|Ask the Experts|
January 3, 2020
Rework for OSP Assemblies
Should we modify any rework or repair procedures for circuit board assemblies fabricated using OSP (Organic Solderability Preservative) bare circuit boards?
|Expert Panel Responses|
I would use a slightly more active flux and clean off any residue remaining with a good cleaner. This should give you a better chance to wet the copper if it has aged and oxidized. If the PCBs are straight off production and need rework then you should be OK with standard no-clean wires.
Technical Sales Manager
BLT Circuit Services Ltd
According to Enthone, the manufacturer of the Entek Plus 106A OSP coating, the coating is more susceptible to certain types of "solvents" than others. They state that alcohol will strip about 70% of the coating, water will remove about 15%,but Kyzen Lonox only about 3% of the coating.
Picking an appropriate solution is key to prolonging the life of the coating. In the event that the coating is stripped using alcohol or other classic solvents, they recommend that the soldering be completed within 12 hours. Fitting the proper wash material as well as a limit on time between the soldering would be applicable to the repair / rework process. Once solder has been applied to the pads,the repair or rework process should not need to be modified.
Director of Marketing
I can think of one scenario where procedures might need to be modified; when rework needs to be performed on the first-assembled side of a two-sided SMT assembly, care needs to be taken not to degrade solderability of the second-side pads.
This would be of concern where a BGA or other bottom-terminated component is to be reworked, since repair stations can aggressively heat the opposite side. You would need to evaluate the effects of the rework station on solderability, or alternatively do rework only after second-side reflow. The second option may be less than desirable, depending on the design.
If your assemblies contain through-hole parts that need to be soldered after reflow operations are complete, you should also take care that heat from rework does not degrade solderability in these areas.
OSP (Organic Solderability Preservative) boards have been around for many years. As to whether the rework or repair process should be modified, I would not think so. One thing to consider is the solderability of the board after a period of time after their manufacturing date.
The OSP coating does have a shelf life and to repair the product by removing and replacing components might require a more aggressive flux. If a more aggressive flux is used then a cleaning process must be incorporated and a check of cleanliness is strongly suggested. So other than using a more aggressive flux, I don't see the need for any additions or modifications in the process.
Vice President, Technical Director
OSP is a coating that protect copper. Once you install the components, pads are soldered and tinned. When exist the needing to rework components, an uninstallation will take place, which involves removing the component and absorbing the remaining solder residue on the pads.Luis Alberto Garcia, Sinectech Training
Therefore, from this perspective, we can understand that pads are tinned so it would not be copper exposed, actually it may contain better solderability than the original form of the pad with OSP. I do not consider it necessary to make any procedure change because the tinning of pads is to increase solderability.
You can make tests with the approved flux of the original process and if it does not give negative issues, use it, but if it results in a negative issue, you will need to make a flux change that does not affect the solder mask or adjacent surfaces.
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