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Advice for the "Harder" under fill?

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  • Advice for the "Harder" under fill?


    Just a quick post to see if anyone has any advice to deal with the "harder" under fill that Apple uses.

    Just to be clear I'm not talking about the softer one that is found near all the screen connectors on the top of the board.

    I've been hesitant on doing jobs with IC or components surrounded with the harder under fill. I have been able to deal with it when I had too, but I do find it challenging. So far I usually remove the top coating of the chip if there is one, and use an iron to melt the solder balls of the chip by putting solder on the iron and placing it on top of the chip I want to remove, but this only works if the chip is small enough. I will also cut the underfill around the chip with a blade.

    I will sometimes use a small tip iron to remove the excess under fill, but I have mixed results with this technique. I will also use the same technique than the softer under fill, which is simply using heat with a small tweezers, or blade and scrap it off, but with a higher temperature than the softer one.

    I find using higher heat works good, but I find I must heat the area a lot for it to come off easily.

    I did not tinker around with the harder underfill to much, so I might just need more practice, and get the feel for it.

    Any advice? or those are pretty much my options and I just need to get more experience with it?

  • #2
    I am not sure if this is right, but I will tell you what I do.

    On the normal underfill that I encounter near the connectors I use around 160C. For the harder underfill that I would encounter around the CPU, I up the temps to around 200 to 225C, and that seems to help.

    I should note that could be completely full of shit, and may be wrong on this one, but I think upping the temps is the answer, as long as you don't approach the melting temps of the un-leaded solder on the chips that you are working around.


    • #3
      I call the substance under the chips underfill--this is what you are calling the harder stuff. I call the other stuff near the connectors "silicone waterproofing" and I do not think the purpose is the same and I don't think that is underfill at all.

      So for underfill, I use low 'hairdryer heat' about 220C and use the thinnest thing I can find, which is the tip of my one pair of green tweezers. I have tried using a lancet or needle--anything super super fine. I then carve a border, gently, all the way down to the board all the way around the chip. I move away some caps or something if necessary to get some space so that i can insert an ic remover tool (which is a thin flexible spatula much thinner than an exacto blade). I warm up the chip to normal soldering temperature and then slide the ic remover tool under the chip, and twist until the chip pops off.

      Then I mow down the solder balls with an iron so that they are not "hot air balloons" likely to tear at the stalk. Then I might braid a little bit depending on the scenario to remove some of the underfill, then clean with alcohol and then just pick it gently with the same 200-220C and thin tweezer tip. It will flake away. If there is underfilled chips nearby I will consider using the big tip method to take the chip off--maybe iPhone 5 tristar, but for big chips I do it as described.

      It really isn't that big of a deal with practice---I just did an iPad 3 PMIC on Friday and it worked out fine.


      • eDigitCom
        eDigitCom commented
        Editing a comment
        Would it be possible at all for you to do a training video on this method Jessa I personally am able to do a procedure much better if seen it in a video then just by the written word----Thanks

    • #4
      Thank you both for your feedback.