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November 1998

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Wed, 25 Nov 1998 13:56:07 -0600
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Back in the good old days! Here we go all over again.

I do considerable work with R/F design types insisting thou must have vias in SMT pad centers to
ensure specified performance. Again, in the good old days, I designed and fabricated stuff whith
which most everyone in this forum is familiar.

An oversimplification is to fab two double sided boards with vias in SMT pads and relaminate
(sequentially) the finished outer boards to whatever multilayer construction is required. Then, do
the final drilling and plating with the holes in the SMT pads now "plugged" with epoxy during
relamination. Also, the electroless and electro-plating process covers the entire pad surface
leaving no signs of the hole ever having been there (ideally).

If you're going to place holes in pad centers, or adjacent to them without neckdown feature
requirements being met, soldering is next to an impossible chore - certainly not through reflow or
wave processing. This is true notwithstanding the never ending touch up and device cracking
problems as no solder fillet is possible.

Enter micro vias. However, they don't always satisfy R/F engineer's needs as electrical and high
speed performance unless you place several in one pad. Back to square one.

One note of optimism concerning R/F, black magic stuff points to most designs, with which I have
bee associated, working very well with noles next to pads connected properly with specified
neckdowns. It's often a hard sell, but using good DFM stuff, it gets done.

Earl Moon

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