|Ask the Experts|
February 10, 2021
What Causes Solder Balls During Rework?
During the rework of SMT components on conformal coated boards, we see solder balls appearing at neighboring components. How can this be eliminated?
|Expert Panel Responses|
As to solder balls during reflow, you may find you need a bit more solder conditioning via preheat before processing through the reflow temps.
This will allow the solder to better bake off the volatiles and liquids while still keeping the flux active for reflow. Solder balls can be caused by out-gassing from rapidly expanding liquids in the solder.
R&D Technical Services Inc.
I would need to see samples to be sure, but I would bet that the solder balls are forming because the solder in the joints is being reflowed and extruded through the coating. There are two sure ways to stop this:
If you melt solder underneath conformal coating it will expand in volume and normally burst out of the coating and form small solder balls. This does, of course, starve the solder joint and create more rework. The solution is to ensure that no coated solder joints are subjected to temperatures above the melting point of the alloy.
This can be achieved either by stripping coating from the heat affected zone or by reducing the HAZ to a minimum by careful selection of the rework method. For instance, hot air rework using the correct nozzle size can minimize the area of the assembly reaching melting point.
This is very design dependent and also on the type of coating used since some are easier to remove than others ! Prudent use of heat shields and masking tapes is also advised.
Principal Engineer - CMA Lab
One very common issue is the absorption of moisture, especially in environments with high humidity levels.
The use of a pre-rework baking process using a batch style oven could be considered, typically 120C at 8 +/- hours for assemblies and replacement parts. If the problem persists, at least the moisture variable would be eliminated.
Regional Sales Manager
This typically happens during hot gas rework when the coating is not removed far enough away from the nozzle. If the solder under the coating reaches liquidus temperature it will build pressure and pop out of the coating creating a solderball out on the board surface where it landed.
Look around the rework area and you should find insufficient solder joints - this is where the solder balls came from.
SME Production Technical Excellence Staff
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