Reworking DPAK FETs from double sided FR4 PCBs with much thermal copper?

Hi,
Please help with removing and replacing DPAK FETs from an FR4 PCB with much thermal copper on/in it? Please help more with soldering the DPAKs back on the board, as this was harder than removing them.

So, Today I went for a practical interview for a soldering job. It was required to remove 5 DPAK FETs from a PCB , and then replace them with different DPAK FETs. (The PCB was a 100W, 36V battery charger SMPS. This PCB was in production, but the FETs were said to be counterfeit, and thus in need of replacement) I failed the assessment, and so didnt get the job.

I’ve now searched the web for “the way” to do this, but all the ways recommended wouldn’t have worked on this job.

The DPAK FETS were generally close together near the PCB perimeter. The PCB was double sided populated , so not generally conducive to placing on a hot plate…but the PCB area directly under the DPAK’s had no components , so therefore that “Bit” of the PCB could have been somehow placed on a “bit” of the edge of a hot plate. (however, no hot plate was available)

So, the PCB was……
8cm diameter round PCB,
It was 2mm thick and of FR4 (I did not have access to gerbers, schem or PCB layout files)
Each DPAK FET had many 0805 resistors and capacitors in the close vicinity…ie, only about 1mm away.
There were 12 thermal vias in each DPAK pad.



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Anyway…now for getting the replacement DPAKs on to the PCB……

The company had the Tenma 21-10125 hot air gun...

This is Farnell 2064551.
https://uk.farnell.com/tenma/21-10125-uk/hot-air-station-550w-220v-uk-plug/dp/2064551#

Do you think this is good enough for this job? (hot-airing the DPAKs back on the board)

Also, please can you recommend a head/nozzle for it that would be suitable? (or a range of possibles?)


Also, how do you know that the entire DPAK tab area is correctly soldered to the entire DPAK drain pad area?...you can’t tell?...most of the the joint is under the component.


How should I have done this work? What tools should I have used?
(I have searched google for “DPAK removal tools”, etc, but drew a blank)

Thanks
This video makes soldering a DPAK on look so easy. They just use a soldering iron alone…

However, at 00:26 they swab the DPAK with what looks like a cotton wool bud, which cant be good from an ESD viewpoint?

At 00:39 they use “AIM no-clan gel flux” on the pad, but no solder tinning of the pad whatsoever…(why no solder fillet placed on the drain pad?)

Can any "no clean gel flux" be used here?

At 01:30, they solder the DPAK drain from the end of the tab only, but the solder appears to wet all round the side of the DPAK.
Do you know what power of soldering iron woudl be preferred here?
It makes you wonder how the solder managed to flow under the DPAK?

I wonder if pre-tinning the pads with solder paste would be best?..then the DPAK could be pressed into the paste?

The trick is to use a BIG soldering iron so it doesn't cool down on contact with the board. I usually snip the pins first, apply heat to the tab then pull it away with needle nosed pliers while gripping the stubs of the pins. Use 'de-soldering braid' to mop up excess solder on the pads so they are level then lay the new device in place, solder its pins first so it can't escape then use the big iron again with a little solder to fix it back down.

Capillary action will make the solder flow along the back of the tab but make certain the pad is flat before mounting the device or the new solder won't 'wick' up the gap cleanly. Optionally, you can use solder paste on the flattened pad then use either that big iron or a hot air gun.

Brian.

Thanks, for soldering the DPAK on.....do you reckon an 80W Metcal iron with a 2mm chisel tip will be ok?.....i think we will aim hot air at it at the same time as it is thick with copper planes. And preheat it with the same hot air.

Funny video. The DPAK has effectively no thermal connection, thus easy to solder. The small tool wont't work with usual heat sinking copper pours and planes. Hardly for assembly and not at all for rework.

If hot plate isn't an option (not available, too large PCB, double side population), I'd prefer hot air. Need to preheat the device vicinity, beware of overheating the device package top.

Hi,
The manager of this company has now emailed me to say he has bought a "new Pace re-work unit, expensive but it should make life easier as it sucks up the solder at the same time as melting it."

..Can you think what this means?

Ive never heard of a rework station which sucks up solder...surely this wouldnt be wanted?

No, doesn't make sense. I guess the manager doesn't understand the stuff clearly. The rework stations have bottom side IR heating + top side hot air.