Tony

A 0.2 cents worth: the TCE of silicones are ginormous (e.g. 300-350
ppm/°C) compared with solder (c. 16 ppm/°C) and I suggest you may be
right about worrying about it. Furthermore, silicones have a unique
property that, however squausshy (thanks, Graham, for the term) they
feel under the thumb, they are rock-hard when subjected to a shock
(hence super-bouncer balls and bouncing putty). The combination could
result in a squished silicone at slightly elevated temperatures which
would place a tension on the BGA-ball-pad combination, which could
increase by orders of magnitude should it be subjected to a small shock
at the same time. Any gap in the underfill, especially if accompanied by
flux residues or other hydrophilic contaminant, could even fill with
water under humid conditions of use. I feel vacuum impregantion would be
a must.

A few pages are devoted to silicones in Bartholomew's classic book on
underfilling in the chapter on properties of encapsulants, emphasising
the moisture permeability. He suggests that the TCE problem may be
reduced by silica filling, but that high filling ratios may result in
micro-cracking during high temperature cycling. Otherwise, I don't think
he devotes much space to silicones as underfills, compared with other
resin systems.

Brian

Tony Whitfort wrote:
>
> We are dip coating a product with a BGA part on it.
>
> I am concerned that the coating under the BGA may cause reliability
> problems due to thermal expansion.
>
> We are using a silicon based coating and the BGA part is a standard
> 1.27mm pitch.
>
>  Can anyone point me to some literature on this.
>
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