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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