How can I test a BGA (Ball array) chip, in circuit?

Thread Starter

camerart

Joined Feb 25, 2013
3,028
Hi,
I'm making PCBs for AK8963C chips, shown here: https://forum.allaboutcircuits.com/...ectronic-errors-oshonsoft.187539/post-1741319

When I've soldered them onto the PCB, obviously I can't check the balls as they are under the chip, so how are these type of chips tested, after soldering?

So far I've used a meter and continuity mode and diode mode. With diode mode, I connect the +V lead to GND, and check the tracks in turn, if there is no result, I either swap the leads around, or switch to continuity mode. Usually, I get some reading, which isn't open or short. Today I made 9x PCBs 5x good 4x bad.
EDITED.
Cheers, Camerart.
 
Last edited:

ronsimpson

Joined Oct 7, 2019
2,161
so how are these type of chips tested?
We test then with JTAG. JTAG
Most large ICs have a JTAG port so you can test the pins. In test mode the normal functions are shutdown and the pins are made to be I/O for testing. You can set pin 52 to out and set to 1 or 0.
On a mother board all the ICs are set to test mode. You can set a pin on the CPU to 1 and read back from a memory pin if the CPU really did go to 1.
If there is only one large IC the another way is to toggle each pin one at a time and measure the amount of signal you can see on an antenna. A disconnected pin produces very little noise while a long connection makes a large amount of noise.
There are many other ways to test but this is a start.
 

Thread Starter

camerart

Joined Feb 25, 2013
3,028
We test then with JTAG. JTAG
Most large ICs have a JTAG port so you can test the pins. In test mode the normal functions are shutdown and the pins are made to be I/O for testing. You can set pin 52 to out and set to 1 or 0.
On a mother board all the ICs are set to test mode. You can set a pin on the CPU to 1 and read back from a memory pin if the CPU really did go to 1.
If there is only one large IC the another way is to toggle each pin one at a time and measure the amount of signal you can see on an antenna. A disconnected pin produces very little noise while a long connection makes a large amount of noise.
There are many other ways to test but this is a start.
Hi R,
I suppose that the JTAG PIN will be TST1 here:

For my level of skills, my method of 'suck it and see' may be best for me. I'm getting better that 50% pass rate, which may be upped, by re-floating the chips again. I only need a few more to cover the near future.
Thanks.
C
 

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ErnieM

Joined Apr 24, 2011
8,311
When I've soldered them onto the PCB, obviously I can't check the balls as they are under the chip, so how are these type of chips tested, after soldering?
Where I work we check the solder balls by looking at them.

It sure helps to have a real time industrial X ray machine for this.
 

Thread Starter

camerart

Joined Feb 25, 2013
3,028
Where I work we check the solder balls by looking at them.

It sure helps to have a real time industrial X ray machine for this.
Hi E,
An x ray machine would be handy.

I changed the title, by adding 'in circuit', meaning is it possible to test by resistance or similar?

So far, if all of the PINs show a high resistance (High K Ohm or M Ohm) then they ususally work, but as mentioned I'm getting >50%.
C
 
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