Hi Jack-

I think the issue comes down to the generic board stress which comes up from time to time.  It's a complicated situation, and some kind of careful analysis (such as Finite Element Modeling) may be required to see how much bending stress the fastener could generate locally.  Many board mount fasteners only restrict movement of the board in the vertical direction, allowing enough "float" horizontally to allow the board to grow and shrink with respect to the chassis without adding bending stress to the board.  

A more common case where closeness to a feature makse a huge difference is with the distance between a firmly mounted component such as a capacitor, and an edge which is going to be cut with a "pizza wheel" type shear cutter.  This process has been known to break even small capacitors.  You could use the same guidelines recommended for that condition to pull parts away from mechanical fasteners and probably be fine even if the fasteners are rigidly anchoring the board down.  

So use floating fasteners, or take a chance.  If you have the potential for an extreme case, do the detailed analysis.

Wayne

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jack Olson
Sent: Tuesday, January 15, 2013 10:37 AM
To: [log in to unmask]
Subject: Re: [TN] Mounting Hardware to QFP Component Lead

True, but I can't help but wonder if I'm missing something...
Of all the issues that the IPC committees have discussed over the last few DECADES, why can't I find anything about mounting structures to component clearance?

hmmm...  dar's gotta be sumpn 'bout dat!

thanks anyway,
Jack

.
On Mon, 14 Jan 2013 19:16:27 +0000, Wayne Thayer <[log in to unmask]> wrote:

>If it is a new QFP with the pure copper/very high copper leads then I wouldn't worry about it.  Those leads are so compliant they can absorb amazing amounts of stress.
>
>Wayne Thayer
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Jack Olson
>Sent: Monday, January 14, 2013 11:36 AM
>To: [log in to unmask]
>Subject: [TN] Mounting Hardware to QFP Component Lead
>
>To the assembly professionals out there:
>
>Have you ever had reliability issues with mounting screws if they are near a Quad Flat-Pack? My 144 pin land pattern is pretty close (100mils) but we are mounting the board into a very flat structure (no bowing) I can't imagine the compression of the board material could affect gull-wing style lead connections (maybe BGA?) but I don't know...
>
>thanks,
>Jack
>

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