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I am really confused with working with BGA chips.

I am new to this but have been following other guides so I thought I should give it a try. I am using a DIY pre heater that has a 800watt IR bottom heater and for the top heater I am using a hot air station with 40x40mm nozzle that's about the size of the BGA chip I am trying to rework.

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I am desoldering an Intel JG82855GME chipset chip. I left the board on the preheater and set it to 100°C for 9-10 hours to remove any moisture. Afterwards I removed the BGA by setting my preheater at 220°C and hot air station at 350-380°C.

It's interesting to note that even though the bottom preheater is set to 220°C, the PCB only reaches about 120-130°C.

Why is that so?

The PCB is about 10x10cm in size while the heater is 12x12cm so the whole PCB is covered completely by the heater.

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After I removed the BGA chip I probed its VCC and VSS pins and found a short on those pins. Is the chip dead, or is there still some hope left for it? Is the hot air station a bad choice for this task?

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  • \$\begingroup\$ What was your measurement method for finding the short? How much voltage or current did you apply, and what current or voltage limit did you use to decide there was a short? \$\endgroup\$
    – The Photon
    Commented May 31, 2020 at 16:43
  • \$\begingroup\$ Sometimes I don't understand people at all. A photo, Ok. A screenshot of a photo, WTF? \$\endgroup\$
    – JRE
    Commented May 31, 2020 at 17:16
  • \$\begingroup\$ @ThePhoton I didn't apply any power to check the short. I used continuity mode on my multimeter to test it. It's giving about 0.2ohms reading which it also gives when I touch the two probes together. \$\endgroup\$
    – speedstr
    Commented May 31, 2020 at 17:31
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    \$\begingroup\$ This chip is dead as i was lucky enough to find another board which had an identical chip. The measured resistance accross that chip is about 216.5 Ohms so i think there's definitely some problem with this one as its resistance should've been atleast greater than 216.5 ohms \$\endgroup\$
    – speedstr
    Commented Jun 1, 2020 at 12:33
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    \$\begingroup\$ Good question, and your setup is pretty nice! You have a lot of initiative! \$\endgroup\$
    – user57037
    Commented Jun 2, 2020 at 4:51

1 Answer 1

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Based on your 2 questions :

  1. Question 1 - Is the chip dead, or is there still some hope left for it? Answer - Hard to say if device was damaged due to the rework process or it was not-functioning even before
  2. Question 2 - Is the hot air station a bad choice for this task? Answer - No, the hot air station is OK, but at any rate...

...you should "mask" the card bottom side, in such a way the bottom heater heats up only the area where the BGA is located (preserving other components nearby); however, 120-130°C is a temperature good enough for the card bottom side

...as per the top side of the board, you should thermally protect/isolate components nearby the BGA device to rework

EDIT : BGA rework implies 2 thermal stresses, and you certainly know how much the thermal stresses are important. Just for this reason, one of the principles on reworking of massive components (specially on massive boards), is to depopulate the host board as much as possible, before going to desoldering and to soldering a new component on it. Also, you have to consider that is if you want to recovery the board functionality due to a damaged component by reworking it, you must keep in mind not to damage or thermally stressing the other components on the board. So, in principle, the rework process should be conducted in a way to stress the host board as less as you can.

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    \$\begingroup\$ I appreciate and agree with the points you laid down but I have a question relating to the mask you said for the bottom heater. Wouldn't that defeat the whole purpose of that bottom preheater ? Like what I've been following is that its used to make all the parts of the pcb reach an equilibrium temperature and saturate the thermal mass (Heat sinks and Copper planes) so that once the top heater is turned on, Its heat wouldn't be sinked to nearby elements. \$\endgroup\$
    – speedstr
    Commented Jun 2, 2020 at 0:47
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    \$\begingroup\$ See my added considerations in the edited answer. \$\endgroup\$
    – barrow
    Commented Jun 2, 2020 at 8:31

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