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soldermask...who needs it?

Lock2002

This is a question regarding soldermask between pins of fine pitch devices.

The majority of our production designs end up with at least 2-3 mils of copper thickness on the external layers due to blind vias, wrap plating requirements, etc. Our current design practice has been to add soldermask between pins unless the webbing is less than 4 mil.

One of our PWB/CCA experts says we should gang the fine pitch devices because the webbing is more of a liability than an asset. He suggests that since the soldermask is less than a mil thick it would not act as a dam to prevent solder bridging between pins (2-3 mil copper) and has the potential to flake off.

Thoughts?

  • budnoel

    I would agree with your PWB/CCA expert.  Here's an excerpt from IPC-7351A (Paragraph 3.5.2) :  "A solder mask may be used to isolate the land pattern from othere conductive features on th eboard such as vias, lands or conductors.  Where no concuctors run between lands, a simple gang mask opening can be used as shown in Figure 3-24.  For land pattern designs with routed conductors between lands (the solder mask pattern must completely cover the conductor.  A more precise registration is necessary becuased of the tight tolerance needed to cover the conductors without encroaching on the land area. Printed board manufacturers are required to keep the solder mask material off the land.  Clearance conditions can vary from 0.0 mm to 0.1 mm (.004 in)"

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