Enza,

I do not agree with your recommendation of overprinting as you
described (either by wave or stencil apt/thickness increase) to
relieve insufficient solder on pads with via holes.  I had
experience of trying your techniques years ago.  All I got was
insufficient solder and insufficient solder.  I was wandering at
that time where all excessive solder went.  All solderpaste/
solder filled prior to the reflow went thru black holes and never
been found.

Later I found some of evidences that these excessive solder
landed on the bottom heat panel of reflow oven and a lot went
thru heat holes in the heat panel.

I agree some of responses using off-centered  and smallest via
holes in smt pads.  This restricts some degree of molten solder
going down thru black holes.  It is better to have conductive epoxy
filled via holes or tented via holes from the bottom side.

re
Matthew Park
NII Norsat International Inc.



Hello Geoff,
My experience with "via in pad" has been this...during reflow,
solder
tends to scavenge into an unfilled via leaving an "insufficient" or
starved solder joint.  We had overcome this on older board
designs by
filling the PCB vias with solder over a wave solder pot to prepare
the
PCB for the SMT process.  Now, I'm not very familiar with blind
vias and
such, but you might be able to compensate for the solder which
will be
lost down the hole by over-printing solder paste.  Overprinting
can be
accomplished by selectively modifying the stencil apertures for
the pads
which contain the via so that you do not put excess solder all
over the
board (i.e., by using a thicker stencil.)

That's my 2 cents worth.

Enza

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