Importance of electrical silicone sealant replacement?

[Andyy]

Hi all,

I've just repaired a Samsung BN44-0514A power supply (No Vs or Va, "exploded" QS802, shorted QS804, open RS819 & RS833). Replacing these parts restored Vs and Va and the board is working, at least on the bench with a 100W light globe as the load on Vs.

Removing the MOSFETs was a pain though, each had their legs encased in a large amount of soft white silicone that was cut up and scraped away.

Question: How critical is it to re-encase the legs of the new MOSFETs in silicone?

I'm assuming the silicone is to prevent flashover between legs (the legs are already spread apart on the PCB, including an air-gap slot cut into the PCB between the legs). But given how close the "thick" parts of the legs are immediately underneath the TO-220 case, I find it hard to imagine squirting a bunch of silicone in there would help.

Also, I don't have any silicone

[dmill89]

Re: Importance of electrical silicone sealant replacement?

The silicone is usually just there to hold the parts to the board before they are soldered at the factory. It certainly wouldn't hurt to put it back, but likely isn't necessary.

[NCSailor]

Re: Importance of electrical silicone sealant replacement?

But if you do put it back, it is important that you select the type carefully. Most silicone sealers are acetic acid based (smell of vinegar) and these do have some electrical conductivity when partially cured and can cause shorts. Also the acetic acid formulations are very corrosive and may cause problems down the road.
Best to try to find an alcohol based version. Silicone itself is non conductive and safe to use as long as it is fully cured.

[keeney123]

Re: Importance of electrical silicone sealant replacement?

I would say contact Samsung Engineer department and ask them what was the purpose of encapsulating the legs of this Mosfet. They could of wanted to make sure it's heat was restricted to a certain area. Also the dielectric constant may also be important in the selection of the compound. They may not know, but they may be able to give you the name and part number of the compound. With this you can look up it's specs and find a duplicate replacement.

[diif]

Re: Importance of electrical silicone sealant replacement?

I've never replaced the silicone or other type glues after repair and never had any issue.
With the randomness it is applied I doubt it's for insulation or heat purposes and solely for securing components to the board pre soldering.

[keeney123]

Re: Importance of electrical silicone sealant replacement?

I've never replaced the silicone or other type glues after repair and never had any issue.
With the randomness it is applied I doubt it's for insulation or heat purposes and solely for securing components to the board pre soldering.

So I do not understand why they would use it for that reason? Are these not thru-holes where the lead can be bent slightly? Even if they wave soldered them a slight bend would keep them in place. Also, a slight bend would create physical contact with the thru-hole eyelet which would create a better solder joint. The leads could then easily be cut by a machine afterwards. It would seem an added expense for nothing.

[diif]

Re: Importance of electrical silicone sealant replacement?

So I do not understand why they would use it for that reason? Are these not thru-holes where the lead can be bent slightly? Even if they wave soldered them a slight bend would keep them in place. Also, a slight bend would create physical contact with the thru-hole eyelet which would create a better solder joint. The leads could then easily be cut by a machine afterwards. It would seem an added expense for nothing.

Squirting some goo is quick and easy and cheap. Not all legs can be bent easily, some components have thick rectangular legs not thin little ones like caps/resistors.
The goo isn't very accurate so I doubt very much it's used as insulation.

[NCSailor]

Re: Importance of electrical silicone sealant replacement?

The sealant actually serves a real purpose,although at somewhat higher cost, the same could be achieved other ways.
In the OP's power supply, just imagine if those Mosfets were not secured. They are mounted to two large heatsinks that would be floating. Every Time that power supply is moved or especially if dropped, the heatsinks will stress the Mosfet leads and solder joints.
Sealant is often used on the large electrolytics in power supplies for the same reason. Due to their height, they are usually mounted horizontal and have almost no lateral support. Even mounted vertically, their mass and height will stress the leads and/or case in vibration or shock situations.

Do I always replace the sealant? No, but I also accept that the unit will be prone to damage from dropping or vibration. If this is not acceptable in my application, I do use a sealant.

[keeney123]

Re: Importance of electrical silicone sealant replacement?

Usually heat-sinks are soldered to the board and the components are mounted to the heat-sinks with screws. In the ops case the legs of the fet were in silicon. I would think if one wanted to secure the fet one would secure the body not the legs. The legs are already secure with the solder.

[keeney123]

Re: Importance of electrical silicone sealant replacement?

Squirting some goo is quick and easy and cheap. Not all legs can be bent easily, some components have thick rectangular legs not thin little ones like caps/resistors.
The goo isn't very accurate so I doubt very much it's used as insulation.

So in this case we are talking about a fet. Also, are these being bent by hand or machine? If this is being done by hand one could put all the flat small components on than put a flat piece on anti-static foam which is mount to a hard flat surface on top of the components and flip the entire board over and solder all components. The larger components one can put in the board have a cover that fits on the components and then flip the board over and solder. If this is done my machine it becomes even easier. With a machine this would be done in a process. Actually one could create an mold using an injection molding machine for the top cover of the boards components. With machine solder and component fill the mold could be press down on the components as they are being soldered which would secure their position without glue.

[stj]

Re: Importance of electrical silicone sealant replacement?

it could be to keep dust from building up between the legs.
it could also be hiding ferite beads.

[ReeceyBurger123]

Re: Importance of electrical silicone sealant replacement?

it could be to keep dust from building up between the legs.
it could also be hiding ferite beads.

+1 dust in plasmas can cause some analog noise issues especially on the buffers guessing the same for the switchers but who knows

[keeney123]

Re: Importance of electrical silicone sealant replacement?

This is why I find it best to ask the person who decided to squirt the goo. Many techs I have know will not track down a answer. They usually figure it is OK because someone else made a decision. That is seldom a good choice as in the end the outcome effects everyone and it will not improve production.