I've designed using a .010 diameter via in the middle of land attachment
pads. We would tent the vias on the farside of the board. We did have
some problems with contamination in the via. Prior to tenting, the
solder would flow out of the pad. We designed our solderpaste pad to be
larger than the attachment pad by the volume of the via. This along with
tenting the vias gave us the best results.

> ----------
> From:         Matthew Park
> Sent:         Friday, February 20, 1998 3:04 PM
> To:   [log in to unmask]
> Subject:      Re: [TN] Vias in SMT pads -Reply
>
> 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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