Steve,

        O.K.  Agree you can't solder to gold.  You have to solder to the Nickel.  Others probably remember the intermettalics that are formed.

        Disagree on the thickness, (depending on the application of course).  Now, I know there is some disagreement, but the values I've seen quoted for gold embrittlement have been between 3% and 4% by weight.  Since this issue always comes up here, I had to write a spreadsheet on it.  For 3 mils solder, post reflow, 0.2 mils solder on the component, I calculated 60 uin gold can be applied before you pass the 4%.

        Kirkendall voiding is a separate issue.  This is intermetallic diffusion of one metal at higher rate than the other when bonded.  Case history has been with Gold wire bonding on Aluminum pads or vise versa.  See this web site for good info http://www.kns.com/resources/articles/influence.asp

Glenn

P.S.  Congrats Ryan!

-----Original Message-----
From: Steve Abrahamson [mailto:[log in to unmask]]
Sent: Tuesday, August 21, 2001 9:02 AM
To: [log in to unmask]
Subject: Re: [TN] Solder vs Gold Pads on SMT

Yes it is impossible to solder effectively to 30 micro inches of gold.  15
micro inches is also very difficult to solder to.  I believe the voiding
mechanism is refered to as Kirkindall voiding, although I was unable to
confirm this in any of my books.
Not to drift off the point here, but I was not that interested in replying
on this topic as much as I am interested in what the heck Ryan Grant is
doing replying to this topic.

RYAN, you do not show up for work yesterday because your wife is in labor.
Perhaps you might notify your friends at work- heck and even the guys on the
technet about the birth announcement of your child.  I understand the
importance of proper gold thickness and all, but C'MON man.  Was it a boy
and did you name him after me?  Maybe out of respect to the technet and your
colleagues, you should name the child Dilbert, or even APEX.

Steve A

> -----Original Message-----
> From: Ryan Grant
> Sent: Monday,August 20,2001 8:49 PM
> To:   [log in to unmask]
> Subject:      Re: [TN] Solder vs Gold Pads on SMT
>
>  Ken, that is very crafty of your PWB supplier.
>
> It is actually cheaper and easier for them to supply your boards with a 30
> micro inches of gold finish.  You see, you already demand the 30 micro
> inches of gold for your edge fingers, and getting ENIG (which is
> different)
> on the rest of the board means they would have to do two (count them 2)
> different surface finishes.  The extra cost of the gold is practically
> nothing compared to the cost of the chemicals that carry the gold and the
> process cost of applying it.
>
> Do not worry about gold embrittlement.  You will have to get more than 70
> micro inches of gold on the pad before that happens.  Unless, of course,
> you
> are not at reflow temperatures long enough such that the gold does not
> homogeneously distribute itself through the joint.
>
> However, the thick gold may cause soldering problems.  Thick gold has been
> reported by some to cause voids and/or grainy joints.
>
> If it were me, I would tell them NO WAY, do the board right.
>
> Kind Regards,
>
> Ryan Grant
> Advanced Technology Engineer
> MCMS
>
> -----Original Message-----
> From: Chafin, Ken G.
> To: [log in to unmask]
> Sent: 8/20/01 12:30 PM
> Subject: [TN] Solder vs Gold Pads on SMT
>
> We presently use SMT printed wiring boards with 60/40 tin-lead solder
> finish
> on the pads.
>
> Our PWB supplier has offered to supply our boards with a 30 micron
> electroless gold finish on the pads at the same price.
>
> Does anyone have any views on the relative merits of the two finishes?
> Solderability?  Durability?  Reliability?
>
> We manufacture electronic control and signaling systems for the rail and
> rapid transit industry and our equipment is often in harsh environments.
>
> Thanks for any information on the subject.
>