TEKTRONIX DPO 6GHz scope issue - broken BGA ball connection? - Page 1
Forum section: electronics / repair

author: ZBIQ
author_rank: Contributor
timestamp: April 30, 2023, 07:30:28 pm
content: 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

author: hanakp
author_rank: Regular Contributor
timestamp: May 01, 2023, 10:46:20 am
content: 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.

author: ZBIQ
author_rank: Contributor
timestamp: May 01, 2023, 12:18:08 pm
content: 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?

author: mikehank
author_rank: Regular Contributor
timestamp: May 01, 2023, 11:19:47 pm
content: 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.

author: ZBIQ
author_rank: Contributor
timestamp: May 02, 2023, 07:52:27 am
content: 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

author: mikehank
author_rank: Regular Contributor
timestamp: May 02, 2023, 02:39:03 pm
content: Hi ABIQ,Yep I've worked on these scopes for years fix dozens maybe hundreds of board/scopes. To remove the heatsink is actually pretty easy. Get a heat gun and a small screw driver. Place the screw driver between the fins of the heat sink. Begin to heat the heat sink from above while wiggling the screw driver. Once the epoxy gets to a certain temp the heat sink will start to move and come off. If you are heating for a long while stop and take a break and let the board cool. We don't want to flow the balls under the BGA. It takes a little practice but it not to difficult.

author: ZBIQ
author_rank: Contributor
timestamp: May 02, 2023, 03:28:35 pm
content: Great!Would you consider using a small 0,6 kW preheater just under the faulty IC? Or should I find some real reworking preheat to cover most of the acquisition PCB?Edit:One more thing. I measured the Rdc of the golden traces incoming to the broken IC. They are always 28 Ohms, even if the spikes are still visible (I switched of TEK and quickly measured resistance on the chip still being hot and faulty).Cheers,Zbiq

author: mikehank
author_rank: Regular Contributor
timestamp: May 03, 2023, 07:59:05 pm
content: Hi,You don't really need to pre-heat the board for the heat sink removal.Removing a BGA hell yes, you will need a BGA replacement machine and a trained guy.

author: ZBIQ
author_rank: Contributor
timestamp: May 04, 2023, 09:30:06 am
content: Thanks, I've already started to look for a professional reworking guy.Anyway, from what I noticed, the TEK hardware is prepared for much higher bandwidth and sampling rate, maybe even up to maximum in its 70000, i.e., 33 GHz. So I was wondering if it is possible to squeeze a bit more of scope's analog bandwidth. Of course, sampling rate will be limited, but working in the Equivalent Time sampling mode maybe could show much higher frequency response. Has anybody ever tried to hack such scope?Mateusz

author: ZBIQ
author_rank: Contributor
timestamp: October 20, 2023, 09:09:43 pm
content: Hi Folks!I started to dig a bit more into DPO debugging and I have a short question. Where I can find extended selfcal/test files of the DPO 7k series? One of the colegues pointed, that I should look for c:/TekScope/calibration, but there is no such folder in my device.My scope is running one of the latest software versions on win7, so probably files architecture has changed...Cheers,Mateusz

author: ddrl46
author_rank: Regular Contributor
timestamp: October 21, 2023, 09:02:10 am
content: Quote from: ZBIQ on October 20, 2023, 09:09:43 pmDid you enable "show hidden folders" in explorer? From what I can remember from the Windows XP units it should be there.

author: ZBIQ
author_rank: Contributor
timestamp: January 03, 2025, 04:13:43 pm
content: Hi guys, it took me almost 2 years but I finally repaired my DPOI will try to summarize it shortly how I did it.1. Careful diagnose.As I mentioned before, the faulty BGA connection was under one of the HFD204 chips. I was probing the PCB very carefully with the power on and off. It was clear that this was the only chip affected as one of it differential lines were showing >200 Ohms while others showed <100 Ohms (measured deferentially).It is important, however, to provide adequate cooling for the PCB, as Illia from Xdevs told me:"There is no thermal protection and all those chips burn lot of power during operation. I know of a cases when people opened up scopes, ran them for "quick" testing for maybe 20 minutes to do some minor issues testing and then ended up with broken ACQ board because damn ADC blew up from overheating since there was no adequate cooling airflow. Scopes are designed to have air pulled by fans with all covers in place."I placed a large cooling fan directly blowing on the ICs. It made the job.

author: ZBIQ
author_rank: Contributor
timestamp: January 03, 2025, 04:16:15 pm
content: 2. BGA reworkThe main PCB is approx 400 mm x 400 mm, >32 layers and 5 mm of thickness. It requires a big and reliable reworking station, since replacing PCB, as well as getting another chip, is not an option. I currently live in Switzerland and for sure I would find the proper workshop. Unfortunately, reworking this PCB would cost me for sure >1..2 kCHF. BUT, 1 year before I moved to CH I purchased for myself in Poland a Chinese BGA reworking machine - LY HR-506C. Quite a decent one, but I knew nothing about BGA rework...So, I started to train and learn how to reball PCBs. I reballed couple of PCBs found in trash, but the main learning came from a large PCB: 400 mm x 400 mm but only 2 layers and 2 mm of thickness. Probably behaves way different from the DPO PCB, but better this than nothing. I purchased 4 point thermocouple sensor/logger which helped me a lot during my research.

author: ZBIQ
author_rank: Contributor
timestamp: January 03, 2025, 04:16:58 pm
content: I made 5 trial runs with different profiles, but actually simple ramping, soaking and reflowing was good enough - see pictures.

author: ZBIQ
author_rank: Contributor
timestamp: January 03, 2025, 04:23:32 pm
content: Then it was time for the real game. I noticed that my test PCB flexes a lot during reflowing thus my DPO PCB got a stiffening frame. It was built from 15x15 mm2 V-slot construction profiles. I though it might be an overkill, but during the process, PCB bent no more than 2 mm. I truly recommend doing so for any large area PCB.I also protected other components with aluminum and silicon foil. I didn't want to stress other ICs on the board.In the end, IC removal was quite smooth and I did it on the first shot

author: ZBIQ
author_rank: Contributor
timestamp: January 03, 2025, 04:30:56 pm
content: The reballing process went also quite smooth. One or two BGA pads from the PCB were missing, but they were dummies. Some other lost a bit of soldermask so I applied a bit of UV mask as well.I applied 0,4 mm balls to the chip with the help of a standard BGA screen. This time, however, I used SnPb balls so I had an idea to lower the profile temperatures by 15 deg. Unfortunately, this caused the chip to not solder properly. Only one side of the chip got a proper reflow... So I had to do this again, with the v5 profile. I was a bit scared that I might overheat the chip or delaminate the PCB. Also, the flux might have been activated before, so there was a bit of room for an error. But, luckily, the chip got fully soldered this time:)BTW. I used NC-559 flux. Original one. Good stuff.

author: ZBIQ
author_rank: Contributor
timestamp: January 03, 2025, 04:35:59 pm
content: 3. Final touchesAfter reballing I tested DPO for about 30 minutes and it showed a perfect behavior at any settingsNo more glitches and perfect 6 GHz pulse response! BUT, after some more tests, DPO died. It couldn't boot for sever times, but I knew this behavior from last time I used this scope. Apparently, after digging some time in the HDD and motherboard area, one of the RAM chips showed no response. After removing it, and working at half the memory, it boots every time. Happy to catch itThe final touch was to apply some thermal adhesive to the analogue chips. I used two-part MG Chemicals 8349TFM.And this is it. DPO now sits in my rack and it is ready to measure some stuff!

author: inse
author_rank: Super Contributor
timestamp: January 03, 2025, 04:39:45 pm
content: Thats a real crime story!Thumbs up for solving that cold case

author: ZBIQ
author_rank: Contributor
timestamp: January 03, 2025, 05:34:43 pm
content: I also captured two reballing attempts: too cold one and proper, final reflow.

author: CaptDon
author_rank: Super Contributor
timestamp: January 04, 2025, 02:48:51 pm
content: I admire the effort you were willing to put into this project!!! Sadly, at the company where I worked, sending that scope to Tektronix for repair would have ended in 'Not worth repair' since they probably would have charged 50 to 60% of the cost of the scope when new. They may perhaps offer a 'trade up' program giving us about 30% of our scopes original purchase price as credit toward a new scope with a much higher purchase price. Way back when the outlandish pricing of a Tek scope came with the benefit of owners manuals, programming manuals and service manuals with in-depth verbal descriptions of every circuit and waveforms and voltage test points for practical repair. Although one of the biggest drawbacks of Tek products were the custom hybrid I.C.'s which became un-obtainium about a year after the product went to market. Today the insane pricing of Tek products becomes a financial risk to small companies when our supply of 'highly degree'd engineers' who absolutely suck when it comes to their technician and troubleshooting skills can blow up a $6000.00 probe or destroy a $150,000.00 scope with a simple 'ooopps' "I forgot that part of the circuit was mains referenced". Yes, we still get college educated engineers trying to measure the source impedance of an operating power supply with an ohm meter or measure voltage while switching the DMM through the amps range! Common sense, caution and troubleshooting are not taught at the university level anymore. Sadly, 90% of our blown up stuff just goes in the dumpster. Glad I was able to snatch and repair a TDS644A and a TDS644B from the dumpster!!

author: morzh
author_rank: Newbie
timestamp: January 04, 2025, 04:25:06 pm
content: My hat's off to you for the effort and the success.Very few people will go to this length in order to fix a device, even a very expensive one.I once modded a board I designed (a first prototype) with a reasonably large BGA (AMD Geode CPU) on it; I needed to solder to a second row ball which had no drop via under it. I was able to heat a strand of wire, routed between the balls, to hot enough temp to start melting the ball and solder too it. (The alternative was spinning the board). It was like 18-19 years ago. But that does not even come close to what you've done.You should be proud of yourself. It has been quite a feat.BestMike.