Soldermask misregistration will result in some process issues
Surface Mount Technology (SMT) needs smaller, higher input/output components for new electronics applications, which requires reduced size and higher density interconnect for new area array packages. However, there are still many challenges for assembly, including solder short prevention and detection.
Some process issues can be caused by soldermask misregistration that occurred at PCB fabrication. Soldermask misregistration may result in issues with exposed adjacent traces and/or vias underneath array packages, increasing the opportunity for shorts. When an adjacent trace is exposed due to soldermask misregistration, solder bridging could have formed during the solder paste screen printing process. The acceptable soldermask registration criteria must be much stricter than standard PCB soldermask fabrication requirements.
Soldermask Registration for Pad-to-Trace separation needs to be taken into account. Considering soldermask registration, the soldermask openings should make about extend 3mils beyond the copper on pads. In this way if there is 3mils of registration error between the mask and the copper, there will also be no soldermask on the pad. This is very important because solder resist on the pad may cause assembly defects. The soldermask opening is usually 3mils, adding up the 3mils soldermask registration tolerance, so it means minimum pad-to-trace gap must be 6 mils.
In order to encure the soldermask registration, the followings can be noticed:
(1) For accurate film alignment, the expansion and shrinkage of film should be exactly measured.
(2) The recommended temperature of the no-dust room is 18-22 °C, the recommended range of humidity is 45-65% to ensure the film stability.
(3) When lots of boards are in production, the film needs a protective layer, to prevent film rubbing, resulting in ink residue.