Released on September 19, 2014, this 4.7" screen iPhone is the smaller version of the iPhone 6 Plus.

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cpu chip heats up

hello friends , this is the 2nd time i am trying to fix the back light for iPhone 6 but while removing the shield where CPU chip is this phone gets shorted i set my heat gun to 350c

and i am being very careful but now i dont know why the CPU chips heats up and phone doesn't turn on can anybody explain please???

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Hello Jamil. Can you let me know what type of heat gun you are using? Is it something like this or this?

There is an important distinction here. The melting point for lead-free (SAC) solder used in iPhone logic boards is ~217C, depending on the actual alloy. However I suspect the solder used for the shields is a different alloy with a slightly lower melting point so that when they wave solder the shields they don't melt the solder for the components.

At any rate, that means that when you apply heat with forced air, your goal is to get the shield solder joints up to the melting point. You can't just put your gun or station at ~215C; too much heat is lost via air convection and the rest of the logic board also acts as a heat sink. Now if you set a heat gun to 350C and force too much air for too long, you will eventually get the board too hot and the solder balls underneath the IC will melt and blow out, causing shorts and disconnections.

A hot air station set to the same temperature will throw a lot less air and it will be more localized. You still have to be careful about transferring too much heat but the process will be slower and give you more time to accomplish your tasks.

Either way, the secret to successful use of hot air on an iPhone logic board is getting the right amount of heat transferred. Too much and you can kill your board...which is most likely what happened here.

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thanks for the reply , i am using a regular smd station with heat gun and iron which looks similar to 2nd pic you showed me . i am using a 6mm nozzle . so what do you suggest now ? and also what temp i should be using from now on to remove chips and putting back ?

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Hi Jamil, if you are using a hot air station with a 6mm nozzle, then you have the right tool for the job. Your temperature setting is about right but every station is different and low-cost stations can have wide variances. Now you just have to practice getting "just enough" heat to the board. You can also maybe lower your airflow but the only way to know the proper settings is to practice on dead boards.

As for your current board, you probably overheated the CPU. Look for tiny solder balls around the CPU; that is a sign that you applied too much heat. You could also probe the PP_CPU/PP_GPU voltage rail. Let me know what resistance and voltage you measure.

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