I got an unibody macbook pro 2008 Motherboard 820-2330-A. I do take al precautions knowing how thermal cycles affect the solder joints. So my computer was in such a position that the lid had perfect airflow for the fans. In fact I had all the fans both at 5900rpm. I was running Windows 7 natively.
So I used my macbook plugged in on AC with the removable battery. And honestly i never swtiched it off . But at times when the electricity went off it was the only time the battery drained entirely and turned off.
Now what started happening was that once that happened the lap would not turn on. I searched all over the net and someone at Ifixit said you had to press power button for 10 seconds.. plug magsafe keep pressing it for 10 seconds more.. depress and press again . To my astonishment that made it work.
Now , I never really cared much about that..didn't pay much attention .. thought it would be something that might happen a couple of times.
But now I dissasembled it to clean the heatpipe radiator fins I disconnected the fans vacuumed them and then IT DIDNT" START! I folllowed the exact sam eprocedure as before.
SO what should I do , I managed to get the schematic for my motherboard. I thought about doing a manual SMC reset from the logicboard pads which I did.
-I DID FIND ! A SMALL RECHARGABLE BATTERY ON THE LOGIC BOARD which was in bad condition like as mall button cell which was soldered and said F8430 on top on schematics it's labeled PP3V3_G3_SUPERCAP could this affect the power on sequence?. i'm thinking this might be the reason some times the other procecure worked but I don't know what voltage this button cell is
Picture for the button cell is on this link

http://oi39.tinypic.com/2u6cshi.jpg
http://postimg.org/image/uhhr2vyl7/full/

So going on with the story, I started to touch some of the QFN IC's to see if any of them started getting HOT to the touch. So I did find an IC from intersil ISL6269 (U7750) which is running extremely hot to the touch, even without pressing the power button. I just had the logicboard out of the case just with the magsafe board plugged psu and it was burning hot to the touch. I did also reflowed the solder on the EFI chip just in case as I've found one guy on youtube saying solders were dry , but nothing.
Something to do have into consideration is , I do repair notebooks as well and I'm used to GPU BGA's to be desoldered cause of cracked solder joints and thermal cycle stress. So my macbook in question , always had fans at a full speed 5900rpm.. the lid was in such an angle that airflow was precise. I can't believe any of the bga's might have desoldered from the board.
So I ordered a sample from intersil ISL 6269 desoldered the old package soldered the new one, and the machine started. it was that damn intersil part which supposedly managed the PWM pulse width modulation driving the fans.. I'm suspecting it might have come faulty. Cause I mostly used my mac with bootcamp win7 natively overriding SMC controller running fans at full speed.
This happened a long time ago, but it started to happen again, machine doesn't turn on with the power on button.
Now please pay attention to the following.

I managed to heat up the area surrounding where the EFI IC it's soldered with my rework station slowly for a couple of seconds. MAGSAFE not connected. I plug MAGSAFE and machine turns on. So seems there's something that with heat make's the logic board power on work .
Question is , Can you guide me as to check what could be going on here. The original battery of my macbook started to get bloated but charges fine. I ordered a replacement battery from OWC plugged it in but it doesn't charge. With the old swollen battery it charges , this really complicates me.
This is the area I heat up

http://oi57.tinypic.com/102tixg.jpg

I don't know if the problem could be the Intersil part I soldered back ago (it's not hot to the touch anymore) , if It could be the EFI bios chip, SMC BGA .
I have the schematic and Board View just to let you know.

New aftermarket battery might be faulty. I had the same problem with battery before, So I ordered used oem battery from ebay and it worked just fine.

hi,
just for reference on this MB there is something that looks like a button battery, but it is a SUPERCAP that give 3,3V.
For reference on schematic is C2800.
First of all check it maybe you'll see some acid leak around it, anyway check voltage of it and surely you'll see it will be much lesse then 3V (in mine was around 0,6V!)

1. Now take away magsafe and main battery and push keep it pressed for at least 10 second power button.
2. Connect only magsafe and goes on to keep pressed power button for at least other 10 seconds.
3. Next stop pressing power button and press normally it and notebook will power on!
Maybe there will be some strange things likes fan will turn fast or keyboard could not light up but nevermind about it.
It will goes on to works properly if you doesn't remove the magsafe, if you remove it you've to follow the 3 points above again.
If you keep magsafe connected and you pull in again main battery it will not charge.
So the exact sequence will be to put first battery and next magsafe. In this case magsafe will turn orange and battery will start to charge.
Leave it charging and for me machine will start for itself and all will works properly, and you can power on it normally.
If you will leave battery draining you've to follow again this steps, that's why if battery drains too much and supercap doesn't keep 3,3V!
As suggested changing main battery will fix this issue.