Paul, I respectfully offer counter-points to your points from a fabricators perspective:
1. When you factor in the costs of having to treat waste streams containing tin/lead (as opposed to tin/lead-free waste streams), any potential cost benefit associated with fused solder vx. HASL is minimized, if not eliminated altogether.
2. Poorly or partially fused tin/lead (usually what you get when you try to fuse a too-thin initial co-deposit) loses some of the solderability and shelf-life advantages it may have over HASL.
3. The problem with acheiving the proper co-deposit of tin and lead is largely a matter of the current densities associated with a particular design. While the nominal deposit may be within an acceptable range, high and low current density areas on a particular board can result in significant variation in the thickness of the deposit, as well as the ratio. On finer line designs, the variation in thickness alone can wreak havoc with yields, resulting in plugged holes and small solder balls forming on the fine traces.
Additionally, one of the huge selling points of HASL in its early days was the reduction in bridging-type problems with finer line designs. I guess this is a non-issue if your designs are still 0.015" lines and spaces, but as a fabricator, we don't see much of that any more.
Regards,
George C. Toman
Circuit Center Inc.
phone: 937-435-2131
fax: 937-435-7698
e-mail: [log in to unmask]
----------
> 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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