Sayed,
In addition to Steve's excellent comments, I would add:
1.  In addition to considering the high CTE of silicones, you also need to
consider the Young's modulus of the material.  At the SMART Conference in
the UK this last fall, this came up in a conversation that sometimes it is
not the CTE that kills you , it is the elasticity of the material as well.
 Ian Fox I believe was the speaker.

2.  Will you be conformally coating over the material.  Coatings often
don't like to stick to silicones and often dewet during application.

Doug Pauls



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01/17/2012 11:47 AM
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Re: [TN] silicone-based underfill. Resending in plain text






Sayed,

We can read you!!!  What did you change?  Can you tell us?


Just a few quite generic suggestions...

First question I would ask yourself is whether you wish to be able to
rework
the device or not.

Next would be method of application.  Pre-applied [no-flow/flux
underfill],
or post applied [capillary flow].  The pre-applied is sexy.  Comes out of
reflow and its done!  Can have problems, but so can the capillary
underfill
approach.

Third question would be whether you had any cure time/temp limitations [if
you go post-applied].


You have a very low I/O count.  What is the approx ball size?

You can go to any of the large adhesive manufactures for assistance and
some
of the solder paste manufacturers as well.  Ones that quickly come to mind
are Henkel [which is a combination of Loctite, Ablestik, Emerson & Cuming,
et. al.], Cookson...

You don't provide enough info on the other conditions, substrate,
constraints, reliability requirements etc. to really be able to provide a
slick answer.  In general, however, I am always cautious of the high CTE
of
neat silicones [generally in the 200-300PPM  range].  Yes, it will be
heavily filled to modify the CTE, but do you gain anything on the cured
system?  Silicones also tend to be more expensive than semi-generic epoxy
based systems.  Just do your homework...

I suggest you stay with the available epoxy based systems as a simpler,
easier solution unless your device, substrate, or requirements are out of
the ordinary.

Steve Creswick
http://www.linkedin.com/in/stevencreswick




Depending upon the other conditions of your project
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ahmad, Syed
Sent: Tuesday, January 17, 2012 12:20 PM
To: [log in to unmask]
Subject: [TN] silicone-based underfill. Resending in plain text

While trying to gather info on possible options for an underfill for an
approximately 2 x 2 mm chip with about a dozen solder balls, we came
across
a silicon-based underfill online. Any experience or info about its use in
an
RF system? A good source for it if it is a viable option?
Thanks.
Syed Sajid Ahmad
Center for Nanoscale Science and Engineering (CNSE)
NORTH DAKOTA STATE UNIVERSITY
www.ndsu.edu/cnse