Using a solder-mask covered ground plane as a heatsink?

A contract engineering company made us a 15W lamp.

It uses a tapped buckboost converter where the drain voltage goes up to 120V.

The drain of the Metal-drain-tabbed TO220 FET has a heatsink attached to it, and the FET is layed flat, and its heatsink is screwed to the PCB such that the heatsink lays over the solder-resist-covered ground plane.

...is this OK?

I mean, will the thin layer of solder resist be sufficient to prevent the 120V drain voltage from "flashing" over to the ground plane?

Also, the FET gets hot, and so too does its heatsink, so do you think the solder-resist will melt as a result of having the fet heatsink screwed to it?

An intact soldermask layer will have sufficient dielectric strength and not melt under regular conditions according to it's 155 °C temperature class.

Nevertheless it's not designed to be used as insulation film. I won't do it.

Solder mask withstands heat pretty well, but it is easily scratched. Although the FET is screwed in place, it is still liable to move around (especially with vibrations during transport, or during its lifetime), and thus I think the FET's drain tab is likely to scratch through the solder mask in some units. I think it's a clever idea for reducing radiated EMI (they avoid having a large heat-sinking conductor which is attached to a switching node), but for reliability purposes I would avoid it.

There are commercially available insulating sheets, and I think it's a better idea to use these as an interface between the FET and the PCB. These insulating sheets, which are called Mica insulators, are specifically engineered for use as a heat sink interface material and are thermally and mechanically robust, and have good thermal conductivity.

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Solder mask withstands heat pretty well, but it is easily scratched. Although the FET is screwed in place, it is still liable to move around (especially with vibrations during transport, or during its lifetime), and thus I think the FET's drain tab is likely to scratch through the solder mask in some units. I think it's a clever idea for reducing radiated EMI (they avoid having a large heat-sinking conductor which is attached to a switching node), but for reliability purposes I would avoid it.

There are commercially available insulating sheets, and I think it's a better idea to use these as an interface between the FET and the PCB. These insulating sheets, which are called Mica insulators, are specifically engineered for use as a heat sink interface material and are thermally and mechanically robust, and have good thermal conductivity.

Hello grizedale,

FvM and ZekeR have wrote, that the solder resist widerstands the heat, but it's easily to scratch. Better is to use a isolated pad from the copper layer.

A contract engineering company made us a 15W lamp.

Click to expand...

I think, after I read all your posts about this lamp, that it is not an engineering company, that is a "Learn by doing" company.

Better you put this company into a rocket an shot them to space, so that they never come back again. :grin:

Regards

Rainer

If the FET is insulated from the heatsink, then there is no problem if the heatsink touches the ground plane.

FET heatsink is at drain potential

Sorry, I misread your post; I thought the FET didn't have a heat sink attached to it, and was using the PCB as its heat sink.

You should use a mica isolator between the FET and its heat sink, as suggested by godfreyl.