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May 2, 2012

Solder Mask Mis-registration Issue

Solder Mask Mis-registration Issue
I have a situation where solder mask was mis-registered and is covering a portion of the pads for a micro-bga.

What percent reduction in pad area is acceptable while maintaining a reliable solder joint for commercial applications?

R.L.

Experts Comments

Let's start with the industry specifications; IPC-6012 states that no encroachment of mask on non-soldermask-defined BGA lands is allowable, except of course at the conductor attachment point. The reason behind this is that the BGA solder joint takes up all the thermal and mechanical stresses between the component and board, unlike leaded parts where the leads absorb some of the relative movement. For this reason, things that reduce the solder joint area, or create "stress risers" (sharp internal corners) are to be avoided.

The reliability impact for your specific situation depends on a lot of factors, and a detailed analysis would need to be done to determine the level of risk. The bottom line, though, is that if the PWBs were purchased to IPC-6012, they do not conform.

Suppliers can nearly always hold soldermask registration within a true position of 0.003". Suppliers with more modern imaging technology may be able to hold 0.002". You don't say what pitch your part is, but it looks to me like the soldermask registration may be close to accepted registration tolerances. You will need to look at the design vs. the supplier's process capability to determine whether changes to the design may make it more producible.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
I would say there should be no tolerance to resist bleed onto BGA pads. Mostly because there are Plasticizers that are very clear that can creep out from the main body of the resist and reduce actual wetting to the pad. But obviously the BGA ball will still melt and may look perfectly fine but be a future issue electrically and mechanically.
Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has twenty two years of service in Electronics industry. York has installed over 350 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.
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