Remember though that we are trying to create a simple rule of thumb relating
acceptable gold thickness to the newly appearing super small packages. Looks
like if we are in reflow for 30 seconds or so the error that you and Werner
mention is not important.
But I wonder, from observing under SEM we see that the gold is not evenly
distributed in, for example, a solder ball. or even in the heel fillet of a
QFP. There is a lead rich area adjacent to the tin and gold needle
structures that form near the tin nickel intermetalic. It seems important if
we are trying to keep gold below 3 or 4 percent anywhere in the joint.
Does any one have a copy of Wolverton's Paper? "Understanding Gold
Embrittlement in Surface-Mount Soldering,"
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Wenger, George M.
Sent: Monday, August 22, 2005 10:47 PM
To: [log in to unmask]
Subject: Re: [TN] max thickness electroplated gold
Ryan,
Your dissolution rate for gold is off by about a factor of 3. The best
source is Wally Bader's paper in the Welding Journal which lists:
Au = 117.9 u"/sec.
Ag = 43.6u"/sec.
Cu = 4.1 u"/sec.
Pd = 1.4 u"/sec.
Ni = 0.05u"/sec.
Pt = 0.01 u"/sec.
Regards,
George
George M. Wenger, Andrew Corporation
Reliability / FMA Engineer
Base Station & Subsystems Group
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 [log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ryan Grant
Sent: Monday, August 22, 2005 6:16 PM
To: [log in to unmask]
Subject: Re: [TN] max thickness electroplated gold
Hi Guy,
My mistake confusing wt% and volume%. Glen is correct, it should be
wt%.
Gold embrittlement is associated with AuSn4 which is why I incorrectly
assumed alloy content is important. However, after a little research, I
found that AuPbx intermetallics are "possible", and besides, the 4% rule
applies to Sn/Pb bulk anyways.
The percent of solder is 50%, because you stencil print solder as a
function of calculated volume, not weight. (Now that I mention it, my
formula is further incorrect in that the volume of printed paste is
never 100%, rather something like 50-70% depending...man, I'm full of
errors!) The 50% comes from the approximation that neatly stacked
spheres will outline a shape roughly the same as neatly stacked cubes.
The spheres are the solder powder, the space between the spheres is the
flux vehicle and the rectangular shape represents the shape of the
stencil aperture the solder power will be forced to conform to. The
volume of the sphere inside a cube of the same dimension is roughly 50%.
O.K., technically its 47.7465...% because of the 3/pi. ([4/3 pi r^3]/8
r^3)=volume of a sphere divided by the volume of a cube.
Or, you can back calculate from the density of the 90%wt solder compared
to the density of the 10% flux to arrive at approximately the same 50 -
50 ratio of solder powder to flux by volume. But to get the allowable
thickness of gold, weight has to be converted to volume to get
thickness.
Since weight=Volume * density, the density of solder and gold must be
added to the equations.
V_g = l*w*height_gold*density_gold
V_s = l*w*.5*height_solder*density_solder
where height_solder is assumed the stencil thickness
max_height_gold = 0.04 * stencil_thickness * .5 *
density_solder/density_gold
Density of gold = 19.3g/cc
Density of solder = 8.46g/cc
Fortunately the units will cancel each other so we can apply the
forbidden mixing of inch and metric units. (My Physics professors would
kill me for this.)
Solving for height_gold=0.04*.005inches*.5*8.4grams/19.3grams=44uin
assuming 100% paste release.
Assuming 70% paste release...we're down to 30uin.
I've lost my good references of gold dissolution rates into tin at
reflow. But the one source I have is 36uin/sec at reflow temp. Since
joints are typically molten for several seconds, I'd imagine the gold
would dissolve pretty completely...but, I have heard several sources
indicate they've found a gold rich region in gold containing
solderjoints.
Concerning Dave's comment questioning 10 uin of gold being insufficient
to protect solderability, I have to agree, if we are talking about
electroplated gold. Under the high power magnification of an SEM, holes
can be seen in the electroplated gold and the underlying nickel can
often be seen. Immersion gold is a completely different story. It's
self limiting for a reason...
Ryan
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