Best methods for removing a microcontroller.

[jerryk]

I'm looking at a control board failure in a high end NOCO jump pack (GB500+).  It died early on with almost no use.  I am about to conclude that there is an internal short in the microcontroller itself but here is the part I don't get.  The resistance between VSS and VDD is 4.6 ohms which says it's shorted to ground somewhere.  I cut the VDD trace under decoupling cap and the short is on the microcontroller side.  So the microcontroller is the prime suspect.  However when I reconnect it, apply power to the linear regulator, and check for heat signatures the microcontroller does not heat up to any significant degree.  I would think if it's the load in this case it would get fairly warm.  Of course the linear regulator is warm and its output voltage stunted to 2V.  There is another area that gets warm with no clear path to ground that I can find.

So I'm at the point of last result and want to remove the microcontroller to confirm that the problem is in the microcontroller itself and not elsewhere on the control board.  My concern is inflicting harm on it and screwing up the programming. 

Any suggestions or cautions on how to safely remove it without doing damage? 

Jerry

[jpanhalt]

It would probably help if you told us whether it was in a DIP socket or a soldered leadless package and anything in between.

[jerryk]

Sorry about that. It's a Holtek micro in an LQFP48. Also I have a hot air station and decent Haako soldering station.

Jerry

[jpanhalt]

Hot air is the answer.  There are specially made irons for those devices, but hot air, while slower works just as well.  If you have a way to preheat the board, say to 150°C, it will help.  Crinkled aluminum foil (kitchen type) is what I use to protect other components from the blast. A little Kapton tape may help and is quite temperature resistant.  How do you plan to grab the chip to remove it?

I am assuming you do not have the program to use with a new chip, so if the chip is bad, the device is junk or pay $$$ to get it repaired, which may not be possible after an attempted DIY repair.  If the chip is good, then you are wasting your time and risk ruining the ship.  Those threads that pass for pins are quite fragile as are the pads.  Bottom line, I wouldn't do it to test a hypothesis, and if the chip is bad, you are wasting your time anyway.

[AndyBeez]

In theory, you should be able to use hot air to rescue the uC from the pcb. It would take no more than 20 to 30 seconds to liberate this small part.

Cover everything else nearby with kapton tape and foil. Pay attention to any plastic parts, including capacitor sleeves. Place plenty of liquid or gel flux on the uC pins.

Using a fine nozel, set the temperature to around 300C. This should be enough for RoHS solder, but drop to 280 if you know it's leaded solder. You can always add a little extra heat. Use enough air flow to get heat onto the pins, but not too much air flow to blow the heat away.

Do not force the uC off the board, otherwise the solder pads will always rip off. Instead, wait for the uC to start 'floating' on liquid solder. Solder turns 'bright and glossy' as it melts; and that's your cue to remove with tweezers. Beware, the air flow can now blow the loose uC into adjacent components.

Replacement is best done by micro soldering. A real challenge on a 10mm square footprint.

Check out YouTube.

Have fun