Why Test for Solder Mask Under-Cure?

Solder mask under-cure can be a precursor to product reliability issues. Under-cured solder mask (either inadequate cure time or temperature) can trap processing residues and contribute to electrical leakage or electrochemical migration failures.

The Indicator

Performing a C3 localized extraction test on a printed circuit board will reveal instances of under-cured solder mask.  The under-cured mask is porous, and steam from the C3 test will penetrate the mask. The result is an opaque white circular remnant as seen in the image below.

Bare board prior to C3 extraction

Bare board after C3 extraction

The Fix

Fortunately, mask under-cure is an easily correctable issue. By exposing the board to heat (or UV depending on the cure mechanism) the mask can be driven to a fully cured state. Once the solder mask is properly cured, the resulting non-porous surface is no longer at risk for absorption of subsequent processing materials and residues.

Verification

If a board identified as under-cured is exposed to additional heat (or UV bump) the white remnant will vanish and a subsequent C3 test will not visibly affect the board. Not only is the C3 an effective cleanliness analysis tool, but it can also be used to identify solder mask under-cure conditions.