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LED driver shouldnt be mounted on same heatsink as LEDs?

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treez

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Suppose we have a 150W LED lamp, and the FR4 LED driver PCB is all surface mount.
Now suppose the LEDs and the driver PCB are both placed on the same heatsink.
This would be advantageous for the DPAK Power FETs, because they will have a heatsink beneath them that they can be thermal via’d to.
-But what about the small SMD resistors and capacitors?
Surely, with them being mounted on the PCB on the hot heatsink (that gets used for LEDs too), they will heat up more than they otherwise would and stand a greater chance of cracking due to thermal cycling?
 

If your is a commercial unit, you should run for a MTBF calculation, to know the expected life of your unit.
Consider anyway that every 10°K of temperature increase, the expected life of components is halved.

Usually the problems will arise from electrolitic capacitors and from semiconductors, resistors and ceramic capacitors usually are not an issue.
 

In absence of running some obscenely detailed statistics on MTBF based on every single component, I would just choose the thermal design that gives the coolest hot spots for the entire board. This would mean using a heatsink under the entire board. If you're worried about thermal cycling, then you should check that the CTE of your SMD components are matched fairly well to the PCB substrate.
 

Mah, considering that is a small unit, the MTBF will be calculated fairly quickly, there are some programs that will give to you any detail about the components installed.

Maximum will be 100 components, quite simple and not complicated.
 
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If you're worried about thermal cycling, then you should check that the CTE of your SMD components are matched fairly well to the PCB substrate.
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Thanks, i imagine that the 1206 resistors that are blowing up on our board have a CTE nothing like FR4?
 

Probably not, since most chip resistors use a ceramic substrate, and there's not much that can be done about it. This is one reason why using smaller footprint parts can be beneficial (when possible): less stress on the terminals due to bowing and thermal expansion.

But I wouldn't assume that your failures are due to thermal cycling without some very strong evidence of it.
 
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