Gold is indeed very soluble in liquid phase tin-lead solders at normal
pot tempertures. Virtually all the gold exposed to a wave for >2 sec will
dissolve, and is widely considered a contaminant.
A presentation at the 8/95 SMI show by Dr. Jennie S. Hwang indicated the
dissolution rate of gold into solder is 111 microinches/sec at 232
degrees C. Given that most plated finishes are below that thickness, and
that the operating temp of your wave solder pot is normal, you may expect
to find dissolved gold in the pot in short order.
Since gold-tin intermetallics are considered highly detrimental to solder
joint fatigue life, this may be a concern.
The other side of the argument may be that the fast dissolution is
actually good.
If you plan to use gold as a solderability protective (or just do so
without planning) for the PWB, perhaps you can use a very thin gold
coating and tolerate the dissolution, as the pot may have say 200 kg of
solder alloy and the dissolution can continue some time until the pot
reaches a detrimental solution level of gold.
Manko recommends that the amount of copper contaminant plus _twice_ the
gold should not exceed 0.35 % (wt/wt); he also states that gold as low as
0.02% has been reported to cause wetting problems.
Immersion coatings of as little as 5-10 microinches are possible (but at
that thickness, I wonder if gold may not provide the protection against
base metal passivation that is apparently desired). So I recommend that
you calculate surface areas and production rates, see how many you can
run before you have your pot at a level which may affect the metallic
properties of all joints, or even the wetting you'll see. I would
definitely accelerate a regular pot testing program.
See _Solders and Soldering_, 3rd ed. by Howard Manko for a good
discussion of intermetallics, and lots of other related info.
I have heard some recent proposals of the use of Ni/Au as a solderability
preservative, but there would seem to be some fundamental metallurgical
concerns which make it sound less than fully safe for me. This said, I
know that some bizarre sounding approaches have been shown to be workable
in the past. I know some good companies who required gold plated finish
on PWB's in the old days (meaning more than a few years in this
business).
Seems strange that we now have some people saying that this plated finish
is desireable, when for years I seem to remember people twisting my arm,
raising prices, and using other techniques to get me to accept no gold or
various gold replacements.
Perhaps you could simply dump and replace your pot at regular intervals
and tolerate a level of say 0.02%. If so, assuming a 200 kg pot, you can
have about 0.0002 x 200,000 g or 40 g of gold. At a thickness of 10
microinches (0.25 um), that represents 40 g = (0.000025 cm) (area, cm2)
(19.3 g/cm3); or the area in cm2 you can dissolve before reaching 0.02%
wt/wt is:
40/.000025/19.3 = 82,900 or 13,000 sq inches.
That may be a lot of units you can wave solder, depending on the total
surface area of gold you see on each unit. Since you have a dual wave
machine, maybe the second pot will see relatively little gold, and the
joints are actually being formed by the second (less contaminated) pot.
Keep in mind that 40 g of gold is more than one troy ounce, which may
warrant recovery.
You will need a better metallurgist than me to predict the possiblity of
harmful intermetallic formation using solder having low level gold
contaminants, or what may happen with uneven dissolution and resultant
higher concentrations at joint sites.
AND, the above is pretty straightforward, but what about SMT joints where
ALL that gold will dissolve and STAY in the joint with normal printed
paste/relow joining soldering techniques...
Lastly, keep in mind the Cowboys blasted the Giants 35-0 last night, this
may correlate with higher than normal residual ethanol levels in my
bloodstream and affect my calculations for a day or so. Check those
numbers.
I'd like to hear what your experience is with this finish in regard to
the actual solderability and joint formation.
Jerry Cupples
Interphase Corporation
Dallas, TX
214-919-9150
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