TECHNET Archives

August 2005

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Condense Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Content-Type:
text/plain
Sender:
Subject:
From:
Glenn Pelkey <[log in to unmask]>
Date:
Fri, 19 Aug 2005 13:01:29 -0700
MIME-Version:
1.0
X-To:
Reply-To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Parts/Attachments:
text/plain (132 lines)
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?



---------------------------------------------------
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
-----------------------------------------------------

---------------------------------------------------
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
-----------------------------------------------------

ATOM RSS1 RSS2