Silver Epoxy for Ground Paddles

[krapht]

Hello,

I'm currently designing a 915 MHz oscillator. I want to use a small DFN RF amplifier in my circuit. I've never hand-soldered a part with a thermal paddle - and normally I'd just pick another part, but since this project is on my own time, I'd like to explore outside my comfort zone.  I do have a hot air gun.

The recommended layout has many vias to ground underneath the ground paddle. If I laid it out according to manufacturer recs, I don't think I'll ever solder the part correctly with my setup. One, the solder would wick down the vias, and two, my ground plane is an enormous heatsink.

Instead, I was thinking of using silver epoxy on the ground paddle and my hot air gun to reflow the leads. Has anyone done this before? Would it work?

[richard.cs]

My suspicion is that 1) thermal and electrical contact will be pretty poor and 2) the epoxy may be damaged when you solder the rest.

One way to hand-solder these is to heat the ground plane on the back with a hot plate and then solder from the front with an iron - the copper pad should extend past the end of the chip so you can get the solder to wick under onto the pad. Some people put a large plated through hole on these to hand-solder the thermal pad from behind. You may be able to get away with a hot air gun if you do it before you put other components down but I suspect you'll still need a hot plate to pre-heat the ground plane.

If you haven't made the PCB yet I would strongly suggest you deviate from what the manufacturer recomends as that will be for machine assembly and you can make your life a lot easier. So long as you have a low (electical and thermal) impedance path to your groundplane it should be fine. I would suggest extending the copper on the top layer above and below the chip and one large plated hole behind the chip instead of the many vias.

[SArepairman]

I read that the solder coverage area of thermal pads is not as linear as I would think, I think the difference in heat dissipation ability from 50% coverage to 100% coverage was like 20%.

There might not be any need to fuss about it.