Defective Pads?

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Defective Pads?

Postby Molly O'Hara » Mon May 14, 2012 9:30 am

Question: "We recently needed to repair a defective PCBA that required removal of an SOIC. See the attached photo.
We were able to simply lift the SOIC off the PCBA using tweezers with very light force, and NO HEAT.
The pads on the PCB are bare copper. This PCBA was originally manufactured using SnPb HASL finish and the SOIC was soldered with SnPb solder.
What do you think happened?"
P. T.
Answer from Sunstone Circuits: From the photo, it looks like there was some type of undercut of the solder or oxidation of the copper that reduced the surface area where it made contact. It almost seems like there was an oxidation layer or corrosion between the copper and the solder layer that resulted in little to no adhesion. Under normal circumstances, the tin in the solder makes an inter-metallic bond with the copper which is very strong, reducing the chance for a clean separation as is seen in this photo. If the bare boards were provided from the board manufacturer with a solder finish, I am not sure how this could have occurred without some visible solder defects (non-wetting). If the oxide was not removed prior to the HASL process, there would likely be some sign of a solder issue. If the boards were provided as bare copper and the solder was added prior or during the assembly process, it is likely that the copper finish was not cleaned adequately prior to application. Copper in air will oxidize and does not accept solder readily if not properly cleaned just prior to solder application.
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Molly O'Hara
 
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