George, I'm a fabricator too!!
I guess you use tin plating or photo-resist as your etch resist. I was
considering switching to tin a while ago but we still get a steady
demand for IR reflow and so stayed with tin-lead. Tin is better if it
is going to be stripped off before solder resist so there is a benefit
there.

On the effluent side, rinse waters from tin-lead plating are easily
treated and there is no lead evolved from the fusing operation.

Distribution can be a problem with some designs and those with solid
ground planes should be solder masked over and HASL finished. If the
ground plane is not to be covered with solder resist, however, you do
get a better cosmetic finish from reflowed tin-lead.

I am intrigued by the bridging problem you mentioned. On fine line
designs which have say 6mil traces, these can be reduced to 4mils on
1oz copper so there is 50% more tin-lead effectively on these traces.
This fuses back to the trace width and causes lots of little blips.
The work around for that is to reduce the plating time by 30% on these
designs. HASL is far more likely to cause bridging on fine lines and
so these tend to move to ENIG rather than back to IR Reflow.

I am not trying to push the clock back and would not argue that HASL
is the mainstream finish for many good reasons, but there is still a
niche for IR reflow if the customer wants it. I estimate our current
throughput must be around 10% IR Reflow, 60%HASL, and 30% ENIG on high
density fine line work.

Regards
Paul Gould
Teknacron CIrcuits Ltd
UK
www.teknacron.com

-----Original Message-----
From: George Toman <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: 01 December 1998 21:25
Subject: Re: [TN] HASL vs. Fused Solder For CCA Assembly


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]

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