I concur with Werner with the key exception being for pressfit connector
holes.

Our experience dictates that we specify pressfit holes with
a) manufacturer's exact drill size, (provides the ideal "Goldilocks"
envelope for the needed copper deformation -- not too much to crack copper,
not too little and not acheive gas tight joint)
b) minimum copper thickness, (we spec 0.0012" min for older 2.5 mm pitch
and 0.001" min for 2.0mm pitch pressfit) and
c) minimum finished hole size (too small will increase insertion forces
overwhelming the insertion press or causing pins to "Z-kink" onto pwb
surface).

These factors combine to ensure a reliable gas tight joint without
excessive insertion forces or excessive holewall deformation (leading to
post separation or barrel cracking).

It all starts with using the proper (precise) drill size.  The connector
mfg's recommended drill size is the paramount concern.
Ergo, the exception where drilled hole size should be specified.

my $.02,
dw


At 05:41 PM 1/16/2006, Werner Engelmaier wrote:
>Hi Bob,
>The call out of finished hole size is an anachronism left over from the
>through-hole technology era. In modern high-density PCBs you should
>specify the
>drilled hole size (no tolerance) with a minimum plating thickness in the
>hole to
>be verified by cross-sectiong of coupons.
>The reliability of your vias depends on PCB thickness, drilled hole size, Cu
>plating thickness, Cu plating quality (verified by testing co-plated mandrel
>foils for tensile strength and ductiolity--not elongation). Your friend is
>right,
>electrical test is not enough. You should have coupons either IST or
>HATS-tested. This will be even more important with LF-soldering
>temperatures. You then
>might want to specify not only Tg, but Td and CTE(z) as well.
>
>Regards,
>Werner Engelmaier
>Engelmaier Associates, L.C.
>Electronic Packaging, Interconnection and Reliability Consulting
>7 Jasmine Run
>Ormond Beach, FL 32174 USA
>Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
>E-mail: [log in to unmask], Website: www.engelmaier.com
>
>
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