Today, I've undertaken an interesting project where I repaired a Redmi 7 that refused to turn on. An experienced technician could fix this in half an hour and make a decent profit. As I always say, time is gold. So, I quickly disassembled the device, applying heat to the back cover, removing screws, and taking off the upper plastic protector. This gave me access to the motherboard.

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On diagnosing, I found some sulfate on the side and a loose shield, a sign the device had been tampered with. Undeterred, I continued my diagnosis and noticed a dark gray spot above the CPU, likely due to overheating. To confirm this, I connected the motherboard to a lab power supply and pressed the power button. The thermal camera showed the CPU was indeed heating up.

I applied heat and flux to lift the CPU and found out that a section had suffered overheating damage. This results in shiny solder balls becoming dull, leading to cold solder joints which can cause the device to stop turning on. My suspicion is that this could be a form of planned obsolescence.

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Next, I proceeded to reball the CPU, apply flux and solder paste. I used a soldering iron tip at 350ºC, without pressing, to pass over the CPU and then cleaned the excess solder using a desoldering wick. After cleaning the CPU and repositioning the components, I was ready to apply the stencil and the solder paste for reballing.

On cleaning the motherboard area where the CPU was to be placed, I found a broken pad, common in mid-low range devices. I fixed it by scraping the track, applying flux and solder paste, and tinning it. I used a thin 0.01 copper wire to create a bridge, which I then cured with UV mask.

Finally, I applied flux to the area where the CPU was to be placed, and taking note of the notch that indicates the exact position of pin number one, I soldered the CPU using a 380ºC air station and a 30% airflow. After connecting the motherboard to a power supply and checking the consumption, I was thrilled to see that it turned on perfectly.

Fixing this Redmi 7 has been a challenging yet rewarding experience. It's about understanding the problem, the right application of tools, and a bit of patience. If you found this helpful or interesting, don't forget to give this a like. I'm Leonardo, see you next time.


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