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Spontaneous VCC-Main Short After NFC Repair

Hi! I have an iPhone 6s which I did an NFC repair on. Previously, it was working 100% and looked okay although the water stickers were red. Afterwards, the phone had a shorted pp1v8sdram cap which I removed. The cap was opposite the site so I assumed it got too hot and died. There was some solder ball squeezing on the opposite side but not much. The phone worked for about two hours, except for the flash. I went to take a flash photograph and the screen instantly went black. Full VCC main short. I have removed the strobe driver and its input caps and the short persists. At 5 volts, 2.4 amps, the supply is at 0 volts and there is no heat ( because there is no wattage if it’s 0v). Everything is at 0 volts when I connect a battery except for speaker boost which is at 0.5. Any ideas? I think it’s speaker amp but why? What would cause these components to fail?

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Ur solder ball burnt out transistors. If it died on a flash like that after work, it’s done forever.

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Wait which transistors? I removed the ICs and there aren’t any output MOSFETS.

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Here are a few observations, questions and suggestions:

  • What exactly did you repair? Did you replace or reball the NFC IC?
  • Previously water damaged board…always a red flag and adds one, if not more, variable to the mix.
  • Caps don’t go bad just from heat, it was probably flaky to begin with and may be related to the previous water damage.
  • Solder balls squeezing out is never a good sign either, you probably applied too much heat. Sometimes this is of no consequence but in this case, it probably is related to your issue.
  • Taking a flash picture seems to have caused a cascading failure. High current draw through the strobe driver, possible near shorts due to solder ball squeezing, excessive heat and probably bridging of solder balls.
  • I would remove the Backlight driver IC to see if the VCC_MAIN short goes away.
  • You could try connecting a battery instead of a DCPS to get a heat signature but this could cause catastrophic damage as there will be no current limiting. Another trick is to connect the DCPS directly to the VCC_MAIN rail and bring up the voltage slowly. You may get a tiny heat signature on the bad component at 2V before the other IC’s kick into gear.

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I replaced the NFC chip with a known good one, so technically I reballed and replaced it. I will try removing the backlight driver. I appreciate your help!

par

Do what Minho said in his last paragraph. Solder a wire to a VCC main cap. This way it bypasses the charging FET. Use some freeze spray or thermal camera to track down the heat source.

par

Update: I removed the backlight driver and the short persists. Tomorrow I’ll solder onto a VCC Main cap and test it. However, the phone didn’t get hot during my testing even when I connected a battery ( Q1403 was barely even warm ). I really appreciate the help you guys

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Just to let everyone know, the issue was solved after removing FOUR shorted capacitors. Definitely will watch my heat more closely next time!

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Platypus sera éternellement reconnaissant.
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