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
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Friday, August 19, 2005 2:28 PM
To: [log in to unmask]
Subject: Re: [TN] max thickness electroplated gold
I would describe your equation as a rule of thumb. And I thank you for
it.
However, I have a couple of questions about the equation.
max_height_gold = 0.04 * stencil_thickness * .63 * .5
Why do we care what the percent tin is? Isn't the number 4% in the
solder
( and Glen brings up and interesting question)?
Are we sure it (the gold contamination) is by volume, or is it by
weight?
Glenn
Why would the percent solder be 50%, our paste is 90% metal . . . by
weight?
or volume? hummm.
Why wouldn't we need to factor in the rate of dissolution of gold into
the
solder? Since the alloy forming is AuSn(2 and 4). Won't the tin be
consumed
at the interface? Won't the resulting Pb rich region inhibit dissolution
of
the gold through the filet? Wouldn't this result in a region rich in
gold
near the board surface. With a soldering iron we can stir the pot a bit,
but
in a reflow oven there is none of that.
I get 180 uin as a max when the stencil is 0.005 in thick. But, I know
better. In fact, I think the 67 number is way to thick as well. My
experience tells me the number is much lower than this rule gave us. I
think
it is even under the 18 uin in the IPC-6012B.
I don't think we have a reliable model mathematical model here. But,
it's a
start. This old thread from the archive is pretty interesting.
Does anyone have a copy of Mike Wolverton's paper?
Dave, do you still think that we need more than 10 uin of gold to
protect
solderable basis metals? Or were you only speaking to component lead
finish?
Date: Fri, 21 Mar 1997 15:42:08 -0800
From [log in to unmask] Fri Mar 21 17:22:15 1997
In-Reply-To: <[log in to unmask]>
Reply-To: Mike Wolverton <[log in to unmask]>
X-Receipt-From-Agent: true
Resent-Message-ID: <"aC16b2.0.3ED.YtlCp"@ipc>
Resent-From: [log in to unmask]
X-Mailing-List: <[log in to unmask]> archive/latest/11286
X-Loop: [log in to unmask]
Precedence: list
Resent-Sender: [log in to unmask]
Status: O
X-Status:
Gold embrittlement is well understood, but not widely
understood.
The good news is, the risk of gold embrittlement has been
quantified.
It is a function of the solder density and the allowable (maximum)
percent
gold. The maximum ratio of gold finish thickness to solder alloy
thickness
can
be calculated, to help prevent a problem from excessive gold-tin (i.e.,
AuSn4)
compound. For the equation and its derivation, please refer to a paper
of
mine,
"The Use of Capillary Action Measurements for Solderability
Improvement,"
Soldering and Surface Mount Technology, No. 9, Oct., 1991, Wela
Publications,
Ayr, Scotland, Table 5.
It is interesting to note, The Embrittlement culprit, AuSn4 is
29
weight percent gold. So even if one had 100% tin as the solder alloy and
10%
gold were dissolved into the solder, then about a third of the solder
joint
would be rocks (i.e., compound) and the remainder would be soft solder.
Do
not
count on the rocks to bond to the solderable surface. Hence most
recommendations I have seen have been in the 3 to 5 weight % gold range
for
the
maximum allowable percent.
Gold is dissolved by molten tin based solder, but the gold does
not go
into solution (i.e., the result is not a solid solution). For more
details
on
the mechanism and explanation, please refer to a paper of mine:
"Understanding Gold Embrittlement in Surface-Mount Soldering," ISHM's
1st Joint Technology
Conference, San Diego, March, 1990 (//WWW.ISHM.EE.VT.EDU). Incidentally,
ISHM
changed its name to IMAPS, (703) 758-1060.
The above represents my own view, and not necessarily that of my
employer.
---------------------------------------------------
Mike Wolverton
Texas Instruments
[log in to unmask]
------------------
Original text
From: "ddhillma" <[log in to unmask]>, on 3/20/97 5:50 PM:
To: [log in to unmask]
Cc: [log in to unmask]
Hi Tom -
The words gold embrittlement really mean job security for
metallurgists! You would think that as well researched and
investigated the gold embrittlement mechanics of failure has been
over
the last 20 years we would stop making the same mistakes. Two rules
of
thumb to follow:
a) Is there enough solder volume to allow the gold to uniformly go
into solution?
b) Is the soldering operation of sufficient temperature and time to
allow (a) to occur?
The thickness of the gold finish isn't the only deciding factor of
having a solder joint embrittled by gold/tin intermetallics - the
process is equally important. Also, you can avoid the whole gold
embrittlement issue by keeping the gold below 10 microinches but
you
then have to deal with poor solderability on the components if this
thin gold plating is of poor quality (and plating a quality gold
finish below 20 microinches requires good control and knowledge of
one's plating system).
Dave Hillman
Rockwell Collins
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Ryan Grant
Sent: Friday, August 19, 2005 1:25 PM
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?
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text
in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at:
http://listserv.ipc.org/archives
Please visit IPC web site
http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100
ext.2815
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
|
