Electronics > Repair
TEKTRONIX DPO 6GHz scope issue - broken BGA ball connection?
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ZBIQ:
Hi guys,

Currently I'm resurrecting a high-end scope (especially back in the days) TEKTRONIX DPO 70604C. After replacing its HDD and ATX PC board it finally boots (it fails to do so from time to time, but whatever...).

Right mow, however, I'm facing acquisition board issue. On the first channel, after switching the full sampling rate (from 12,5 GS/s to 25 GS/s) I see large spikes appearing. This tells me, that some additional sampling circuit kicks in and starts to show bad behavior. There is 320 ps of separation between the spikes, thus it is 25 GS/s divided by 8. Makes sense - there are multiple ADC for a single channel for sure.

I tracked it a bit more and found a chip under glued heatsink - see arrow in the picture. This is probably the CH1 multiplexer. After pushing it, especially on one of the crones as my pencil shows, or by spraying some freezer - the spikes stop and scope behaves as new. Is this a broken ball behavior?

If it is, is it worth to try heating up and pray for the IC to reach the PCB? I am quite afraid about doing so, because:
1) The board is about 32 layers and 5 mm of thickness.
2) The thermal addhesive underneath the heatsink is solid as a rock - very hard and quite impossible to remove. I tried tu cut it a bit with a dental floss but without much luck.
3) The suspect IC is surrounded by other delicate ADC ICs.

Is it worth trying to re-solder this IC?

Best!
Mateusz

hanakp:
How strong mechanical force is needed to "fix" the IC? If it's not too big, I'd just install a spring that pushes the IC against the cover. It won't last forever (solder tends to creep), but you'd "fix" it for a few months or even years.
ZBIQ:
I would say approx. 1,5 kg. Quite a bit. but not dramatic.

If I could only remove this heatsink... Are there any 0,1 mm OD diamond wires on the market to cut this epoxy?
mikehank:
That IC is an A2D HFD204 ASIC to be specific.  If you are having spikes during acquisitions it certainly will need to be rebelled.  You can actually measure the problem by probing the gold traces.  If the A2D is open you will get 60 or 28 ohms i cant remember.  Compare to the other A2D's.  BTW the chip that the traces are gong to is the track and hold.  Please ask more questions if needed.
ZBIQ:
Hi mikehank,

That's a lot of information for me in a single post! Have you used to work with or service those scopes?
My questions are mainly about the servicing procedure i.e., how to remove those heatsinks properly and what PCB preheating is required? I wonder if 20x20 cm2 heater will be good. If no, due to potential PCB stresses, I will look for some BGA guru nearby.

Cheers,
Mateusz
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