Questions about Silastic

[airlomba]

Hello all,

In the part of my life of unmounting electronic gear, I have seen a lot of sticky stuff on PCBs. Stuff like some kind of wax holding inductor coils in old radios, sponge bits with some goo, silicon-like stuff, many kinds of adhesive tapes, etc. But it was in Dave's videos that I first heard about Silastic. By the way Dave, I love your channel! Thank you. Keep up the good work! Best regards.

So, silastic, emh? Well I searched the forum and found some references to it, I googled about it and found its origins and just that it is a good silicon compound used in many applications. But I feel the need for more. So here's "couple" of questions to those that use Silastic or really know about it. Thank you.

1) What is the primary need of Silastic on a PCB? One can see some of it in almost all power supplies, next to the big electrolythic capacitors.
2) When does Silastic get applied to the PCB? Before soldering the components or after?
3) Anyone knows a "clean" way of removing Silastic from a PCB in order to replace the capacitors, or other component embedded in it? Normally I try to pull it of using some plier.

I think that having the answers to the above questions, I can reply myself to the following one, but here goes anyway.
4) If I remove the Silastic in order to replace some faulty component, do I have to put back some Silastic?

A last one:
5) Does Silastic have some kind of electronic friendly spec, like EMI reduction/shielding, temperature dissipation, etc?

Thank you all for any help.
Best regards,
Emmanuel.

[wraper]

It's just a silicone adhesive, may be two component or one component with air moisture cure. It mechanically fixes heavy or flimsy components to the board to prevent them from moving because of vibration, and breaking off the legs or cracking solder joints. I either break it off when possible or cut with a scalpel. If you want to replace it, never use acetoxy type silicone, it emits acetic acid while curing and metal underneath will corrode. Use neutral or electronics grade silicone.
Of course it won't shield any EMI.

[Brumby]

There are several products you can use.  Silastic is one.  Hot melt glue is another.

The first reason for using something is for mechanical support.  Large capacitors may need something more than their leads to hold them in place.  Add a bit of silastic between the capacitor body and the PCB - or another component (sometimes another capacitor) and you will get a more rigid assembly.  This is especially important for equipment that is portable and/or may be subjected to vibration.  With a heavy component being supported by comparatively thin leads, the leads could weaken and break.

Another reason is for reducing acoustic issues with inductors.  Windings around a ferrite core may physically oscillate.  Burying them in silastic or hot melt glue will dampen these.  Same deal for components that might be microphonic.

A third use is for cheap mounting of things or dressing of wires.


Is there anything special about this stuff? .... No .... but there is one very important thing you need to get right - only get neutral cure stuff.  A lot of silicones are acid cure - and they will eat up things you don't want eaten.

[wraper]

There are several products you can use.  Silastic is one.  Hot melt glue is another.

Hotmelt is useless in PSUs because of the heat, and nearly useless everywhere else. I yet to see a hot melt glue which did not at least partially came loose after 5 years.

Another reason is for reducing acoustic issues with inductors.  Windings around a ferrite core may physically oscillate.  Burying them in silastic or hot melt glue will dampen these.  Same deal for components that might be microphonic.

If used for this, completely wrong thing was chosen for the task. There are special resins for this, which will work really well unlike silastic.

[SL4P]

Just keep in mind that some types of 'silastic' / silicon adhesive do two things that are of no help at all..
As they dry out, they crumble, and worse - the nastier variants seep a liquid (acetic acid i believe).

Not such a big deal when breaking gear down, but can make a mess when your initially building the device!
Use simple epoxy based adhesives,
.

[wraper]

As they dry out, they crumble, and worse - the nastier variants seep a liquid (acetic acid i believe).

Silicone adhesives don't dry out or crumble. And they do not sip any liquid. Acetic acid is emitted only by acetoxy type silicone while curing only, and it's not in a liquid form, just evaporates causing acetic smell. However that type of silicone is never used with electronics. Also silicones easily endure >200^oC temperatures. Many are stable even at 300^oC.
Actually it is the best common adhesive which may be used in electronics as causes least of the problems when compared with others.

[Inflex]

DOW CORNING  744 WHITE seems to be the most commonly sold silastic/RTV for electronics repairs. 

If you don't use a lot, then you can transfer to 10~20mL syringes, so long as you keep the air/moisture out it should last a long time ( and easier to apply in smaller syringe form ).