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  1. #1
    Join Date
    Sep 2011
    Posts
    22

    help...smc replacement

    does anyone here know how to remove and resolder an allegro a3977 chip???

  2. #2
    Join Date
    Oct 2005
    Posts
    2392
    Yes.

    Use a strong Stanley knife (Exacto?) to cut the legs off one or two at a time by pressing the knife tip hard down on the legs where they join the body. That's why you need the strong blade and preferably a strong metal handle.

    Then use a fine tipped soldering iron to remove the legs and solder.

    Then use a clean, new, solder braid ("wick") to remove all solder and leave the PCB pads clean and flat.

    Then place the new chip on the PCB and hold two corners with blu-tack. Position with a magnifier as needed. Then solder a couple of pins on the other free corners.

    After checking positioning, solder all the pins, do it quick and don't worry about getting bridges or multiple pins joined.

    Then finally use a bit of wick (sparingly) to remove bridges. Check with the magnifier to make sure all pins enough solder and there are no bridges.

    Then curse very loudly because you put the IC in backwards.


    Seriously, it's not that hard IF you are pretty good at soldering and have a fine tipped iron and wick etc. But if you are not good at soldering then you will find it a nightmare!

  3. #3
    Join Date
    Jan 2006
    Posts
    2985
    It may also be possible to make an aluminum foil nozzle to fit a heat gun and heat up the whole part and remove it in one piece. You have to be careful not to overheat surrounding components or scorch the board this way but it can be much quicker and easier than cutting the pins. It is the only way if there is a thermal pad soldered on the bottom of the chip. I have used both methods successfully. Getting the old chip off is the hard part, soldering the sufrace mounts is easy once you figure it out. I get a nice little ball of solder on the iron and then drag the tip across the entire row of pins. They are so small that they are soldered almost immediately and if you are lucky the excess solder will stay with the ball on the iron and you will not have to wick off the excess. The wick will tend to leave you with a dry joint which is not as reliable as one with a nice little fllet of solder between the pad and pin.

    Matt

  4. #4
    Join Date
    Sep 2011
    Posts
    22
    wow thank you for the fast replies. one other thing tho ... the allegro uses a j-pin connector. i cant even see any solder???

  5. #5
    Join Date
    Jan 2006
    Posts
    2985
    Is it like this: Chip carrier - Wikipedia, the free encyclopedia

    You can still cut the pins as described. You will be soldering in the very corner between the pins and board.

    Matt

  6. #6
    Join Date
    Sep 2011
    Posts
    22
    no its a j connector the connection solders under the chip....its prolly done by robotics and all soldered st the same time??

  7. #7
    Join Date
    Jan 2006
    Posts
    2985
    You can always use the thermal removal method rather than the mechanical method. Can you provide an example of the pins? I am not sure what you are referring to.

    Matt

  8. #8
    Join Date
    Sep 2011
    Posts
    22
    well i got it off... just need to figure out how to solder on a new one. if you look up the A3977 chip you can see that the pins curl under it and on the circuit board the pads to solder to will be under the chip not on the edge

  9. #9
    Join Date
    Oct 2005
    Posts
    2392
    Sounds like a QFN package.

    To put a new one on you will need solder paste and some fussy SMD reflow soldering. Google for "DIY hot air SMD reflow" or "heat gun SMD soldering".

  10. #10
    Join Date
    Jan 2006
    Posts
    2985
    I only see two package variants, neither of which is QFN and both of which should be hand solderable.

    Allegro MicroSystems - A3977: Microstepping DMOS Driver with Translator

    Matt

  11. #11
    Join Date
    Oct 2005
    Posts
    2392
    Maybe it's the PLCC one then? They are not easy to hand solder.

  12. #12
    Join Date
    Sep 2011
    Posts
    22
    yes its the plcc...im clueless on how to go about this unless i run small jumpers to each connection but is going to be very time comsuming

  13. #13
    Join Date
    Oct 2005
    Posts
    2392
    Put some solder paste on the PCB pads, place the IC on top, and (carefully) use a heat gun.

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