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Heat Sink bonded to PCB???

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Advanced Member level 2
Dec 24, 2005
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I have a new design that uses lots of power MOSFETs as linear regulators. The MOSFETs are a powerpak8 package that are optimized for dumping their heat into the PCB. The problem is that I am forced to have 32 of these circuits and each can handle up to 3.5A. This dumps lots of heat into the PCB which I do not have a good way to extract.

My thought was to arrange the MOSFETs into groups of four around small square copper pad. Onto the copper pad, I want to mount a small square heatsink meant for a small BGA package. My hope is that numerous localized heatsinks will pull the excess heat from the board.

Boss man is afraid of the thermal tape on the heat sinks. He is concerned that they stick to BGAs, but will not stick to copper foil. Does anyone have any info of thermal tapes and what they will stick up? So far, the heat sink folks and the thermal tape folks have not been too helpful.


---- Steve

I don't see problems with adhesion of thermal tape on bare copper (most likely tinned or otherwise galvanized) pads. You should however estimate the expectable thermal resistance. Horizontal thermal conduction isn't that large. In most cases vertical thermal conduction (with help of a sufficient amount of thermal vias) can be better.

For a top mount heatsink, there are also solderable versions, that can avoid the additional thermal resistance of thermal tape and also simplify the assembly.

I had a similar issue used an array of thermal vias, 16X 0,30mm hole on 1,0mm pitch to carry the heat through the PCB then used a thermal epoxy to bond an extruded H/S to the PCB. Like the previous poster says expect very poor spreading on the surface layer. If you are limited to same side Heatsink & PowerPaks then maybe consider bonding to the PPaks directly.

There are many thermal managment options, several application parameters decide about the best choose.
For lowest thermal resistance in conjunction with topmount heatsinks, i would particularly check package types as IRF DirectFET.

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