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levistubby 7th October 2012 11:00 AM

PCB and chip mounting
Hi Guys, I'm about to do the final assembly of my chip amp using Brian GT PCBs for LM3886 chips. Is it considerd acceptable to hang the PCBs just off the chip rather than also using the mounting holes in the PCB? After all the chipleads take all the weight of the components and wire when point to point wiring techniques are used.

Also are there demonstrable benefits in clamping the chip to the heatsink with heavy aluminum or copper bar stock as well as a bolt through the mounting hole, assuming that the heatsink itself is of adequate size?



sofaspud 7th October 2012 11:29 AM

I'm unsure what you mean in your last sentence of the first paragraph, but as far as "hang[ing] the PCBs just off the chip" I would have to advise against it. Surely there is some way to support the board, even if it must be improvised.
For the heatsink, if the bar stock improves the thermal conductivity then by all means. If it doesn't, then why do it?

levistubby 7th October 2012 12:09 PM

I meant that if a PCB is not used then the components just hang on the chip pins and pins take the weght. So hanging the components plus PCB is not a lot heavier.

In relation to clamping the chips, my questionwas really "does clamping give better therma transfer from the chips to the heatsink?"

sofaspud 7th October 2012 01:55 PM

You could hang everything off the pins, sure. I just can't imagine it as a design requirement. Those dead bug -style circuits you see are done because there is no PCB. Shock and vibration are much more of a concern as you can probably guess. Is there some obstruction in your build that necessitates hanging the boards?
Clamping provides more mass and more contact with the chip. It makes that good-enough heatsink better. I'm unaware of any other benefits, but those are enough if that's what you're looking for.

Arty 7th October 2012 05:30 PM

supposedly -!supposedly!- chips that have more than 2 holes to screw them to the heatsink and are suspect of getting hot (huge instant current pulses, aka big sub amp chips) can be damaged by the flexing that occurs from the different thermal dilettation coeff.
-witch if one would ask me is b/s-
so, they clamp the heatsink onto the chip in a fashion that allows the tab on the chip and the heatsink to react in size by the heat without stressing eatch other.
and a nother benefit is -witch may not be b/s if you have the guts to sandpaper the plastic on the chipp flat- to have a larger contact area, to be able to get rid of heat faster.

actualy if one stays whitin the safe area of operation and uses a properly sized heatsink (read -> 2x whatever is told to be sufficient) it makes no sense to clap/sand whatever..

sofaspud 7th October 2012 05:42 PM

Chips that have more than 2 holes? T03?
Forget the sandpaper; buy thermal compound.

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