We also experienced such a serious problem earlier--secondary reflow on
topside during wave soldering. Curreently we are coating the vias on
solderside with epoxy to shield the heat passing onto the topside. we
also turn off top preheaters prior to wave. We did tried to increase
solder by experimenting on stencil apertures.It helped but you may need
to run DOE and identify the cause.

Another thing for looking at is --package outline of components.

Best of luck.
> ----------
> From:         Valli Robert[SMTP:[log in to unmask]]
> Sent:         October 3, 1997 11:00 AM
> To:   [log in to unmask]
> Subject:      Re: [TECHNET] TEchnet
>
> We have seen similar problem caused by smt component reflow during
> selective wave soldering.  Have you done thermal profiling of your
> product through all of your processes to verify that topside board
> temperatures of fine pitch smt components during wave solder do not
> cause secondary reflow of the soldered connections?
>
> RAV
>  ----------
> From: Martin Bourke
> To: [log in to unmask]
> Subject: TEchnet
> Date: Thursday, October 02, 1997 9:07PM
>
>
> Hi,
>
> I work with a SMT process which requires us to build boards ranging in
> size
> from (6"*4") to (18"*14"). These boards can be single or double sided
> with
> component counts ranging from 100 to 2,500. We palce large quantities
> of
> 20 thou fine pitch devices and BGAs'. Several of the boards are double
> sided
> reflow and are selective soldered at the wave solder machine.
>
> We currently print using DEK and MPM machines with a 6 thou thick
> stencil
> and a water soluable solder paste on to Hasl finish boards. Our
> biggest
> cause of defects is insutticient solder and dry joints found both at
> the
> end of the SMT area and at ICT. Has anyone done any work what the
> optimum
> apperture size for a specific device type should be ? Are there any
> changes
> which can be made to apperture design to allow better release of the
> solder
> paste ? Any information that anyone has on stencil design would be
> greatly
> appreciated.
>
> We also use a View Engineering SVS 8100 solder paste inspection
> machine
> to
> measure the area and volume of solder paste deposited. Is there a
> theoretical
> minimum volume of paste required to create a good solder joint for a
> specific
> pad size, component lead size etc. Our current inspection criteria on
> the
> SVS
> uses the theoretical Area and Volume of the specific apperture with a
> +/-
> tolerance. Has any one come across a way of calculating  an actual
> expexted
> value which would be close to the actual mean values measured by the
> machine ?
> This would allow us to use much tigher tolerances, and hence have a
> better
> process window to work within.
>
> Any information that any body can provide on the general
> screenprinting
> process
> and the inspection of same would be greatly apperciated.
>
>
>
> Martin Bourke.
>
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