Are we sure it is by volume, or is it by weight?
I've used the 2%, 3%, or 4% rule by weight. (Threshold depends on
application and use environment.)
Glenn
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ryan Grant
Sent: Friday, August 19, 2005 10:25 AM
To: [log in to unmask]
Subject: Re: [TN] max thickness electroplated gold
Hi Guy,
Its not a rule of thumb, its science... Gold embrittlement happens at
4% gold per volume of tin for eutectic Tin/Lead solder. Clearly this
rule isn't a sharp "edge" so I'd recommend a safety margin.
Now lets do the math...
Assume the component pad size is a 1:1 ratio to printed pad size.
Volume of the gold (V_g) = l*w*h_g (length times width times gold
thickness)
Volume of the solder (V_s) = l*w*h_s*.5*t_r
where .5=close approximation of solder powder volume to printed
paste volume and
where t_r= tin ratio of the alloy.
Since we are looking for the ratio where V_g/V_s = 4% as the drop dead
ratio, and
Since we assume component pad size is 1:1 to printed pad size, l*w
cancels out.
We now have h_g/(h_s*.5*t_r) = 4%
Solve for h_g = .04*h_s*.5*t_r
If we assume a 5 mil stencil
h_g = t_r*.0001
Assuming Tin/Lead solder at 63/37
h_g = 0.000063 = 63uin
That 63uin includes both component and PCB pad gold thickness.
That said, your 50 to 100 micro inches is bad news...
By the way, if the component is BGA, it comes loaded with a lot more
solder, thus the calculation will need to include the volume of the BGA
ball.
If you are using Lead-Free solder, this rule doesn't apply and
transferring principles becomes complicated. The 4% rule is known to
apply to silver as well, but you will notice Lead-free alloys are
already at a 4% ratio of silver to tin but do not exhibit the same
embrittlement failure implied with Tin/Lead embrittlement. The specific
point that gold, or silver, embrittles a Lead-Free solder joint has not
been studied and published, as far as I know. (If anyone knows
otherwise, please let me know the reference).
Good luck,
Ryan Grant
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Thursday, August 18, 2005 2:38 PM
To: [log in to unmask]
Subject: [TN] max thickness electroplated gold
Many of our customers are using the old MIL-STD-275 to specify gold
thickness for Hard Gold. They are ordering this because they are doing
both
wafer and package testing at the die and device level. So, we are seeing
50
to 100 uin of gold over nickel. It hasn't been much of an issue because
we
build most of the boards by hand soldering methods.
We are starting to see components in the parts list for these assemblies
that cannot be installed with hand soldering methods.
When we solder with printed paste and reflow methods we see dull, grainy
joint that are clearly contaminated with gold.
When we hand solder this condition is not evident. (I believe the
condition
may actually be worse because the dissolved gold is more concentrated
near
the bottom of the fillet. But, that is another discussion).
Does any one have a rule of thumb for the maximum thickness of
electroplated
gold on land patterns intended to receive SMT components . . . say, 0402
passives or MLFs?
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