For all those interested, this is what I sent out regarding vias in pads:

Let me be a little more specific regarding our PCB design, then, if you have
additional questions, I'll try to answer them.

Our typical design is a multilayer (anywhere from 4 to 8 layers) polyimide
board.  The back layer is also polyimide (no copper), approximately 5 mils
thick.  The vias have been anywhere from .014 to .022" in diameter.

We control TCE mismatch by hard bonding the board to a restraining core (metal
matrix material), therefore we needed to insulate the back of the board from
the core (using a polyimide layer).  When bonding the polyimide layer to the
board, the vias fill with polyimide resin.  Any resin which spills over the top
onto the annular ring needs to be removed.  The board then goes through an
additional copper plating operation, which adds another layer of copper on the
top surface.  The board then goes through photo resist and the normal etching
process.  The result is what appears to be a solid surface mount pad with the
via enclosed.

A picture is worth a thousand words... not much of a picture, though.

                ________________________
                |_______________________|<---- SOLDER
                |_______________________|<---- COPPER OVER-PLATE (.001-.002")
                |______ |       | ______|<---- ANNULAR RING
                       ||RESIN- ||
                       ||FILLED ||
                       ||HOLE   ||      <---- POLYIMIDE (TYPICALLY 6-8 LAYERS
                       ||       ||                   APPROX .060" THICK
                 ______||       ||______             3.5:1 ASPECT RATIO)
        _________|______|_______|______|_______
        _______________________________________ < POLYIMIDE BACK LAYER (.005")

Additional information:

We've been using this SMT design for about 15 years for military avionic
electronics.  Besides being a good approach for BGAs, it also has allowed us to
embed the vias in standard surface mount pads where real estate was an issue.

Jim Marsico
(516) 595-5879
[log in to unmask]

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