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August 1997

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Subject:
From:
peter dahlen <[log in to unmask]>
Reply To:
TechNet Mail Forum.
Date:
Tue, 5 Aug 1997 08:57:57 +0000
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Hi,

Regarding design of pads for internal and extrenal layers, I think the best
will be to have them in the same size, this to better see misalignment on
finish board. However for manufacturing of PCB the internal pads should be
something like 0.1mm larger than extrenal pads this due to misaligment
during lamination of internal layers. One PCB company has the following
criteria: that the internal pads shall be 0.35mm larger than drilled hole
and extrenal layer 0.25mm larger than drilled hole.

For finish board the criteria, will be a larger distance between the pad
edge and the hole edge on extrenal layers, this to be able to have good
soldering (for example 0.15mm). For internal layers it is enough with for
example 0.05mm.

If the manufacturing tolerance is 0.1mm larger for the internal layer but
the criteria for the internal layer is 0.1mm less, so will the result be
that it will be possible to use the same size of pads.


Best Regards

Peter Dahlén
Saab Ericsson Space
[log in to unmask]




At 07:26 1997-08-04 -0400, you wrote:
>  John..
>         I'm still surprised! We have been designing 12-14 layer (standard)
>   smt designs for about a zillion years, maybe not quite a zillion, but
>   a looonnng time and no vendor has ever requested that the inner layer
>   pads be larger. In fact, unless I am looking at the formula/chart in
>  the Mil and IPC specs incorrectly, the minimum annular ring requirement
>  for internal is .001/.002 mils vs .002/.004 mils for external layers.
>  Therefore, by definition, your external pads should be larger. What
>  am I missing???
>                                       Regards,
>                                                  Bob
>


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