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November 1998

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Subject:
From:
"Phillip E. Hinton" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Sun, 29 Nov 1998 14:12:53 EST
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Ken F.

I assume the joint is in tension and not in shear when you say verticle upward
pull.

However, some background first on your choices:
When you use heavy gold plating, which is anything over 0.3 microns, with
solders that have a good amount of tin, such as Sn63, you are very subject to
bittle fracture.  NASA in the mid 1960's reported the problem with the
formation of AuSn4 intermetallic which caused failure even with through-hole
printed boards that were plated with 2-3 microns of gold over copper or over
nickel.  There have been numerous very good reports and information published
on the subject since that time.  Most of these indicate that there should be
no more than 4% gold in the joint, but when you are soldering, the method may
leave more than the maximum percentage at some portions of the interface or in
localized areas, grain boundaries etc.  Therefore, heavy gold platings with
tin-based solder are to be avoided.  This avoidance of heavy gold platings
applies to leads, pins, lands regardless of the underplating such as nickel,
palladium or NI/Pd.

Therefore, you are left with your other choice of tin plating over copper
plating.  Your suggested tin plating of at 200-300 microns (0.008 -012 inch)
thick is not a realistic tin plating thickness,  suggest 6 to 12 microns if
the tin plating is alloyed completely with the solder during soldering.  (Pure
tin platings/coatings on leads or surfaces tend form whiskers later when in
operation).  Suggest you use a tin-lead plating/coating on the pin over the
copper plating instead of the tin.  If you have restrictions on lead in the
plating process, you can also use 3-6 microns of nickel followed by 0.1-0.2
microns of gold or the same with 0.1-0.2 microns of Pd between the nickel and
gold, the platings should be electrolytic (galvanic) until the brittle
fracture problems associated with the electroless platings is sorted out.
Many of the SMT electronic components used today have a nickel plating with an
overplate of palladium on the leads.  My experience is that they solder well
and form a good reliable joint.

I am not sure if the Pd/Ni plated alloy would be any advantage over the other
combinations, but have heard that it is better to solder to than nickel
without a thin gold overplate.

As to strength, since you are using a bare copper land, you will not increase
the strength by switching to one of the more exotic and costly platings.

Hope this has some value to you.

Phil Hinton

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