[JAM, MDA Compact, S100 Software Upgrading] Keeper:
Yes Boris, i also see your point... doing BGA rework ain't easy...i didn't know that the removal of the cpu on that wallaby (thread posted above), was with the ideia of knowing that it wouldn't work again...

Anyway, i know that is possible to use that technique for desoldering the cpu and not killing anything but it's really hard to come by...

I've found this posted on other website :

Our company is experiencing major difficulties with the older 2 mm server boards. No matter how much heat we apply with a heat gun to the surface substrate of the PCB, we are not able to achieve sufficient semi wetting in the plated through hole to allow extraction of the solder with the desoldering unit. The thick server boards dissipate heat so rapidly that we cannot desolder the PTH's in order to remove and replace the defective components.

Put down the heat gun and step away from the circuit board.

Hand held heat guns are useful in a very limited range of applications. Pre-heat of a large, thick circuit board is NOT one of those applications.

A large, thick server board is definitely going to dissipate a great deal of heat. As you have figured out, this is why you are having difficulty removing a Plated Through-hole component from your board. Pre-heat is going to be critical to the process. However, a heat gun concentrates the heat in a small area. The heat is then dissipated through out the board. Unfortunately, in order to get the entire board up to the required temperature (around 140o C – 160o C), the localized area when the heat gun is applied will be damaged.

So what is the solution?

The solution is a board pre-heater. Table top models are available from a variety of manufacturers. A pre-heater is designed to heat the entire assembly at the same time. A time/temperature profile should be made to ensure the proper heat ramp and soak time. To develop a T/T profile, attach thermal probes to the top and bottom of the board at various locations. Monitor the temperatures and adjust the heater settings as necessary to achieve a 2 – 4o /second ramp up. When all the probes indicate a steady 140 – 160o C, the board is ready for component removal.

If a board pre-heater is simply out of the question, an alternate method is to run the assembly through a reflow oven. Adjust the oven zones to achieve the required board temperature. Using heat resistant gloves remove the board from the oven and quickly rework the component. The major drawback to this method is the limited time for rework. As soon as the board is removed from the oven the board begins to cool.

By using either a board pre-heater or a reflow oven, large, thick boards can be reworked and repaired as easily as any other assembly.

Coly, idk what to say to you...only this if you are still willing to try:

Desoldering the CPU

- Use the oven for a pre-baking of the board at 100ºC ( which is necessary in any case of BGA reworking )

- Since you can't heat up the entire motherboard at 225ºC - it would damage every component - you will need a heat gun for the next part.

- Use a heat gun at 200-225ºC around CPU for 3-4 SECONDS AFTER the pre-baking process, so to force the solder to reflow for a few secs and lifting the CPU gently, step by step, one side at a time. This entire process must not take more then 1-2 mins after which the pre-baking efect will disappear and you'll have to repeat the pre-baking process again.

Soldering back the CPU

- To solder back you will need to heat up the solder pads on the board with the heat gun so that they become small round balls.

- Then you will need to do the pre-baking of the board

- Afterwards you will need to remove the board from oven and carefully put the CPU aligned to the board pads.

- Finally you will need to use the heat gun once again at 200-225ºC for 3-4 secs all around the CPU (within the 1-2 mins time frame), so to force the cpu to get soldered, by gently doing a small pressure above the cpu. (Try not to pressure at the center since it's where the flash ROM resides - use two fingers (or a clamp) in diagonal at the corners and pressure it down)

This all process ain't easy so i'll understand if you're not willing to risk it.

I'll wait for your opinion on all this matter.

Keeper