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November 2002

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Subject:
From:
JaMi Smith <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 15 Nov 2002 11:17:15 -0800
Content-Type:
text/plain
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text/plain (177 lines)
Russell,

There have been a number of follow-ups to your initial questions in your
initial post, and most of them have dealt with the "gold embrittlement"
issue to some extent, but have not really explained it very well to you, so
that you can see the importance of asking the customer about his soldering
process, so I will expand a little bit here.

When gold is present somewhere in a joint that is soldered, some of the gold
goes into "solution" and alloys with the other metals present.

The presence of gold in the alloy of the solder joint causes what is known
as "gold embrittlement", which I can best describe as a "crystallization" or
"crazing" of the solidified solder joint that mechanically weakens the
joint, and can even cause physical and electrical failure of the joint. I am
not a Chemist, so my explanation may be off a little, but I am sure that
there are many here in the forum who can explain the exact process much
better if necessary. It is this "crystallization" or "crazing" of the solder
joint that causes it to look "very dull" as you described.

The amount of gold that goes into solution is dependant on two basic things:
First, the amount of gold present in the joint. Second, the composition of
the solder, i.e.: the specific alloy of the solder.

It is this second issue that is being ignored here, and is important for you
to follow up on, and the reason is as follows:

Standard solder, such as Sn60 or Sn63, has the ability to "wick" or "suck"
metals such copper, silver, or gold, into its' own solution or alloy, up to
about 2 percent of its volume. When gold is wicked into the solution, it
causes "gold embrittlement".

To combat this problem, special solders have been developed which have
already combined a certain amount of silver in the alloy (about 2 percent)
which tends to "satisfy" the alloy of the solder so that it will no longer
will "wick" or "suck" additional metal (such as the gold on the pads of your
PCB in this case) into "solution".

So what this boils down to is this: The "gold embrittlement" that your
customer is experiencing could be caused by your putting more gold (i.e.:
too much gold) on the PCB, or it could be caused by your customer changing
his solder from Sn62 (which contains about 2 percent silver) to something
along the lines of Sn60 of Sn63, either of which will "wick" substantial
amounts of gold into "solution".

The point here is that you could actually have less gold on your boards, and
yet still have more gold going into "solution", if the solder was changed.

To compound issues even further, today, due to the industry trying to get
away from the use of lead, especially in solder, there are many new types of
solder which I understand do not use lead. It is entirely possible that your
customer has "updated" his soldering process to a newer type of solder,
which is more susceptible to "gold embrittlement"., and that this is the
cause of your problems (excepting of course the "reflection" issue with the
"shiney" finish).

So, it is imperative that you do in fact address the issue of solder with
your customer, elsewise you may end up chasing your tail and never solving
the real problem.

I am not by any means up to date on any of the new solders that have been
developed in recent years, or the exact reasons for their development, and
as stated above, I am not a Chemist. For this reason, it might be
appropriate for me to ask whether or not there is anyone else in the forum
out there who could point us to a good article or discussion on these
issues, so that we can get some credible source information on the topics.

Hopefully this has clarified some of the issues involved.

JaMi

----- Original Message -----
From: "JaMi Smith" <[log in to unmask]>
To: "TechNet E-Mail Forum." <[log in to unmask]>; "Russell Burdick"
<[log in to unmask]>
Cc: "JaMi Smith" <[log in to unmask]>
Sent: Thursday, November 14, 2002 2:48 PM
Subject: Re: [TN] gold solderaility


> Russell,
>
> It sounds like there may be some "gold embrittlement" taking place in the
> soldering process with the newer boards, among other things, and while
there
> may be some problems on your end, such a little more gold on your newer
> boards, I think that before accepting all of the blame, you need to point
> the finger back at the customer and ask him whether or not he has changed
> his solder, or soldering processes, or anything else in his own
processing.
>
> You need more information from your customer in order to solve the whole
> problem.
>
> Regarding the issue of the "shiney" gold fiducials showing up as "black"
due
> to the reflections, the "average" customer "should" be able to adjust the
> location and intensity of his light source to compensate for the
> differences.
>
> JaMi Smith
>
> * * * * * * * * * * * *
>
> ----- Original Message -----
> From: "Russell Burdick" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Thursday, November 14, 2002 2:04 PM
> Subject: [TN] gold solderaility
>
>
> > Hello everyone!
> >
> > I could use some help concerning soldering to gold.
> >
> > Problem:
> > A customer of ours has given feedback to our salespeople that hard gold
> > boards we made in May 2002 were much better to solder to than what we
sent
> > in October 2002. Biggest difference was stated as the newest boards have
> > fiducials that auto pick and place equipment can't read because under
the
> > lights they look "black". Under normal shop lights both boards look
bright
> > and shiney(sp?).
> >
> > Also, the customer commented that on the boards with "black" unreadable
> > fiducials had solderjoints that looked very dull.
> >
> > Known manufacturing/processing differences between months of May and
> October
> > (aka, confessions of a process engineer):
> > Prior to August 2002, dull plated gold boards were scrubbed to improve
> > appearances. After August 2002 no scrubbing needed when a carbon treat
and
> > proper wetters were performed/maintained in nickel and gold baths.
> >
> >
> > Any and all comments are greatly appreciated.
> >
> >
> >
> >
> >
> > _________________________________________________________________
> > Help STOP SPAM with the new MSN 8 and get 2 months FREE*
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> >
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