Thanks Wayne.  I am more concerned with thru-hole or cuts and jumps.



-----Original Message-----

From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Showers

Sent: Wednesday, March 28, 2018 3:34 PM

To: [log in to unmask]

Subject: Re: [TN] when is there too much rework on a circuit board



To contrast slightly with Dave, 3 reworks was established by procedure and was a direct result of preparing for NADCAP certification.

For BGA's, we established a 1 and done guideline for rework so that the assembly was evaluated if a second rework was to be attempted.  This was done mostly as a function of the component cost, but also to minimize potential pad damage.



Rework limits are mostly driven by the diminishing bond strength of the functional epoxy.



Here are the general guidelines:

Note, Prepreg is chemically almost identical to DEVCON 2-ton epoxy.  This has a bond force of 15.5N / mm2, but each successive heat cycle weakens the bond by hardening (crystallizing) the epoxy.



Hardening (crystallization) at temperatures above 300C, decomposition at temperatures above 350C.  For my non-metrically inclined, 572F and 662F.