Ask the Experts
December 11, 2006 - Updated
July 4, 2007 - Originally Posted

Reflow and wave soldering OSP Boards

In OSP boards, after reflow and wave soldering, if the pads are exposed for about 2 to 3 mils, is this acceptable? We are getting good solder connection between components and Pads.


Expert Panel Responses

Exposed copper around the periphery of the pads is acceptable. With the use of the new lead free solder alloys the basis metal copper is at times visible around the periphery of the pads. This is due to the lack of wetting with new lead free alloys or differences between pad sizes and stencil aperture sizes where the paste deposition does not cover the entire pad location. In these cases the molten solder may not wet to the edges of the pad or land areas, yet this does not impact the reliability of the solder joint. Work conducted during the 80s proved that exposed copper is not a problem and will not deteriorate over time. It is important understand the need to verify the fluxes used are compatible with the end use of the product. Residual flux residues must be benign to not impact the long term reliability of the product and if the residual flux residues are not benign, then a cleaning process must be incorporated to clean the flux residues. Hence, the reliability of the solder joint is based upon how the solder wet the basis metal and the lead, not how much of the pad area was covered with the molten solder. Many times customers relate this exposed copper to lack of solderability and must be educated in the physical behavior of fluxes and molten solder where the solder will not wet to the edges of the pads.

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.

Yes, this is very common with OSP boards and even more so if you are using a lead-free alloy. IPC 610 also supports this as a acceptable condition.

Greg Hueste
Senior Applications Engineer
Speedline Technologies
Greg joined Electrovert in February 1984. Based out of the Electrovert applications laboratory in Camdenton Missouri, Greg has been in the process applications support role since 2000. His primary responsibilities include providing process and machine applications support for the wave soldering lines as well as process, machine and operations training. He also provides applications support for the reflow and cleaner lines. Greg is a PBET certified trainer and holds two patents on wave solder nozzle design.

This is a common issue with Pb-free solder. For the most part, it depends upon your environment and your level of risk acceptability. For most applications, a small amount of exposed copper should have no influence on reliability.

Dr. Craig D. Hillman
CEO & Managing Partner
DfR Solutions
Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification, passive component technology and printed board failure mechanisms.
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