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DIY IC replacement - help!

  1. Oct 22, 2007 #1

    DaveC426913

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    I'm going to try to replace the IC in my stereo like this.

    Here it is in my stereo.

    I've got a small but cheap soldering iron. But I suspect I'll need a solder-puller and some sort of heat sink to prevent the chip from frying?

    I've never used a solder-wick and don't own one. What should I get? Also, how do I heat sink this?

    Thx.
     
  2. jcsd
  3. Oct 23, 2007 #2

    mgb_phys

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    I wouldn't worry too much about heatsinking - it looks like a fairly hefty part.
    I prefer solder-pumps for large de-soldering jobs, other people like solder wick.
    Any desolder pump will do.
     
  4. Oct 23, 2007 #3

    DaveC426913

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    Even though I've got to desolder, like, a dozen adjacent connections?
     
  5. Oct 23, 2007 #4

    berkeman

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    Is the power chip bolted to the heatsink? It's hard to tell from the pic how the chip is held tightly against the heatsink and grease.

    Is the heatsink soldered on the backside of the PCB, or is it just mechanically held down (like with bent tabs on the underside of the pCB)?

    That is single-sided, non-plated-through PCB material, and you have to be VERY careful doing rework on that kind of PCB. It is way too easy to de-laminate the metal traces and pads on the bottom side, if you apply too much heat, or apply some pulling force while the trace is hot.

    For rework on that kind of PCB, I don't think I'd use a solder sucker, because of the added potential for delaminating the pad that you are heating. I'd use solder wick most likely (you can get wick of different widths at your local Radio Shack, Frys, or via Jameco/Mouser/Digikey).

    A couple other tips.... Often it is easier to desolder a joint (whether with wick or sucker) by first applying a little new solder with your iron. That is sometimes the only way to get a clean pad and hole again -- by re-soldering it a little, and then wicking or sucking the solder away. Another great tip if you are using a solder sucker -- use dykes to make two small cuts in a "V" shape on one side of the cylindrical tip of the sucker. The V should be about 2mm on a side, just big enough to fit the tip of your soldering iron into. Put the V cut on the same side of the sucker where you will be holding the iron when you desolder. You use this small access hole to let you put the sucker down over the pad while you are still holding the soldering iron on the pad (at like a 45 degree angle), and you seal the sucker tip on the pad and iron tip and whamo, suck the solder off of the pad while it is still being heated by the iron. This eliminates the possibility of the solder re-solidifying in the time it takes you to remove the iron and hit the pad with the tip of the sucker.
     
  6. Oct 23, 2007 #5

    berkeman

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    BTW, you want to grease and bolt the new chip to the heatsink in place first, and then solder the new connections. If you solder first and then bolt the chip or transistor to the heatsink, this creates mechanical stress in the leads that can lead to early failure.
     
  7. Oct 23, 2007 #6

    DaveC426913

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    Yes, two bolts, which I've removed. The arrow points into the gap.

    Both. The diagram shows where the heatsink is soldered. Out-of-picture on either side are two large mechanical attach points as well. Hopefully, I won't have to remove it at all.

    I'll be extra careful.

    Good thing I bought both.

    The wick is about 2mm in width.


    But it's plastic... won't it melt?

    Thanks very much. This gives me the confidence to proceed.


    More ?'s:
    - I guess I need to pick up some grease too. Don't know what I'm looking for though.
    - Do I apply the wick ON the joint and the solder tip OVER that, like a sandwich?
    - I've got rosin core solder, so I don't need flux, right?
     
  8. Oct 23, 2007 #7

    mgb_phys

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    The tip of the desolder pump is PTFE or somesuch - it's farly heat proof.

    Didn't notice it wasn't through plated. It might be easier to cut the legs of the existing part if you can, then just desolder and remove the remaining tips. It's hard to fully desolder all the legs so that the entire part pulls out.

    The wick you can also melt the solder and apply the wick at the same time - as if you were soldering. I have more luck this way than trying to heat the wick on top of the solder - but I'm not an expert - prefer pumps.

    Correct, with cored solder you don't need flux.
     
  9. Oct 24, 2007 #8

    NoTime

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    Radio Shack has a small vacuum iron catalog #: 64-2060 @ about $10.
    Doesn't look like much, but it works quite well for PCB rework.
    I find it better than the two handed juggling required for a solder sucker or braid.
    Not quite as good as a real rework station, but the price is right.

    Braid(wick) can be tricky to use on some of the cheaper PCB material that is prone to separation, without some practice.

    If you don't have a lot of experience, then cut all the pins off the IC with a small wire cutter as mgb_phys suggested. Pull the individual pins by pulling on them gently with needle nose pliers or hemostat from the top side of the board while heating the solder connection on the bottom.
    The pads will be much easier to clean by any of the suggested methods with the pin removed.
     
  10. Oct 24, 2007 #9

    DaveC426913

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    You know what? That's a great idea. I wasn't looking forward to being so proficient as to be able to get them so clean that they'll all pull out cold.
     
  11. Oct 24, 2007 #10

    NoTime

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    For single side non plated holes this usually isn't a big problem.
    Even with lots of experience some mechanical manipulation of the pins is normally required. The trick being to know when you have it clean enough to break the residual capillary without damaging the pad.
    Multilayer boards are a particular challenge and unless you are feeling lucky, cutting the pins off first is the best option.

    Good luck with the repair.

    PS: If you have some junk boards around a little practice will be good for your confidence.
     
  12. Oct 24, 2007 #11

    DaveC426913

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    I did it! It worked OK (I think).

    Now I just have to buy the replacement chip.

    (My biggest worry is the edge contacts where they join to another board at a right angle. They're fixed, so I have to haul the whole thing around as one very flimsy piece. And the joints creak every time I move it.)
     
  13. Nov 5, 2007 #12

    DaveC426913

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    OK, now I've goen BACK to the store to get antoher tube of IC compound to replace the one I carefully hid last week...

    I'm ready!

    What is the idea behind applying this IC compound? Is it meant to form a layer between the IC and the heat sink, or is it meant to form a seal around the edge? I guess if it's a heat sink then the former - but it's also meant to reduce noise, right?
     
  14. Nov 5, 2007 #13

    berkeman

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    White heat sink grease is used to make a better thermal contact between the power IC/transistor package and the metal heat sink. Congrats on the new skills that you're acquiring!
     
  15. Nov 6, 2007 #14

    DaveC426913

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    I AM A GOD AMONG MEN.
     
  16. Nov 6, 2007 #15

    DaveC426913

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    I'm just puttin' that out there.
     
  17. Nov 6, 2007 #16

    NoTime

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    LOL! Sounds like you were successful
     
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