[HD2 General] motoi_bogdan:
@booze_pistol excellent discovery! I analyzed your method and indeed it seems the best way right now. But i was wandering why my results were different although i was using similar equipment and calibrated probes.

So.. here's the rest of the story.

I've continued researching so here's some updates.

The hd2 bga soldering is made of a Tin-Silver-Copper alloy in accordance with ROHS specifications. While this is the best way to solder a chip like the snapdragon and still be withing ROHS specifications, it has some drawbacks.

The surface area of the solder alloy will oxidize if exposed to normal atmospheric conditions since the Tin itself makes almost all the alloy. The oxidized material has, however, different thermal proprieties then that of the pure solder material underneath it. For example if the soldering is oxidized, it will require more heat in order to partially melt it. It seems that my hd2 motherboards were in a more bad shape then those of booze_pistol's. That's why they were fixed only by applying a more hotter air flux over them. Should that would have been done to one of booze_pistol's motherboards, it could have killed, because without that oxide layer, pure solder alloy tends to melt at a much lower temperature.

The problem is that you can never know for sure the state of the soldering under the chip. I have a board with the cpu removed and still could not have seen that. It only happened to think of this when i saw booze_pistol fixing motherboards with a much lower temperature then that indicated by my tests and wandered why some devices need more heat then others?

So.. my "official" position now is that since we cannot determine the chemical composition of the actual solder material at the time we conduct our heating sessions, the best approach would be heating at lower temperatures, like booze_pistol suggested and ONLY INCREASING IT IF THE MOTHERBOARD DOESN'T SEEM TO RESPOND WELL AFTER TESTS.

In theory.. any heating method should do the job, if booze_pistol got the temperatures right, that should fix most phones. But now we also know that not all phones have the same soldering, some of them may suffer from more wear&tear, some have a thick layer of oxide over those small balls while some still have good soldering. From my experience with smd equipment, i can state that my experiences with oxidized boards were the exception, not the general rule. Therefore at present time, booze_pistol temperature measurements seem to be the default solution for most phones.

Also please note that we're still doing experimental work and as you see by looking over the topic, i've changed my view from the hd2 "cooling" system, to the smd rework station and now to the oven or other heat source and even now we're still trying to find the sweet spot. But i believe that booze_pistol takes us pretty close. It will still be kind of hard to get "almost perfect" results. This is not like rom testings, that's more simpler, a cook releases a rom version and some tens of thousands users can test it with almost no risk and report bugs to be fixed. Here we're just a bunch of people who cracked open some hd2's and trying to figure out what to do with them :D

But as someone here said, we're the real chefs :p trying to find the perfect recipe for cooking out our hd2's.