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Thread: How to unsolder this??

  1. #1
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    Default How to unsolder this??

    Got a power amp device where it's "foot" pokes through a hole in the PCB and solders to a plate of metal, which in turn is itself soldered to the PCB ground plane & screws to a chassis that doubles as a heatsink.
    I've got a replacement power amp FET to solder in there, but need some ideas on how to desolder the old one. The picture below shows my dilemma. The red circle is the thing I need to heat up to get the power FET off (I've already got it's pins off the PCB on the other side... that's no problem). Want to avoid heating the other items in yellow... pretty much everything surrounding the plate I need to heat.
    I've tried a SMD hot air reflow to heat the plate, and can't get enough heat quickly enough into it to allow the power FET to come free. I can't get anywhere near enough heat in from the top side of the PCB either, where I'd have to send the heat through the FET body first to get to it's foot.

    I'd previously done something similar in another device by resting the plate on an upside down domestic clothes iron to pre-heat the plate, then heated it up just that little bit more with the SMD hot air reflow station to get it off. But that was a carrier PCB with nothing else around it. Can't do that here because of the surrounding components.

    Ideas??




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    Hey mitaux8030

    your idea of using a heat plate is a good idea - the iron isnt probably hot enough though

    I have a cheap iPhone screen separator hot plate similar to this -

    heat up the board sufficiently and use the hot air to melt the area

    goodluck
    有段者

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    Would it be possible to hoist an image of the other side?

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    Top side:

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    aha been there done that i used a big solid copper wire thru the holes then made a inductive loop around it with a lump of steel in the middle of the loop on the item you want to unsolder
    then run a shitload of current thru the loop and it heats the steel which releases the solder the steel remains hot for quite a while so its easy to manoeuvre of the board ok don
    . Please treat people as you would like to be treated yourself.

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    I think I'd be reaching for the mini butane torch with appropriate heatshielding of surrounding components on bottomside ; I probably expect the smd component adjacent to fet to fall off during process on topside (to be replaced later) ; inductive loop would work too, same methodology in the end ~ you want to get the steel plate up to solder melting temp quickly as possible.

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    Dremel the crap out of it? You’ll probably find trying to heat, wiggle, heat, ply, heat, swear etc will also stress the board and possible delaminate it...

    What is the FET part number?
    Last edited by Gitch; 11-02-20 at 08:29 PM.

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    It's a NXP AFT05MP075N LDMOS 70 watt RF power amp device.

    After pondering the issue, currently considering using a big plumbers soldering iron with a large broad tip, and slightly inefficiently couple the heat into the plate from the underside, to get it just below solder melting point. Meanwhile, on the top side of the board, I'll use the SMD rework station to heat the top side of the FET to give it that little extra kick to melt the solder bond. I'll have to Dremel the top case of the FET away to expose the foot first, of course. And while that is going on, I may as well have the new FET ready to drop in so the PCB only gets heat stressed once.

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    Package reference ~

    I did consider the 300w iron, but thought perhaps it'd be too slow (to heat things up) ; there's no going to be much heat stress to the PCB there ..lots of thick cladding & vias (which is why I think the iron would be slow =)

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    Perhaps it's time to be brave and burn a bit of butate! (how's that for some assonance?)

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    How’d you go?

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    Been busy with other stuff, hope to get time on Friday to give it a go.

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