If this isn't heatsink paste, then what is it?

[paul8f]

Hoping somebody might know what this substance is needed for (please see photos).

Just repaired an old DELL laptop power supply adapter (high ESR 'Ltec' electrolytics...)

To do the work I had to use a scalpel to cut away some of this flexible white material. At first I thought it was simply thermal paste, but there was so much of it used, together with the fact that it was not still wet (or gone dried & crusty over time) like regular silicon heatsink compound.

The main bulk of it was used near the small chopper transformer. Could it be possible that this white material was to "bulk out" some of the internal space to make the power adapter more resilient to bumps and vibration (it is used with a portable device after all...!)? Thanks.

[paul8f]

Silastic

   -   I'd never heard of this stuff. So it's like a PCB-safe silicone sealant that hold components in place during the manufacturing process? I'm guessing its thermal transfer properties are nowhere near as good as heatsink paste?

[Berni]

Yep its essentially silicone safe for electronics use

It doesn't really provide much of a thermal advantage and its usually put on after the board has been manufactured and cleaned. The main purpose of the stuff is to glue together tall bulky components to make them resistant to vibration and impact.

Its very commonly used in power supplies, most often on large electrolytic capacitors because those tend to be the bulkiest components on a board that sit only on two pins. This gives them a tenancy to wiggle cross axis to those pins and you can actually remove a capacitor from a board by simply grabbing it and wiggling it back and forth a few dozen timed until the pins break. The glue keeps the capacitors from wiggling like that.

[paul8f]

Thanks for all the help. I went to the Farnell website and did a quick comparison between a Silastic product and a Servisol product:

SILASTIC® 9161 RTV - Silicone Elastomer. Thermal conductivity = 0.25 W/mk.

200001000 -  Heat Sink Compound, Highly Conductive, Silicone tube. Thermal conductivity = 0.77W/mK

[paul8f]

I didn't remove much of this Silastic stuff during the repair. Most of it is still in place.

It's a 90 watt power supply, so gets fairly warm during normal operation. Would the integrity of the unit, or the lifetime of components be reduced by not re-applying the Silastic that got cut out?

[David_AVD]

As others have said, the silicone is there as a purely mechanical function.