Perhaps a more simpler way to look at it is, you know you've done enough reworks when the thickness of all the bonding, glue, lifted pads held down with glue, is too hard to remove for the next re-work... :-) OK, maybe this was a Friday thing, but hey, we have it off this year so I'm early!
Later...
Scott Decker – Staff Engineer, PCB Design Services CID+ – Electronic Systems Center
UTC AEROSPACE SYSTEMS
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of BEV CHRISTIAN
Sent: Wednesday, March 28, 2018 12:59 PM
To: [log in to unmask]
Subject: [External] Re: [TN] when is there too much rework on a circuit board
Phil,
BlackBerry’s engineers (all gone now ☹ ) would agree with Bhanu and Dave Hillman about the # of reworks in one spot. However, if we are talking about DIFFERENT components I believe, so tof remember that BB allowed 10. Why anybody would do that many for something as cheap as a cell phone, I don’t know, but there it is. Joyce, can you remember if I am correct?
Now if we are talking jumper cables, I can remember a 12” x 10” board at Nortel that had 32 jumper wires on it – and that was for a long life telephone switch..
Regards,
Bev
Sent from Mail for Windows 10
From: Nutting, Phil
Sent: Wednesday, March 28, 2018 3:53 PM
To: [log in to unmask]
Subject: Re: [TN] when is there too much rework on a circuit board
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.
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