|Ask the Experts|
March 2, 2020
Concave Via Fillets Causing Problems
When we wave solder our circuit boards some of the vias have concave fillets giving them a dimple effect. The boards functions fine, but we are having problems when we test using our flying probe system. What can we do to create flatter fillets on via holes?
|Expert Panel Responses|
Maintain your pad size,but reduce the via size to the smallest diameter your board house can provide without driving-up cost.
Circuit Connect, Inc.
You could always try to reduce the dewetting causing the concave joints by speeding up the process just a little which will reduce the drag off or alternatively run the bath cooler.
Technical Sales Manager
BLT Circuit Services Ltd
In order to reduce or eliminate the dimples, we first need to list the root cause(es):
For example, the volume change of SnPb solder during solidification is approx. -4%. For an 0.062" board, that means that we might have a dimple as deep as 0.0025" due to solidification shrinkage alone. If we have entrapped gas bubbles prior to solidification, these will shrink in volume rapidly as the liquid solder cools from the soldering temperature toward the solidification temperature. Both of the above effects are important causes of the observed "dimpling."
We cannot do much about the first issue, since all solders exhibit this behavior to some degree. The second effect can be mitigated by minimizing the effects of trapped gasses. Low-solids fluxes and control of PWB quality and moisture content (to ensure that PWB outgassing is minimized) can help greatly. The probability is that even with all available mitigations in place, we will still have the issue to some degree.
At that point, we need to ask what we can do to make our test process less sensitive to the issue. Some possible actions are:
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