Sometimes the phone not turning on is caused by CPU/UFS faulty, hence we must to refurbish them to fix the phone. However, it is difficult to take down the CPU/UFS, including adhesive removal. Therefore here we share the adhesive removal and teardown of the MTK chips.
The Basic Idea
For the glued motherboard CPU\DDR\UFS desoldering repair, the key point is to control the temperature and heating time. The heating should be sufficient so that the motherboard or IC will not drop the solder joints when the IC is pried up. The temperature should not be too high, and the heating time should not be too long. If not, some PCBs with poor boards are prone to blistering and delamination, which will cause them to be scrapped.
Keep that in mind and we will demonstrate the detailed operation afterward. We choose an OPPO A11 for the demonstration.
The Faulty Analysis
The OPPO A11 cannot boot up. Firstly, we plug-in a USB cable, but no response. Then we connect it to DC power supply and power on, the current goes to 30mA small current but reduce to 0mA soon. Under such circumstances, we conclude the key power supply circuits have been short-circuited. By judging from the phone’s repair history, it is probably caused by CPU exploding solder joints.
We need to refurbish the CPU/DDR&UFS.
Take Down CPU/DDR&UFS
To refurbish CPU/DDR&UFS, we need to take them down in the first place. The key points here are the temperature and wind power setting. Hand them one by one in the following steps:
- Take down the motherboard and fix it on the PCB holder
- Remove the waterproof adhesive around the SIM card
- Set temperature to 380℃, wind power 40, use the biggest muzzle, and then heat up the cover shield and take it down
- Then set the temperature to 200℃, wind power 30, remove the IC edge glue
- Then remove the back adhesive with tweezers and use a brush to clean it off, so that avoid picking up the other small parts when prying up the IC. Please do it gently, or the PCB circuits will be damaged.
- Set temperature to 380℃, wind power 40, and heat up the IC for 30 seconds to melt the tin
- Lastly, pry up the CPU at a suitable position, and then take down the DDR&UFS chip.
Bonding Pad Refurbish
It is high time to refurbish the bonding pad so that the CPU/DDR&UFS can be installed well. Do it in the following steps:
- Apply soldering flux on the bonding pad and trim the extra welding spots at 360℃
- Dip the brush with PCB cleaner, and then stack up the anti-static glass wiper to remove the remaining soldering flux
- Apply soldering flux on the bonding pad again, then trim it with solder wick to clean to the bonding pad
- Next, handle the IC adhesive, trim the welding spots with the soldering iron
- Set temperature to 320℃, wind power 30, and remove the black adhesive with a repair-blade when blowing. Be careful, the blade should not touch the IC welding spots
- Apply soldering flux on the IC welding spots, and trim it with solder wick
- Then handle the DDR&UFS chip, scrape a layer of a black adhesive gap from the edge without welding spots, and split the back adhesive carefully
- Lastly, trim the welding spots with solder wick, and clean the two ICs
It is time to BGA the CPU/DDR&UFS. The methods are the same:
- Apply 217℃ welding paste evenly on the BGA stencil
- Set temperature to 320℃, wind power 30, and heat up until the welding paste is melted
- Apply welding paste again, and heat up the tin balls on the IC surface.
Tip: Better BGA twice for each part, since the tin balls sometimes are not full or even.
Install Refurbished CPU/DDR&UFS
After CPU/DDR&UFS refurbish, we need to install them back on the phone.
- Apply soldering flux on CPU, and align it with motherboard according to the polarity
- Set temperature to 360℃, wind power 40, and heat up CPU. When tin points melted, the CPU returns automatically. It means that the CPU has been welded well.
- Apply soldering flux on the DDR&UFS, and align it according to the polarity. Set temperature to 360℃, wind power 30 and heat it up for 20 seconds. Touch it gently with tweezers, if spring-back, then the welding is OK.
After the motherboard cooling off, we install the phone and test again, the phone boots up normally. It has been repaired well.
For the detailed operation, please check the video below: