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Subject:
From:
Yuan-chia Joyce Koo <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Yuan-chia Joyce Koo <[log in to unmask]>
Date:
Wed, 28 Mar 2018 17:06:13 -0400
Content-Type:
text/plain
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text/plain (65 lines)
you mean the 10x I usually disagree with MFG chaps?  it usually  
either a MEMS or RF shield can that need touch up... not full blow  
processor BGA (MEMS gyro or accelerometer that required absolute  
flatness, or large funny shape RF metal shield can that warp during  
reflow because the vendor forgot to stress relief after cold work)...  
or new parts that wrap  like potato chip ;-). parts replaced are  
usually throw away - almost feel like someone practice their skills  
for so many rework... ;-(
  or your CM paid as extra for rework rather than paid by achieve  
minimum 1st pass yield (good old days, without minimum 1st pass yield  
you don't go to production... but I guess since the design/MFG  
decoupled, it not apply now). - logistic issue regarding contractual  
agreement... - CM on the forum, don't get any idea... ;-(... ;-(
On Mar 28, 2018, at 3:58 PM, BEV CHRISTIAN wrote:

> 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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