Hi Larry,
although containing a nominal 2% silver and despite that we normally see
silver as a shiny metal, it doesn't go necessarily that a 62/36/2 alloy will
give better cosmetics. In fact in some cases it will exhibit 'worse'
cosmetics by exhibiting a rougher surface. A 62/36/2 alloy won't be eutectic
it will be off eutectic, melting and solidifying over a temperature range.
Albeit there is a tight range, any deviation from the nominal 62/36/2
through over charge of the elements or through contamination (Sb, Ce, Fe
.... yes there are limits but they're changed or overlooked, sometimes for
good reasons), will potentially increase the likelihood for cosmetic
deterioration.
With respect to palladium finishes, it's been shown that the joint produced
over palladium alloys/plating is formed through the soldering alloy
dissolving the component finish and hence wetting the component. Note that
it's also know that silver finishes give a nicer joint with silver
containing alloys, work I was involved in indicated that even with the
limited contamination of the silver finish into e.g. 63/376 solder alloy can
create a differential cooling profile in the joint producing a solder joint
with a rough surface. Strictly speaking as 'scientist' I don't have the info
on other finishes (OK maybe I know a little about soldering to an alloy 42
finish), however the dissolution of gold pad finishes into solder joints is
well known, therefore I draw a similar conclusion of dissolution of gold in
the formation of solder joints over gold.
With respect to joint strength being better with 62/36/2, there is a lot of
data available apparently showing this to be the case. I state 'apparently'
since 'joint strength' is a very vague statement. Alloys tested under one
set of conditions can exhibit the reverse or inverse results under different
test conditions, so it's a bit of a 'mythological fact', docusoap style, not
exactly incorrect, but not the whole truth.
Now here's my myth for the record, I believe the thick film story is the
correct one, however it was before my time so can't categorically state it's
true. You also might find that someone has spec'd 62/36/2 alloy to overcome
some apparent soldering issue that disappeared through using this alloy, it
is significantly different that 63/37. A lot of specs come about through
chasing down a solution rather than an explanation.
Does that help you?
Probably not eh?
Andrew Hoggan
BBA Associates Ltd
www.bba-associates.com
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Larry Koens
Sent: 26 July 2001 15:05
To: [log in to unmask]
Subject: Re: [TN] solder paste with 2% silver
Thanks for the input you guys have given me.
I had heard some of the same things, that the 2% is used in the Hybrid
industry and that the 2% is suppose to give shinier (cosmetic) solder
joints. But, I guess my question is, does the 2% give me a just a better
cosmetic joint? Is there no real mechanical benefit?
Thanks,
Larry
-----Original Message-----
From: d. terstegge [mailto:[log in to unmask]]
Sent: Thursday, July 26, 2001 8:32 AM
To: [log in to unmask]
Subject: Re: [TN] solder paste with 2% silver
From "Soldering in Electronics" by Klein Wassink:
"Note on solder alloy composition:
Solder paste in hybrid circuit technology (for thick-film circuits)
usually has a metal composition of tin62-lead36-silver2. It should be
realised that the addition of silver is not at all necessary for the
silver (-palladium) metallisation of the components, but for the much
thinner silver-palladium conductors on the tick-film substrates. The use
of the more expensive silver containing alloy, instead of the common
tin60-lead40 alloy, for (relow) soldering on printed boards with copper
solder lands is not based on technological necessity, but sometimes on
better availability of this alloy (in the form of paste) and in most
cases merely on habit. For the rest: with silver loaded alloy no harm is
done !"
Kind regards,
Daan Terstegge
SMT Centre
Thales Communications
Unclassified mail
Personal Website: http://www.smtinfo.net
>>> "Kasprzak, Bill (sys) USX" <[log in to unmask]> 07/26 1:54 pm >>>
To all:
I've been reading, with great interest, the responses so far to the use
of
2% silver paste. I am not being critical of the folks who responded so
far
but, each one cites a "reported" or "understood" advantage to using 2%
paste.
I have also heard some of same reasons for considering a switch to a 2%
silver paste. Can any of the metallurgists on this forum confirm some of
these reported advantages and perhaps recommend that given x,y, and z,
one
should consider using 2% silver solder alloys?
Good topic.
Bill Kasprzak
Moog Inc., Electronic Assembly Engineering
> -----Original Message-----
> From: Eric Christison [SMTP:[log in to unmask]]
> Sent: Thursday, July 26, 2001 4:20 AM
> To: [log in to unmask]
> Subject: Re: [TN] solder paste with 2% silver
>
> > I always understood that you had to use a paste with 2% silver if
you
> > were using components
> > with terminations containing Palladium. The 2% silver is also
supposed
> to
> > five you a shinier
> > finished solder joint.
> >
> > Steve.
> >
> > Larry Koens wrote:
> >
> > > Got a question for everyone,
> > >
> > > I just switched companies and the new company that I'm with now
uses a
> > > 62/36/2 solder paste. I've always used the 63/37 formula. The guys
who
> > > decided to use the 2% silver are no longer with the company. My
> > question
> > > to you is, why would they want the 2% silver in the paste? They
know
> > > something I don't?!
> > >
>
> The silver should give the alloy higher strength, better creep
resistance
> and a higher melting point. Perhaps your application benefits from one
of
> these differences?
>
> Regards,
>
>
>
> Eric Christison
>
>
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