In a message dated 08/07/2000 2:38:05 PM Central Daylight Time,
[log in to unmask] writes:
> Hey all-
>
> We got some surface mount connectors in that have a white,
powdery,
> dull
> appearance to them. The soldering results were not real good. After some
> research, the supplier says that the parts are tinned with a 93 tin/ 7 lead
> alloy. Seems to me, with that high a percentage of tin, tin oxidation
would
> be a problem. He claims that they have made millions with no reported
> problem. Is this a standard lead finish? Could oxidation be a problem
with
> that alloy, or is that enough lead to stop oxidation? We need too much
> volume to dip them ourselves before assembly. The supplier won't dip them.
> Thanks for any input.
>
> -Ryan Jennens
> TelGen Corporation
> (517) 887-6666 x151
Hi Ryan!
Actually, it is a quite common finish...I checked a few web sites for
connectors, sockets, etc. and they spec out SN93/PB7 plating as a finish.
Here's a few othere things I got from the web for ya'..
From: http://www.nbplating.com/finishes.htm (New Brunswick Plating)
Q- Are there many problems associated with Tin Plating?
A- There are many problems associated with tin plating. One of which is
whiskering, like cadmium and zinc, thin needle-like crystals known as
"whiskers" form within a period after plating that may vary from a few weeks
to several years. A whisker may measure up to .0001" (2.5um) in diameter, and
grow spontaneously to a length of 0.375" (10 mm). Conditions that tend to
promote the growth of whiskers are compressive stresses and uniform
temperatures for long periods of time. In most applications, these slender
microscopic crystals would be unnoticed and harmless, but in closely spaced
electronic circuits they are capable of carrying sufficient current at low
voltages to cause serious short circuits or a corona discharge.
When the formation of whiskers is known to be a potential problem, the
condition may be prevented by specifying that a small amount of lead be
included in the tin deposit. While 1-2% lead is adequate to substantially
reduce the risk of whiskering, it is customary to specify a 93% tin - 7% lead
alloy to assure that the alloy remains sufficiently high in lead under all
conditions of electroplating to prevent the formation of whiskers. Small
quantities of antimony, copper or nickel in the tin deposit have also been
reported to prevent the formation of whiskers.
From: http://www.pfonline.com/articles/pfd0023.html
When talking about Electrolyte Formulations, it states the following:
Inorganic Impurities
The importance of high-purity anodes cannot be overemphasized. Antimony
arsenic and bismuth will cause a heavy black sludge on the anode. This can
cause roughness and lower anode efficiency. Copper is one of the most harmful
contaminants. A solution concentration of over 10 ppm can cause reflow and
soldering problems. Iron, zinc, aluminum and cadmium are other harmful
impurities. Copper is easily removed by low-current-density electrolysis (one
asf).
-Steve Gregory-
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