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IC replacement

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  • IC replacement

    When using a new bga chip to replace one in a crowded board I presume the new chip will have lead free balls. Does Iphone Rehab reball the new chips with eutectic leaded solder to ensure the chip and surrounding components do not have to be subject to hight temeratures especially as the board acts as a heat sink.

  • #2
    In general, no. But for some special cases—yes. Namely, iPhone 5 tristar chip, and iPhone 7 audio ic. I can’t think of any other examples where we reball a new chip other than those two.

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    • #3
      Thanks for the advice Jessa. Do you normally heat the boards before reflow to reduce the time the hot air has to be on the replacement chip? Watching many streams I have noticed how many people have to 'nudge' the chips into position and then rely on surface tension to locate the chip in final position. This is because the pads are sherical as are the balls on the chips and they dont sit on top of one another properly. I rub (very gently) the ball side of the chip on 3,000 grade polishing 3m paper which put small flats on the solder balls and ensures they are all at the same height. When the chip is placed on the board pads they stay in position whilst heating up. Regards

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      • #4
        The movement of the chip breaks the surface tension between the ball and the pads and is the “tell” that we use to determine that the solder is melted. Add about 5 seconds of time above liquidus. We don’t preheat the board for most jobs. However if I’m working on top of the CPU then for some chips I will.

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        • #5
          Unfortunately on taking a shield off I ripped a small IC off a bord that was stuck to the shield with underfill. I lost one of the pads off the board and watched video streams of repair using the jumper wire spiral method. Looked a bit fiddly and the resulting pad looked higher than the rest so tried a different method. Using a polished steel ball (ball bearing about 10 to 12 mm diameter) held in vice or similar lay the wire over the top and tap lightly with a small polished tool. Scrape the insulation off the flat so formed, tin with low melting point solder and use a scalpal to cut the 'flat' in half. Result - two wires with pads on each end the same thickness as those on the board. Some stick the wire to the board with UV glue which seem a good method before moving the new pad into its final position under the chip location.

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          • #6
            That sounds like you’re spending a lot of time worrying about details. What board are you talking about? What chip?

            Are you sure that the missing pad was necessary? What is it’s function?

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            • #7
              I was looking for a short on a Samsung S7 edge G935f. The shield had underfill covering the Bucks booster IC and pulled it off as the shield lifted. Yes the pad was necessary. Only made the post as an alternative way to easily form a new pad. Although I have ZXW the information generally on the web is for Iphones which increases the difficulty for 'newbies'. Thanks for your interest. Kind Regards

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              • #8
                Ok--I understand. Making a "pad" isn't necessary at all, but making electrical connection is. In general when we have to work in an area with a missing pad, we solder a wire to the corresponding circuit and let the wire "hang" loose in the space of the former pad. Then apply new chip and the molten solder ball will embrace the loose wire like the stem inside a lollipop and restore connection to the chip.

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                • #9
                  Referring to a samsung S7edge Logic board G935F the chip reference U7011 (Max 77854) is noted as the main charging chip however it does much more than this supplying voltages to other chips on the board in particular U7000 main power IC and bucks voltages. All power to the rest of the board from both battery and USB input runs through this chip. If you take the chip off there is no voltage anywhere on the board. My question is has this chip any voltage/ current protection as logic would suggest that if there was a bad short anywhere on the board it would destroy parts of U7011 before one had any chance of discovering where the main fault is located. Is the solution to put 4 volts on the main power rail and therefore bypass the Max 77854? and then look for hot spots on caps etc. If there is a short in any of the other pmics which tend to get warm anyway does one have to remove each individually and look for the short to be eliminated?

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                  • #10
                    Yes! That’s the strategy. Figure out where the vccmain is by finding a big coil where one side is short. Then inject and see what gets hot.

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                    • #11
                      Sorry Jessa but I need to understand where to find Vcc main to be absoluteley sure where and at what voltage to inject, I would not wish to trash boards injecting either in the wrong place or with too much voltage. I have looked at ZXW Schematics and there are a large number of coils ( inductors marked L????) on the board for Samsung Galaxy G935F which show as 'BATT' which will be 4volts or 'VDD' which will be about 1.5 volts. Are there no indications on the schematics where vcc can be identified for definite? Looked on youtube extensively but all references I can find are for IPhone's and the repairers already know from experience where vcc main is and which caps to inject on. Samsung are not very popular phones in the states and there is little repair information on them Appologies if im being 'thick'. Regards

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                      • #12
                        You will have to guess. That’s just the way it is.
                        The people successful fixing Samsung boards begin with a deep knowledge of iPhone problems and then apply that experience to Samsung’s.

                        From iPhones, we know that the charging coil and backlight coils are always on vccmain. If you know in your Samsung that vccmain is short, then find a large coil that also has the same short ohms to ground. Inject there.

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