Ask the Experts
July 18, 2011

Defective Pads?

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?
Defective Pads?

P. T.

Expert Panel Responses

It appears as though there are a couple of things needing to be investigated. As stated the boards were HASL prepared, this function should have created an intermetallic layer between the base copper and the tin/lead surface of the HASL process. Secondly, the component appears to have unreflowed solder paste along the bottom edge of the photo, which leads one to lean towards the reflow process parameters being incorrect. The question also stated that a light force was used and the component popped off. With this thought, care must be taken however when using tweezers with small tips or objects to pry components as the tips or other tools in reality applies a large amount of force within the given point area. This is a lever effect which converts the light force into a high force at the point location and it is very difficult to determine the exact amount of force which was initially applied. In any case, it appears there may have been some contamination on the base copper material thereby impacting the ability of the HASL coating to create an intermetallic layer. After the solder paste deposition and the component placement, the reflow dwell time was not long enough to recreate the intermetallic layer allowing the solder to wet the copper pads and subsequently the solder paste only fused itself to the HASL coating. I say this because if the joint had broken within the thickness of the intermetallic layer there would be more evidence of tin in the copper pad and from the picture provided it appears as though there is no tin on the pads. I would recommend a review and verification of the thermal profile to certify the board temperature did exceed the liquidus temperature of the solder paste and the dwell time was long enough to complete the creation of the solder joint. I would be willing to discuss this further if needed.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

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.

Matt Stevenson
Director of Marketing
Sunstone Circuits
Matt Stevenson has over 20 years experience in the PCB industry. Serving in roles as a Chemical Lab Technician, Process Engineer, Quality Engineer, Quality Manager, and Marketing Manager. He has proven himself to be an invaluable resource.
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