FAQ :CPU bga reballing

I've got the gist of the procedure. Do correct me if I am missing anything?

1. Cover everything but the cpu socket with aluminum foil
2. Use pen type flux to flux the area around the cpu socket and let if flow between the socket and motherboard.
3. Use heat gun to heat the cpu socket to ~200^C.
4. .........
5. Profit!

Questions:

1. Won't the CPU Socket itself melt at 200C?

2. Won't the solder at the back of the motherboard melt?


"I've got the gist of the procedure"
Looks so to me as well. I'm just going to add a bit more details or tips if I have any on some of the steps. I don't think I have an exact step-by-step process, since there can be set-up and tool availability variations.


"1. Cover everything but the cpu socket with aluminum foil"
It's a good idea. But it's better to just put a "wall" of aluminum foil just a few centimeters around the CPU socket and not cover anything (except possibly capacitors, if there are any that are too close to the CPU socket).
Also, do something similar on the under side of the boards as well - at least if we are going to reflow from both the topside and the bottom side (more on that in a bit).


"2. Use pen type flux to flux the area around the cpu socket and let if flow between the socket and motherboard."
Good idea. But since there is great variation of fluxes, and they all perform differently, I would recommend you first put a little bit of that flux on the surface of a broken/junk motherboard and a piece of solder next to it. Then blast it with your heatgun and see what it does. Ideally, you'll want the solder ball to melt and the flux to still be active. Some fluxes (especially cheap liquid fluxes) will just boil/sizzle and leave a brown residue way before the solder melts - you don't want this! Such fluxes are only good for general soldering, but not for reflowing/reballing and BGA work.
One flux that I can recommend to you that is very decent for BGA work is Kingbo. If you can find smaller quantities so that you can find it cheaper (generally, it comes in 100g jars), then definitely get some. It's not like the 100g jars are overly expensive and you can use it for any type of solder work, but still something to keep in mind.


"3. Use heat gun to heat the cpu socket to ~200^C."
This appears to be a post-RoHS era motherboard, so it likely uses lead-free solder. In that case, you would need to get the solder up to at least 220C (around 230C is best, but don't exceed 250C as some components will not take the heat too well).
If you are using the bog-standard type-K thermocouple that came with your temperature meter (assuming you got one), then keep in mind that it will take some time before the thermocouple picks up with the board temperature. Generally, the larger the thermocouple, the slower it will respond to the heat changes. So by the time the board has reached 230C, you may as well have some areas reach 240C or higher if the thermocouple is big. Just keep that in mind.
Last, and most important, I would recommend you heat the board from both sides to avoid warping. The easiest way to do this is to put the board on a oven-type grill or some kind of metal screen and suspend that off the edge of a desk or table. This will give you access to both sides of the board. However, make sure the grill/screen is solid and won't warp from the heat or allow the board to warp from the heat (add aluminum foil balls as supports as needed around the CPU socket). Once you have that set up, start heating the board from the bottom. Make sure to move the heatgun nozzle around so that it doesn't create hot spots or end up overheating certain areas... At around 150C+ temperature near the CPU socket, you may stop the heating and proceed to adding flux around the CPU socket (but make sure you don't get any in the socket!). Putting flux on a hot board is better, because the heat will suck the flux under the socket. After this, go back to heating the board from the bottom. When it reaches around 180C, move to the top side and heat the CPU socket from above. Go all the way until at least 220C (as mentioned above). Once you do, hold the temperature there for about 10 seconds, then you can turn off the heat and let the board cool.

Alternatively, if you don't have a grill or metal screen to put the board on, then put the board on a flat surface and add aluminum balls as supports - especially around the CPU socket. This is done to prevent the motherboard from warping from the heat. The repeat the above procedure, but by only heating from the top side.


"1. Won't the CPU Socket itself melt at 200C?"
Shouldn't unless you heat it up way too much. It may become a bit more yellow, though.


"2. Won't the solder at the back of the motherboard melt?"
Yes, but that's a good thing. It means that the solder under the CPU socket has successfully melted as well (so higher chances of the reflow being successful).