Ken Patel:
- It is my belief that filling vias with epoxy will have no beneficial effect
on reliability and may in fact degrade reliability.  As Werner has pointed
out, the beneficial effects from filling a via are most noticeable when the
environmental stress on the pwb is extreme.  A study done (in the distant
past) by Roger Wild of IBM showed that an unfilled via was most reliable, a
completely filled via was almost as reliable as an unfilled via and a
partially filled via was not at all reliable.  Roger's study was conducted by
subjecting pwb's to several (thousands I think) fairly stringent  thermal
cycles and monitoring for both an increase in resistance and an open circuit.
As I recall (been some time ago), one of his conculsions was that the
relatively high reliability of a solder-filled via was due to the solder
flowing up over and therefor reinforcing the "weak knee" area of the pth.
DOD, for whom I worked at the time, became (unfortunately) dogmatic about
having complete hole fill.
Just my 0.02 worth.
Jim Moffitt
Moffitt Enterprises
Electronics Manufacturing Consulting Service.

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