[HD2 General] motoi_bogdan:
yep.. heat is the begining of the problem. then comes the bad cpu soldering.

if the modifications are made at an early stage or on a working device, i don't see how the problem could appear in the future.

Back some years ago when i designed a cooling system for the first revision of the macbook air, a pc magazine from my country approach me and ask if i could design a liquid cooled custom made pc, more precisely a computer that would run fully submerged in a cooling fluid. Yeah, i admit, that would be cool, sf-ish and although it was done in the past with mineral oils, i felt that a different approach could be taken using less viscous liquids. Some time ago i designed a cooling liquid based on some industrial solutions and some market available products and felt i could give it a try.

However, before i actually built the pc, one problem emerged. Yes, the liquid transfers heat and does it fast. But in a liquid environment heat transfer also becomes a real problem, because be it liquid or not, we've got a static environment. The liquid near the cpu will have a far greater temperature then that of the liquid in the distant regions of the enclosure. Even if the liquid is recirculated, the bga matrix is large enough for liquid to get underneath it but dense enough to actually trap it there. That region will become overheated and the soldering will eventually fail. The actual cpu comes mounted on a slot, the gpu on a video card is not. It will most likely fail if placed in a liquid environment, be it viscous or not. The solution was to incorporate a better heat transfer method but alas, it could not be found in such a design so in the end the project was abandoned. Sure it can be done, by all means in a low power pc, but the more powerful configurations won't cope with this.

Heat itself isn't a problem. It's normally occurring with semiconductors. HD2's cpu can withstand 105 degrees in spikes or 90 degrees in continuous operation. I've read cpu datasheets (be them arm,risc based or other sorts). These limits come by the design of the silica pathways inside the chip. Above them those semiconductors will oxidate, the atoms will start to loose charge carrier particles - electrons - thus making them electrically insulant (more simply put, the chip itself burns out). But i repeat... we're talking about 105 degrees in working state. The chip itself can also sustain almost 400 degrees in off state and in fact, factory soldering is done at about 320-360 degrees. So surely, the chip can handle it.

HD2 will however fail because of the crappy cpu soldering. Too much led in the solder mix, coupled with improper heat transfer in a closed environment. The chip won't never make to 80-90 degrees in a normal working phone, but rapidly heating and cooling cycles - dilatation - will start to form microfisures in the solder balls that link the cpu to the motherboard. As they extend, it will affect the phone's functions at more lower and lower temperature levels.

HD2 is a somewhat experimental device, it's also one of the first phones made to feature this cpu. So while the lesson may be learn for future devices, something must be done with the HD2 to make it survive longer. I did what i felt worth doing, understood the risks - got lucky and rescued the phone. I won't risk to disassemble the phone's cooling system again to see what makes it work, if i had a sure answer to that one, i've been already posted a simple 1-2-3 guide and the topic would have been flooded with numerous success stories. Alas it is not. We're still talking experimental things, we're still guessing. But HD2 has a problem alright... something's wrong with it's design. That's for sure.

Although i can manage to get more advanced tools for diagnostics or can go as far to try a complete cpu reball i don't have neither enough damaged phones to experiment nor the money to buy them. If i however could get another damaged hd2 or similar device with same cpu, i could try other cooling systems or variations to this one. I'll post on local forums in my country to see if i can find people with damaged/dead motherboards willing to donate them for a complete cpu removal in order to better understand the nature of the soldering used. If i find another hd2 with heat problems i will try to experiment a more advanced approach, an active cooling system using peltier effect microelements, powered by the hd2's battery.

I also won't mind if my hd2 fails again. There's still a lot of things that can be experimented on it :D