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October 1999

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From:
Gareth Jones <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 27 Oct 1999 16:34:07 +0000
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Bret,

Thanks very much for your reply and concerns of gold migration and
intermetaliic's. You are of course quite right
to say "that large amount of gold intermetallics that will form, since you are
soldering to a gold ball?
I'd highly caution against shipping any product with this metallurgy mix until
you have performed some extensive reliability testing."

This is why we are using a 60/40 indium/lead type 5 paste from Indium are
potentially struggling to get the paste through the apertures due to there size
(100 microns) and a type 6 paste would be nice, but I believe this is currently
unavailable.

The pressure information provided of 10 grams sounds good and we are currently
in the processes of using this parameter for our placement force.

The project itself is designed to supply one of out customers with a specialist
low volume flip-chip assembled board and provide ourselves with the opportunity
to establish our current limitations and the ability to develop new techniques.
Hence this is why we are currently using Gold balls and solder paste, but we are
also developing ACF's and Isotropic to dispensed using syringes.

I would gladly tell you how things go on with this assembly as long as it is
successful (ha ha). I would be interested in hearing about the methods you use
for flip-chip assembly.

Many thanks
Gareth Jones
Celestica

[log in to unmask]

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



A table in John Lau's book on Flip Chip Technology has a value
of 10-30 cN/pad for stud bumps in printed eutectic solder. I think
this translates into about 10 g per bump, but you should probably
check that calculation.

     A couple notes of concern: Are you using normal eutectic
solder for this?  If so, aren't you worried about the large amount of
gold intermetallics that will form, since you are soldering to a gold
ball?  I'd highly caution against shipping any product with this
metallurgy mix until you have performed some extensive reliability
testing.  Also, a note in the same table form Lau warns that flux
residues will cause the aluminum bond pad to corrode.

     From reading your note again, maybe you meant that you
stencil printed conductive epoxy instead of solder?  If so, then
Lau's book states a force of 3-50 cN/pad - I'd guess that probably 5
grams per bond would be sufficient.

     I'd be interested in hearing how your test turns out - I'm thinking
of trying a similar method of flip chip here with gold ball bumps.
Good luck.


Brett Goldstein
EVI, Inc.


Date sent:          Tue, 26 Oct 1999 17:15:16 +0000
From:               Gareth Jones <[log in to unmask]>
Subject:            flip chip placement force question

> Hi Netters,
>
> I've manufactured some naked die into flipchips by gold ball bumping them.
> They are then assembled onto a board which has been stencil printed. I
> need to know the pressure required to assemble the flipchip onto the
> substrate with the paste. I'm using a pico-placer to place the flip-chip
> onto the substrate with a programmable force applicator. The chip has 25
> bumps which are in a periferal formation. The minium pitch is 178 microns,
> with a minium gap of 68 microns. The pad size is 111microns and the gold
> ball is 90 microns across and 35 microns in height. The assembly is then
> to go through a reflow oven which is to be nitrogen purged.
>
> Thanks very much
>
> Gareth Jones
> Celestica
>

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