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February 9, 2018

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?

T.W.

Experts Comments

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.
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David Suihkonen
President
R&D Technical Services Inc.
David is President at R&D Technical Services, a vapor phase reflow oven manufacturer. he has 20 plus years experience in the vapor phase reflow industry.
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:
  1. Shield the adjacent components from heat such that they are not co-reflowed during rework.
  2. If (1) is not possible, strip conformal coating from the adjacent components prior to the rework.
The above are the only *sure* ways of stopping this, assuming of course that I am correct on the root cause.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
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.
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Bryan Kerr
Principal Engineer - CMA Lab
BAE Systems
Bryan Kerr has 35 years experience in providing technical support to PEC assembly manufacturing. His experience ranges from analysis of materials and components to troubleshooting and optimizing, selecting reflow, cleaning, coating and other associated processes.
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.
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Al Cabral
Regional Sales Manager
Finetech
Al Cabral is Regional Sales Manager for Finetech and Martin rework products. His expertise includes through-hole, surface mount and semiconductor packaging with an emphasis on soldering and heat transfer. Al has been a significant contributor to the development and optimization of reflow and rework processes and systems, particularly lead-free transitions and microelectronic applications.
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 solder ball 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.
Linda Woody
SME Production Technical Excellence Staff
Lockheed Martin
Subject matter expert in the field of electronics assembly and soldering.
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