FET troubleshooting

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One method of FET troubleshooting and repair, by Methods, from post http://www.endless-sphere.com/forums/viewtopic.php?p=590253#p590253

When mosfets blow I do this:

  • Grab DMM and measure form ground to Yellow, ground to Blue, Ground to Green. They should all be 10K. If one is 0 ohms we know that bank must be replaced
  • Measure from VCC to Yellow, VCC to Blue, VCC to Green. Same thing - they should all be 10K and if any are short that bank gets replaced.

Lets say the first test failed for GND to Yellow We flip it over and identify the big GND trace. We identify the Yellow pad area

Now we identify the group we want. The number to be replaced will be: 12 fet 2pcs 18 fet 3pcs 24 fet 4pcs 36 fet 6pcs

Some times it is a bit confusing so you can grab a sharpie draw a box around the group of fets that must be removed.

Ok - you probably knew all that and the real problem is the actual physical removal right? Lots of ways to do this part... I will choose tools that you have.

Remove the fasteners from the 2,3,4, or 6 fets you need to drop out. Dont even THINK about trying to swap 1 or 2 of them. SWAP THEM ALL. Not worth f'ing around here. So remove the screws Lean the fets forward a tad to break them free from the heat sink.

Now you can go about it a few ways. First - if you have a heat gun that is probably the easiest. It is a little awkward.. but try to set up the board so that you have access to the bottom Grab a pair of long needle nose or some forceps or tweezers... or tie a little copper wire to the fets... so just something to pull them out when the solder reflows. Fire up that hot air gun and heat up the solder. WATCH OUT -> That shit can burn a hole right through the PCB board so go slow and easy. The whole thing will need to heat up. So you basically just re-flow the solder over the fets you are trying to pull and when you see it go liquid give them a little tug from underneath.

The Ghetto way is to bap the board on the table but... that can cause you trouble so proceed at your own risk.

Ok - that is the hot air method. Works better with a hot air rework station.

Second method involves connecting all 3 of the legs together with a blob of solder. Try to rake back any "beef up" solder to keep it from heat sinking Get a blob across all three pins with your soldering iron Heat those bastards up and very lightly tug, tug, tug from below.

The biggest fail is tugging too hard or too early and ripping up pads on the top of the board. You really have to overheat the shit out of things.

Another method that you might like is the nip-tuck method Get a pair of long skinny wire cutters (buy them - super long and skinny with a little nipper at the end) for tight spots Reach in and nip the three fet legs Then you have to heat them up from the back with a soldering iron and pull them out Some times they have to go through the bottom and some times they have to go through the top... all depends on how you cut them and how deformed the copper is. Dont force either direction.

I hate that method... last hope of a desperate man but I will do it when I have to.

Once the fets are out I repair any damage (about 1 in 10 times I f up the PCB board)

I then try the bake and shake to clean the pads... I get the pads hot and reflow the solder and then shake the board really quick to try and fling the solder off. This saves me on solder wick. Alternately you can make "Ghetto Wick" by using some crappy old stranded wire to suck up the large portion of solder on the pads. Once you have 90% of the solder off the pads you can clean them up with solder wick AND FLUX. Flux is wonder lube - you cant use too much. Buy it in pen form and paint it on everything.

A hot iron and flux for the win. Money well spent. 80W iron with adjustable heat Real flux in a pen Real lead solder

Assembling is easier... Slide them in Screw them in Solder Trim Beef up.