[Amps] LDMOS HEAT SPREADERS

[Manfred Mornhinweg]

Alex,

> I don't know about that, but you have liquid cooled heatsinks for
> high power processors capable of taking care of a few hundreds of
> watts. 

In the PC overclocking scene, a lot of snake oil is being sold. I would 
first do a reality check on any CPU cooler rated for several hundred 
watts, before using it for a real high power application. The fact is 
that PC CPUs normally run at a very much lower power level. The one in 
my PC, a dual core CPU running at 2.8GHz, dissipates an average of 
barely 10W, and when both cores are loaded to full capacity it increases 
to around 20W. There are certainly some CPUs that dissipate far higher 
power, but I doubt that anyone comes even remotely close to the 600 to 
1000W that an LDMOSFET in high power ham service would dissipate.

> What you can do is
> glue the device on the heatsink using silverr loaded epoxy from
> Elecolit or some equivalent.

That glue would need to have excellent electrical conductivity, in 
addition to thermal conductivity. After all the LDMOSFET sources connect 
only through the heatsink. I much prefer soldering.

 > We did dolfder devices to copper
> heatsinks .but that requires special heating plates going up to about
> 240 degrees centigrade, and some eutectic solder, not cheap. Don't
> try it with new devices.

With some care and ingenuity, a thick copper or aluminium plate placed 
on the kitchen range or a camping stove will do fine. Or a heat gun or 
soldering torch can be used to heat the plate. The melting solder will 
tell the temperature. Just heat the plate slowly enough, to avoid 
thermal overshoot to a dangerous level, after the solder melts.

Is anybody here elegant enough to have his clothes ironed? If so, that 
iron can make a suitable hot plate for soldering! Many get just hot 
enough for it, when set to the hottest level they have.

But my favorite method for soldering transistors to heat spreaders or 
sinks, and to do SMD soldering, is the electric oven in my kitchen. It 
easily develops enough heat. I just place the items to be soldered in 
the oven, with some paste solder, then switch it on and watch. As soon 
as the solder melts, I switch it off. I watch the temperature on my 
digital multimeter, which has a temperature range and comes with a 
thermocouple that's fine for such temperatures, and easily enters the 
oven through the door seal. After switching off, the temperature keeps 
increasing for a while, due to the heat accumulated in the heating 
elements. With my oven, when I switch off as soon as the solder melts, 
the air temperature overshoots to about 220°C, which is fine to achieve 
a good solder joint, and is totally safe for LDMOSFETs.

I let it cool slowly, with the oven door closed, until the solder 
solidifies, then slightly open the oven door to speed the cooling.

Unless the LDMOSFET is held in place by some other system, it's a good 
idea to use two heat-resistant objects to clamp it down by gravity. 
Otherwise it tends to float on the solder and move out of place. The 
objects I use are simply bent from thick wire, and have three legs: Two 
go on the heat spreader, the third on the transistor. That's enough to 
keep the transistor in place.

SMD components on a printed circuit board don't need this, because as 
long as the PCB is correctly designed, they will self-center on the pads 
while the solder is liquid.

And here is an example of eutectic, no-clean solder paste that works 
great. This amount is enough for a good number of LDMOSFETS and many 
complete SMD boards, and costs under $14:

http://www.digikey.com/product-detail/en/chip-quik-inc/SMD291AX/SMD291AX-ND/1160002

Don't buy larger amounts unless you run a factory! This 5cc syringe 
would be enough to last a hobby electronician many years! But shelf life 
limits it to only 2 years or so. In fact the data sheet says less, but 2 
years stored in the fridge is realistic. After that it gets pretty 
stiff, but can still be used, applying it with a tip rather than through 
the syringe needle.

Manfred

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