [e-mail]. Her column appears monthly.

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