Immersion tin (white tin) is a particularly aggressive process for bare
board fab - it is operated at high temperature, long immersion times and
uses two chemistries that find it easy to work their way through the
soldermask resin structure, namely MSA and Thiourea. There are lower MSA
versions available which reduce the attack, but we now use immersion tin as
the metal finish to benchmark soldermask performance. We have identified a
soldermask that stands up well to the immersion tin process and meets all
the requirements on SIR and e-corrosion, but surface preparation before
coating, and screening technique do have an influence on the ability of the
mask to perform.
With reference to your comment on the mattness of the ink, their seems to be
a big rift in the understanding of the effect of mattness on contamination,
and too many generalisations made. The way that the matte is achieved is
different with different masks, even by the same manufacturer, so I would
rely on data, not theory, when you want to know if it has an effect. You
can't beat real undisputable facts !
Dougal Stewart
Director of Technology Development,
Viasystems Europe
tel: +44 191 206 2081
mob:+44 780 221 1958
The information supplied here is a personal view, and may, or may not
reflect the view of my employer.
-----Original Message-----
From: Seth Goodman [mailto:[log in to unmask]]
Sent: 23 August 2001 05:23
To: [log in to unmask]
Subject: Re: [TN] White Tin
I have also seen the problem with decreased solder mask adhesion to the
board. Our board vendor had to switch solder masks and print two passes of
it to effectively cover the board (I have no idea why two passes were
required). The results were good except for the matte finish of the new
mask, which we are afraid will ultimately pick up more contaminants than the
very shiny finish of the original mask. Does anyone have any specific
recommendations for a solder mask that is compatible with white tin
processing and has a smooth surface finish?
Our actual goal is lowest SIR and highest reliability over the long term
(10+ years). The environment is a medical laboratory so it is very benign
and we specify class 2 performance. This is an FR-4 board, 4-layer, 0.062"
thick, 1 oz. copper per layer, SMOBC: no rocket science here.
Regards,
Seth Goodman
Goodman Associates, LLC
tel 608.833.9933
fax 608.833.9966
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Neil Atkinson
Sent: Tuesday, August 21, 2001 3:08 AM
To: [log in to unmask]
Subject: Re: [TN] White Tin
I agree.
The question was 'as an alternative to HASL".
As an alternative to HASL immersion tin can be very good with all the
benefits mentioned earlier.
One problem to watch out for is compatibility with solder resist - it can
attack some resists causing problems - use a robust resist and you should
see few problems. Like the man said - try it and see what you think.
It's the old horses for courses - immersion gold might be the right finish
for your application but I would definitely try immersion tin first.
Neil
Neil Atkinson
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