Paul,
        I would like to add a potential drawback for using a fused tin/lead PWB
in an SMT application.  You are correct about the solder mask breaking
down.  I remember seeing large chunks of mask falling off the ground
planes of our fused tin/lead PWAs.  Usually the solder would wet/wick
under the mask, which would help float the mask off or sometimes just
leave the mask wrinkled.  There were occasions when the solder would
wrinkle the mask and then vacate, leaving a cavity.  Then the ensuing
arguments would usually revolve around whether there was potential for
entrapment of contaminants.  I would think this could still be a problem
for wave soldered SMT assemblies.

        On double side reflow SMT assemblies, I would be concern that wicking
would cause some of the solder to disappear up the etch under the mask.

Gary Camac

Paul Gould wrote:

> Sorry Mark but I have to disagree with your three points.
>
> 1.Plated tin-lead post-etch fused is still the cheapest finish. Reasons are
> no need to strip the etch resist, no need to pre-clean and pumice the copper
> before solder mask application, no HASL process. All that is needed is to
> solder condition, flux and IR reflow which believe me is much cheaper.
>
> 2.It is possible to fuse thinner deposits if you are not concerned with
> cosmetics. Very good results are achieved with thinner deposits on fine
> line.
> circuits because of the etch undercut which increases the amount of tin-lead
> over the copper trace.
>
> 3.It should be possible to get close to 63/37 otherwise, as you say, the
> deposit may get into the pasty range and spoil the cosmetic appearance.
>
> The only drawback to fused tin-lead is that solder mask has to be applied
> over it and this can cause breakdown through wave soldering. More and more
> assembly is now IR fused solder paste so this finish could well be worth
> looking
> at again instead of going for even more expensive alternative
> finishes.
>
> Paul Gould
> Teknacron Circuits Ltd
> UK
> www.teknacron.com
>
> -----Original Message-----
> From: Mark Simmons <[log in to unmask]>
> To: [log in to unmask] <[log in to unmask]>
> Date: 01 December 1998 17:32
> Subject: Re: [TN] HASL vs. Fused Solder For CCA Assembly
>
> >Michael, a couple items here might put this in perspective.
> >
> >1) Fused T/L is much more costly to produce for the fabricator.
> >
> >2) T/L does not fuse below 300 micro in.
> >
> >3) The metal is not Eutectic. 60/40 only.  (watch that plastic range)
> >
> >hope this helps,
> >
> >Mark Simmons
> >
>
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