OSP Poor Wetting

[Molly O'Hara]

Question: We are experiencing poor wetting on one OSP coated PCB using a lead-free no-clean paste? We see this problem on just one board part number. All other similar boards reflow fine. Where should we look to solve this problem?
J. G. Answer from Sunstone Circuits: Some of the common solderability issues from OSP can occur:

If the part in question sees multiple reflow cycles. By the third reflow, the OSP coating can degrade and exhibit poor wetting. If there is a thin layer of soldermask on the pads that inhibits the solderability of the copper. This may be related to a defect of the artwork allowing a small amount of light to bleed through, to expose enough soldermask that would cause an issue.
Either of these thoughts could explain why it is only present on a particular part number.