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April 1998

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Subject:
From:
David Anderson <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 22 Apr 1998 14:38:27 -0500
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There are several ways you can go.

1.  You can use aluminum wedge bonding with immersion gold on the whole
board.  The one thing to be careful is to limit the amount of heat that the
board sees.  You can usually get several reflow profiles (>3) before the
nickel can diffuse through the gold resulting in wire bonding difficulties.
 I did see a paper in a very recent IMAPS journal concerning gold ball
bonding to immersion gold, so gold bonding may be possible on the immersion
gold surface, but aluminum wedge bonding is more common I think.

2.  You can use (thick) wire bondable gold (~30 micro inches) on the whole
board.  Solder embrittlement is the worry here.  If you can have enough
solder in the joint and control your reflow profiles, this may work.

3.  I have seen palladium metallizations used.  Sometimes immersion gold is
plated over the palladium to give a nice solderable and wire bondable
surface.

4.  You can get selective gold plating (immersion on solderable pads, soft
gold on the rest).  This would involve another masking operation, and is
usually more expensive, but sometimes the cost is justified.

I'm sure there are a number of other ways to tackle this design.  These are
just the ones that I am familiar with.

I would recommend staying away from HASL or fused solder.  Any
contamination from these metals on the wire bond pads make wire bonding
difficult.  Besides, you need a precious metal metallization anyway for the
wire bonding, so why complicate the board by adding additional steps?

I've been reading the heated debate with regard to the Surface Finishes
Conference.  I think that this may be an application where HASL or silver
may not be the best option.  If one is building boards like this one, the
discussion at the Surface Finishes Conference pertaining to immersion gold
or palladium might be very useful.

Dave Anderson
Medtronic.com

As always, opinions are my own, and may not reflect the opinions of my
employer.

>>> "Chan, Marcelo" <[log in to unmask]> 04/22/98 09:55am >>>
Can anyone describe to me how they would do such a PWB in terms of
processing
order and PWB finish selection?

The application is RF and perhaps higher frequencies, which means that we
may
use materials ranging from Arlon 25N, rogers 4003 all the way to the
traditional
Rogers 6002..few layers...ratio of solderable pads to bondable is about 7
to 1
with about 100 pads on the whole board...1 by 1.5" in size...

Are immersion Au, electrolytic Au, fused solder or  Hasl viable options?


thanks,

Marcelo

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