Author Topic: Repair disaster: Lifted trace on TDS754D  (Read 8309 times)

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Offline TAMHAN

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Repair disaster: Lifted trace on TDS754D
« on: September 21, 2016, 07:20:53 pm »
Hello Folks,
as I just managed to distroy my DPO for real |O / *RIP* .... I wanted to post some postmortem stuff, and also ask for some last help.

First of all: I tried to solder the SOJ ICs like a normal SOIC one. Aka tin the pads first, and then put the IC down. Sadly, this didnt work out as I  had a bridge which I managed to fix by running a needle through the two pins when heated up...but the problem remained in that there was a gap of up to 1mm between pin and tinned pad due to uneven tin distribution.

So I removed the IC again, and took out a trace with it - which is a very critical one, as it carries the A13 signal. The DPO still boots up and shit, but of course can no longer acquire signals.

Now, I have three questions:

a) Looking at the attached damage, do you think that a wire bodge is workable? I was thinking about trying to run a wire from a via to the A13 pin of two of the ICs
b) Do you think it makes sense to buy a hot air station and try my luck again?
c) Anyone an idea where to get a new acquisition board for cheap? Or would you part the unit down yourself and sell the remaining, working guts?
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Offline Refrigerator

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Re: Repair disaster: Lifted trace on TDS754D
« Reply #1 on: September 21, 2016, 07:30:28 pm »
scrape the soldermask off and solder a small wire across, should be anough to connect to a pin.
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Offline TAMHAN

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Re: Repair disaster: Lifted trace on TDS754D
« Reply #2 on: September 21, 2016, 07:33:15 pm »
Hello,
thank you so much for the swift reply. My problem is that this is a structure like:

xxxxxPADxxxxx

So one part of the wire will be below the IC which I have to solder in?
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Offline Refrigerator

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Re: Repair disaster: Lifted trace on TDS754D
« Reply #3 on: September 21, 2016, 07:41:13 pm »
Hello,
thank you so much for the swift reply. My problem is that this is a structure like:

xxxxxPADxxxxx

So one part of the wire will be below the IC which I have to solder in?

Basically this:
                 ______________  <- A wire soldered across the two traces
----------------_(gone pad)_-----------------
trace             PCB                                        trace
Should be plenty to make a decent connection for a simple signal trace.
« Last Edit: September 21, 2016, 07:43:13 pm by Refrigerator »
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Online TheSteve

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Re: Repair disaster: Lifted trace on TDS754D
« Reply #4 on: September 21, 2016, 07:51:15 pm »
I don't think pre-tinning SOIC pads would be considered a valid technique by anyone experienced in soldering. You can tin one corner pad, then reflow it while putting the IC in place. This is very common as it will hold the IC in place and allows for proper alignment. Also the legs should always be touching the pad - you never want a gap.

Anyway, that said the repair should be pretty easy on your scope, worst case you solder the IC in place and then run a wire over the top(30 gauge wirewrap or similar).
Remove all the solder, clean, tin one pad, align the IC and reflow that one pad, then do the opposite corner. When you're happy with placement you can then solder the remaining legs one at a time. Those parts aren't even close to fine pitch stuff.

If it is beyond your skill level perhaps someone local to you will offer to help.
VE7FM
 

Offline TAMHAN

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Re: Repair disaster: Lifted trace on TDS754D
« Reply #5 on: September 21, 2016, 10:31:11 pm »
Hello Steve and Refrigerator,
thank you so much for your swift response and willingness to help! I still feel stupid re failing in this repair.

In the mean time, a small question regarding Reflow. Do you mean using hot air, or using a soldering iron?

Also, how would you go towards removing the solder? My small solder sucker had problems removing too much of it, and with the wick, I did not get far either. So I am always thankful for advice.

Finally, what would you recommend for magnification?

P.S. Forgot to add: my hands are shakey due to a neural damage. Long story...

« Last Edit: September 21, 2016, 10:33:25 pm by TAMHAN »
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Online TheSteve

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Re: Repair disaster: Lifted trace on TDS754D
« Reply #6 on: September 21, 2016, 11:24:58 pm »
I would solder using a hand iron.
For cleaning surface mount pads I wouldn't recommend a solder sucker. I would always use a good quality solder wick. It should be very easy to remove the solder with clean wick. Keep in mind you can't always saturate the wick with solder, once it has gotten hot the flux in it is burnt off and it won't want to soak up more solder. You also don't want to apply too much pressure when using wick. It really is something that requires practice at, and good equipment always helps.
So don't be too hard on yourself. Anyone with a bunch of soldering experience has lifted a pad or two, sometimes worse :)

As for magnification the only thing I have is a work light with a 2x magnification or so, nothing fancy. As I get older I'll likely need more. I used to be fine doing 0.5mm pitch with just my eyes, these days I like the 2x for inspection afterwards.

Again, maybe you can find someone local who is willing to help. I would but I am nowhere close.
VE7FM
 

Offline eKretz

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Re: Repair disaster: Lifted trace on TDS754D
« Reply #7 on: September 22, 2016, 03:39:45 am »
If you're having trouble with the solder wick cleaning up the surface mount pads, try adding some flux to the wick first. I agree with Steve - adding solder to all the pads at once is a big mistake. Clean all pads off and only add a bit of solder to one corner pad, then set the chip on aligned to your pads and reflow that corner with the iron to set the chip in position. Next do the diagonally opposite corner, then do the remaining pins. Run an insulated wire from the pin with the damaged pad over the top of the IC to the trace on the other side.
 

Offline Jwalling

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Re: Repair disaster: Lifted trace on TDS754D
« Reply #8 on: September 22, 2016, 09:43:02 am »


So I removed the IC again, and took out a trace with it - which is a very critical one, as it carries the A13 signal. The DPO still boots up and shit, but of course can no longer acquire signals.


You've only affected one channel with that break, so three channels should still work. If you can't get a signal on any channel, something else is also wrong.
Jay

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Online Ian.M

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Re: Repair disaster: Lifted trace on TDS754D
« Reply #9 on: September 22, 2016, 10:49:21 am »
Pre-tinning SMD pads with a carefully controlled amount of solder is a bodge one can use if one has a hot air station and flux but don't have the right solder paste.  The part should sit evenly on its pads with all pins touching and there should only just be enough solder to make the joints otherwise it will tend to slip off sideways and bridge the pads.  You may need to constrain it (e.g. with two thin strips of Kapton tape) to keep it in position then reflow again |O |O after confirming all pins have reasonable fillets, without constraint to allow it to self-center on its pads.  Its at the same level of bodgery as hand 'reballing' of BGAs in 3rd world backstreet phone repair shops!  :scared: However if you have a ROHS item that must be repaired TODAY in a non-ROHS workshop, you do what you must.

A pro-grade repair would be to cut a new pad from copper foil of the correct thickness backed by heat-resistant adhesive, and splice it to the existing track ends, taking care to keep the actual pad surface flush with its neighbours.

However, if you dont have the right gauge copper foil and adhesive, I'm with TheSteve and eKretz, the lowest risk, best chance of success is a magnet wire or Kynar wirewrap wire bodge wire run from the chip pin over the top of the chip, back to the via.  If you really aren't confident in your skills, don't try to pick up the continuation of the track with the end of your bodge wire, but simply run another bodge wire from the via to the next chip in line.
« Last Edit: September 22, 2016, 10:57:13 am by Ian.M »
 

Offline TAMHAN

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Re: Repair disaster: Lifted trace on TDS754D
« Reply #10 on: September 22, 2016, 06:36:19 pm »
Hello Folks,
thank you all again for the good advice. Given that I have quite a bit of manufacturing, some also SMD, coming up, I decided to shelf the DPO for now and will try again later.

Alternatively, Sir Steve: if I send you either the Acquisiteur board and/or the whole DPO, cover shipping both ways and reimburse you for your effort, would you be willing to take a stab?

Thanks again for standing with me when I fail
Tam

P.S. I will make a post mortem video later...
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Online TheSteve

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Re: Repair disaster: Lifted trace on TDS754D
« Reply #11 on: September 23, 2016, 04:41:09 pm »
Sure, but don't under estimate just how insane shipping rates are to/from Canada.
VE7FM
 

Online PA0PBZ

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Re: Repair disaster: Lifted trace on TDS754D
« Reply #12 on: September 23, 2016, 05:49:27 pm »
@TAM:

Can you clean up the mess and take another picture?
And did you already order the correct part?
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Offline Rasz

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Re: Repair disaster: Lifted trace on TDS754D
« Reply #13 on: September 23, 2016, 07:01:13 pm »
this is easy fix
forget trace under the chip, you have nice huge via on the left, this leaves only right trace to be scraped with something sharp
in order:
-scrape solder mask from the trace on the right from the brake, 1mm is enough
-dont touch solder you already put on the pads, screwing with it might end up in another pad missing :)
-get yourself a hotair(even cheapest $30 858 is perfectly fine for this) and flux
-get few random pcbs from broken/discarded stuff >2000 vintage, practice lifting/soldering back smd chips on those boards, at least one hour, until you are comfy with the tool. You need to learn temperature ranges that work for you and to recognize the point solder grabs/melts
-solder your chip with hotair, kapton on both sides is a good idea
-now use a bodge wire to first solder pin over the missing pad to the scraped trace, then to the nice via on the right side

or find the nearest hackerspace, they will have good tools and usually people proficient with them
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Offline Muxr

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Re: Repair disaster: Lifted trace on TDS754D
« Reply #14 on: September 24, 2016, 01:08:19 am »
Yeah this is fixable.. you can also checkout some Louis Rossmann youtube videos who does a lot of repairs of this nature where he runs into lifted pads or missing traces.

This lady also does some really fine pitch trace repair on iPhones. I mean she will even dig into the PCB and rescue traces from inner layers of the PCB..



 


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