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December 2003

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Subject:
From:
Vladimir Igoshev <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 12 Dec 2003 12:51:06 -0500
Content-Type:
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text/plain (189 lines)
Steve,

That's exactly my point. Nickel will always diffuse into/through gold along grain boundary and porosity shouldn't make it worse (unless it's an "extreme case" like in the Dave's example). 

Have a good one.

Regards,

Vladimir Igoshev
Research in Motion

-----Original Message-----
From: Creswick, Steven [mailto:[log in to unmask]]
Sent: December 12, 2003 12:39 PM
To: [log in to unmask]
Subject: Re: [TN] Cleaning of Wire Bondable Gold


Vladimir,

My last two bits on the topic - Thin gold over nickel will remain wire bondable for a shorter time than will thicker gold.  The same comments holds true when both gold thickness are exposed to heat.

I doubt the nickel needs porousity to diffuse up, there are plenty of grain boundaries for it to occur at

We did see an excess of nickel and salts at pores.  Belief was that residual plating salts trapped in the pores only made matters worse.  How large were the pores?  Don't remember, but it obviously took a SEM to see them.

Thicker gold, only meant that it was more unlikely for a pore to extend all the way from the base metal to the surface of the gold.  (Kind of like why hybrid mfrs would use two separate screens when screening a multilayer dielectric - if there is a defect in one screen, ideally it would not be in the second too)  It also creates a greater distance for any diffusion to occur across, which means greater time to occurrence.

Organics are always a problem.  Tons (either metric or English) of organics come from the air (mainly sulphur commpounds and carbon).  Flux residues add a bewildering amount - even if you can not see it by eye, or by low power micrscope - it IS there.

If it is an organic - a brief oxygen plasma will quickly tell you by 'ashing off' the organic.

Have a good weekend!

Steve Creswick - Gentex Corp



-----Original Message-----
From: Vladimir Igoshev [mailto:[log in to unmask]]
Sent: Friday, December 12, 2003 12:18 PM
To: [log in to unmask]
Subject: Re: [TN] Cleaning of Wire Bondable Gold


Hi Steve,

What kind of "surface nickel/diffusion"  data are you looking for? I'm a little bit "confused" with diffusion "accelerated" (or promoted) by possible voiding in the gold layer. I think the temperature isn't high enough to have something like surface self-diffusion so, it doesn't look like the presence of pores should affect possible diffusion of Ni through the Au layer.  I agree with Joe, that some organic layer may be present on the surface and could cause the problem (let say "develops" over time/temperature exposure). 

Vladimir Igoshev
Research in Motion
(519)888-7465, x5283

-----Original Message-----
From: Steve Kelly [mailto:[log in to unmask]]
Sent: December 12, 2003 11:24 AM
To: [log in to unmask]
Subject: Re: [TN] Cleaning of Wire Bondable Gold


Hi Joe,

I have some answers:
1) soldering is normal no-clean - not lead free . We normally re-flow at
230-235 C (sees this temperature for approx. 40-45 seconds)
2) I am in the process of looking for surface nickel/diffusion. Will know
later today
3) the components are at one end of the flex - wire bond pads are about 4.5
inches away. 
4) I am receiving a lot of opinions about how much gold should be there but
in general people are leaning to more - how much more is a good question
5) we also build another part for this same customer that only sees one
press cycle after gold and no re-flow - construction is a little different
(but gold/nickel/copper is on top of an AP in both cases). To the best of my
knowledge they have no issues with this part for over 4 years.

Regards Steve Kelly

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joe Fjelstad
Sent: Friday, December 12, 2003 11:07 AM
To: [log in to unmask]
Subject: Re: [TN] Cleaning of Wire Bondable Gold

Hi Steve,

Interesting discussion you have brought to the fore and you have gotten some
excellent advice on cleaning. It was not stated but I am assuming gold wire
bonding based on the thickness and as well assuming that the gold is not
selectively plated on the wire bonding pads.

My experience from past lives was that 1 micron was normally enough,
however,
there were not two extra press cycles (with attendant volatilization and
deposition of organic materials and a reflow operation that preceded wire
bonding).

I have worked with folks wherein millions of devices were wire bonded post
reflow assembly using aluminum wedge bonding. The gold for such was about
1/5
micron

Brian has suggested and it has been seconded that porosity might be in the
mix.

I am left with a few questions whose answers that might lead me to ask
better
questions.

Is the soldering leadfree? (higher temperatures? volatilized organics? )

Have you looked at the positity/diffusion issue? (i.e. Is Ni really there?)

Or is the offending layer organic by analysis?

Last (for the moment anyway... ;-) what thickness of gold would be
considered
pore free? 1.5 microns?,  2.0?  2.1?  (I know that there are plating related
issues that mitigate any answer) and would that work for all steps in the
process? (embrittlement has already been cited as a concern but where is the
wall
for this process combination?)

Gold is expensive but not so expensive as the board. I have had customers in
the past that specified 1.5 microns and never had a problem (make that never
reported back a problem)

We all like a good problem but the real objective is to find a way to make
them never show their faces in production. It's pretty boring but very cost
effective :-)

Good luck,
Joe


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