Hand Soldering a DPAK with a Large Thermal Pad


#1

Just had a board spun and I underestimated how much heat the 3.3V DPAK regulator device going to create when under the loads I gave it (at least I didn’t use the SOT-23 package… or the part would have prolly let the smoke out right quickly!).

I “think” I might be able to get away with a filled zone for the tab and via-stitching to the other copper side to keep the device happy.

How large can I make the thermal pad/zone/s and still be able to solder it with a Hakko FX888D before I have to break out the Paint Stripper hot air gun? What if I also choose to remove the solder mask from these zones?

Would I be better off with pad and hole to mount an aluminum heatsink to the copper of the PCB? Only issue there is that the tab is not ground, so the heatsink would be at the 3.3V regulated output voltage. Or, maybe the suggestion would be to use an insulated thermal pad?

General comments welcome, it’s not that big of a deal at the moment But my inner geek is trying to turn me into a real engineer and if you want to try to make my brain hurt I’m okay with that too.


#2

I suspect the soldering task will be a bit cantankerous before the thermal pad gets too much larger than the DPAK’s tab. Whenever I put down a DPAK I automatically grab a second soldering iron and attack it from both edges of the tab, if I’m doing it by hand. My preferred tactic is to put solder paste under the tab and let the skillet do the work for me. I know some guys have special-purpose hot plates that pre-heat the whole PCB assembly - thermal tab, heatsink pad, etc - before you touch it with a soldering iron. I could probably use my skillet that way, but have never tried it.

Your idea about the heatsink is certainly reasonable. The obvious tradeoffs (in addition to thermal performance) are cost, mechanical clearance, and complexity of the assembly process. I’d be wary of any heatsink that wasn’t at chassis potential, but that may be my long-standing prejudice (left over from the days when you kept a close eye on any exposed plate caps when working on live equipment). Insulating pads have come a long way from the days of mica and silicone grease, though that option is still available.

I’m not the one who will transform you into an engineer. I spent decades fantasizing that I could be an engineer myself, until it was too late to become an accountant or some other vocation above the “working poor” category.

Dale


#3

Oh, I doubt that.

An engineer with a piece of paper does not make one an engineer.

It is all about what anyone actually does with the knowledge they have.


#4

As @dchisholm has said, having the pad even slightly larger than the tab will be difficult enough to solder even without vias connecting the pad to another layer.

I would add a hole as you mentioned, and make the pads as large as I had space for and leave bare copper just slightly larger than the tab. Then fix the tab to the PCB with some thermal compound. An extra assembly step but not as bad as adding a heatsink and easier than trying to solder to a large pad. And easier to rework if you ever have to replace the part. There’s no reason to insulate the tab from the pad but you could if you wanted to.

Edit: Instead of a hole for the screw you could also use one of those heatsink clips if it is near the edge of the board.


#5

I’ve been a spoiled brat. Metcal, and with the right tip, I could probably smelt gold bars if needed. I don’t have access to those toys at the moment.

Thanks for giving me things to think about.


#6

How much heat do you need to sink with your heatsink?
How big do you need your thermal pad to be?

Hakko 888 is a 50 to 80W solder station I think?
The size of the tip used ans position during soldering is a very significant factor in heat transfer.
One of the (many) ways to pre-heat a PCB is with a regular frying pan filled with dry clean sand and heated on a stove.
Pre-heating to 150 Celcius or so makes soldering a lot easier.

This can even be used for soldering double sided boards.
First solder one side in the frying pan, then let it cool enough, turn the PCB around and press firmly in the sand to make good contact and keep all components pressed into the sand in their place.