LOADING ...

PCB solder pad repair & corrosion clean up - The epoxy method

433K+ views   |   7K+ likes   |   263 dislikes   |  
24:35   |   Jul 14, 2015

Thumbs

PCB solder pad repair & corrosion clean up - The epoxy method
PCB solder pad repair & corrosion clean up - The epoxy method thumb PCB solder pad repair & corrosion clean up - The epoxy method thumb PCB solder pad repair & corrosion clean up - The epoxy method thumb

Transcription

  • Alright, so here we're looking at an A600 motherboard.
  • Now when it first came in, it looked like this.
  • So what has happened here is that C303 has leaked a bit,
  • causing some corrosion in this area.
  • And also, the owner of this board has made an attempt at repairing this by himself,
  • only to end up making the problem even worse.
  • We can see here that the pad for the negative terminal of this capacitor has been ripped
  • off the board, and the pad for the positive terminal has
  • completely deattached from the PCB substrate.
  • So yet again, this will be a repair where I have to rectify someone else's mistake.
  • Oh God! *sigh*
  • The owner of the board has also put non-polarized capacitors in here for the audio coupling.
  • That is just wrong, I will fix that up later. But the main focus in this video will be on
  • cleaning the corrosion off the board, and also repairing the pads for capacitor C303.
  • Now actually I'm already done with the repair at the time of recording this narration,
  • but in this video, I will show you how this motherboard went from this...
  • to this...
  • ... and then finally ...
  • to this.
  • So here is how this repair was done.
  • I started to prepare the board by first putting it on my preheater and then putting some aluminium
  • tape on there. After cleaning the area with isopropyl alcohol,
  • I then fluxed the components within the corroded area using some liquid rosin flux.
  • After letting the board preheat for a little while,
  • I then used auxillary heating to make sure the flux activates properly during the soak time.
  • Note here that I'm using a temperature of
  • the overhang which is much lower than reflow temperature.
  • We can here see that the flux is starting to activate.
  • I then increased the temperature of the overhang to reflow temperature, and started to remove
  • components one by one.
  • Note here that this procedure should not be done without a preheater.
  • You can not just take a hot air rework station and start spot-heating the board all the way
  • up to reflow temperature without any preheating at all.
  • Doing such a thing can cause all sort of problems because of the thermal expansion properties
  • of the board. We are talking about things like board warping,
  • microcracks in the PCB substrate, increased risk of board scorching etc etc etc.
  • Since this preheater doesn't have a cooling function
  • I usually like to use an external fan for that instead.
  • Okay, now this might look a little bit messy perhaps, but this is actually how a rosin-based
  • liquid flux usually looks like after it has activated in a corroded area.
  • There is no way around it unfortunately. But it can easily be cleaned off with isopropyl
  • alcohol, so really nothing to worry about. But hey, let's stop the video here for a moment
  • because I want to show you something.
  • Okay, so let's have a look at this area right here.
  • Do you see that the green PCB has a certain orange tint to it? That is the flux. All the
  • flux has activated, but this area is so far from
  • the capacitor that is wasn't really affected by corrosion that much. That is also why the
  • flux has not taken on a darker color here - It hasn't mixed with the
  • contaminants from the corrosion. So when our little friend here, C303 (the bastard!)
  • started to leak onto the PCB, it's natural that the most nearby components were
  • the ones taking most of the crap. So in other words, the blackened pattern here
  • that we can find is exactly what is to be expected considering the circumstances.
  • Oh no! My stomach! [Buhwahuaaaa!]
  • Wow, that guy sure likes to puke, doesn't he?
  • So, if we compare these two images, we can clearly see the correlation between them.
  • Here for example, we can easily see that the LF347 is heavily corroded. In fact, if we
  • take a look at it through a microscope while it's out of circuit, it becomes even more
  • obvious. I mean, look at all that crap. It looks absolutely
  • horrible! Of course, reusing this component is completely out of the question.
  • Now I have to say that I think it can be quite interesting sometimes to take pictures before
  • and after every step and then go back and compare them on a computer because,
  • I think it's possible to learn a few things from that actually, so that can be quite interesting
  • sometimes. Anyway, let's continue with the repair.
  • Okay, now at this point, it's very tempting to start cleaning away the flux residues,
  • especially considering that this area is now quite messy.
  • And yes, of course, a good rule of thumb is to clean the area between every step of the
  • repair. But before I clean the area, I will be sure
  • to first remove the solder from the pads with solder wick. Let me explain why.
  • The solder that is left on the pads after component removal can take on different shapes,
  • and in some cases, the remaining solder on a pad might look a little bit like a pyramid.
  • This is due to the cohesive forces of the solder alloy during component removal.
  • Here I have drawn in an exaggerated example of that, where the solder almost has the shape
  • of a hook. If a cotton bud is now used to clean the area,
  • there is a possibility that the fibers of the cotton bud will hook onto the solder that
  • is left on the pad. Since we are making sweeping motions with
  • the cotton bud in order to clean the area, there is a certain risk that the cotton bud
  • will then rip the pad straight off the board. In order to avoid this from happening, the
  • remaining solder is removed before any cleaning takes place.
  • I would like to point out though that the choice of removing solder before cleaning
  • the board is purely based on personal experience. It is not coming off an IPC training manual
  • or something like that. And of course, whether you choose to clean
  • the board or not before removing solder from the pads is completely up to you, but at least
  • now you know about this little risk factor. And this is how I prefer to do it, because
  • after all, my job is not to rip pads off the board - What I do is to repair them.
  • Now it's time to do some cleaning.
  • It's always a good idea to try to keep the rework to a minimum, but here I felt that
  • I needed to add some more flux and go over the pads just one more time with some clean
  • solder wick, in order to get the last corrosion off of there.
  • And of course now, there are flux residues on the board again, so we need to get that off.
  • And in case there is still any corrosion left in this area, this should pretty much take
  • care of that. Here I'm using an ESD-safe brush together
  • with some isopropyl alcohol.
  • A final clean up with a lint free cleaning foam swab will ensure that there are no cotton
  • fibers left on the board.
  • And here we have the board looking nice and clean.
  • Now you might also notice that the pad for the positive terminal of C303 is not there
  • anymore. That's because I removed it. Let me explain why.
  • So here is how the pad looked like before I removed it. We can clearly see that it has
  • completely lifted from the PCB substrate. Now at this stage, we have two options. We
  • could either try reusing the pad by gluing it back onto the PCB, or we could remove it
  • and install a new one. So which one of those two alternatives should
  • we go for here? Well, let's evaluate the situation. Just by looking at this pad, we can easily
  • see that it's terribly corroded. The pad is going to be attached to the PCB
  • through under bonding, that is, a high temperature resistant epoxy is going to be used to glue
  • the pad into place. On the upper side of the pad, a component
  • is going to be soldered into place. These steps can not be performed with reliable results
  • when a pad has this much corrosion on it. In fact, the IPC recommended procedure when
  • dealing with damaged pads is to remove the pad from the PCB and install a new one.
  • An exacto knife is then used to cut the trace, and the pad is removed from the board.
  • However, in this case, the connecting trace was so heavily corroded that it snapped straight
  • off as soon as I touched the pad.
  • Actually, the owner of the board brought the pad for the negative terminal with him. So
  • why don't we take a look at these two pads through the microscope to see how they look
  • like? Let's start with the negative terminal.
  • Okay, so this doesn't look that good, now does it?
  • Alright, so let's move on to the other one, which is the one I removed.
  • Wow, this one looks even worse than the other one! I think the orange stuff on there might
  • be flux, but look at all the oxidation. And also, take a look at the trace where it snapped
  • off. See how corroded that is? No wonder it came
  • off that easily! Now, let's go back to the other pad again.
  • How does it look like on the bottom side? So, okay, let's flip it over and have a peek.
  • Go figure... Kind of looks like it has been thrown into a 5000 degree furnace!
  • Anyway, let's take a look at the bottom side of the other one as well.
  • Yeah... So if I had just glued this pad back into place, that would have turned out to
  • be a perfectly reliable repair, wouldn't it? Umm... no.
  • Here I'm using a metal screwdriver to remove any remaining adhesive and contaminants from
  • the PCB substrate.
  • The site is cleaned with isopropyl alcohol to get rid of any remaining debris.
  • A dental pick is used to scrape some of the solder mask off of these traces.
  • Although it's not necessary, compressed air can be used to make the alcohol dry up a little faster.
  • The old pads are used as a template to create new pads from a 1.4 mil copper foil.
  • Copper can oxidize over time when exposed to oxygen.
  • To ensure reliable results, I am here using a dental pick to scrape off any contaminants
  • from the connecting trace on the new pad.
  • The new pads are put on the board to verify that they are of the correct size.
  • According to proper procedures, the length of the overlap should be at least twice the
  • width of the trace.
  • The traces going to the pads are cleaned with isopropyl alcohol,
  • and then fluxed in preparation for tinning.
  • The traces are then being pretinned before the new pads are installed.
  • Flux residues are then removed from the site.
  • At this point, I decided to fix up some of the traces before continuing with the pad repair.
  • And it looks pretty good. No problem here.
  • Okay, so I think I will have to fix this trace also.
  • And there we are. No problem.
  • Kapton tape is here used to protect the board during bonding of the new pads.
  • The epoxy I'm using for under bonding of the pads is the CW2500 by CircuitWorks.
  • After being fully cured, it can withstand temperatures up to 315 degrees Celsius for
  • 60 seconds, which makes it the highest temperature resistant epoxy I have ever seen on the market.
  • This makes it very suitable for this kind of repair, since the capacitor can then later
  • be soldered onto the new pads without losing much of the bonding strength.
  • This epoxy is ideal for solder mask repair according to the datasheet and meets the requirements
  • of IPC-7721 procedure 2.4.1, but I don't see any reason why it can't be used
  • for under bonding as well.
  • A lint free foam swab is ideal for creating a thin film of epoxy at the site of the pads.
  • Since this epoxy has a tack free time of 30 minutes, there is plenty of time to also repair
  • the solder mask of these traces before the epoxy starts to cure.
  • The pads are carefully put on the board and aligned into place.
  • Kapton tape is then put on top of the pads.
  • I then cured the epoxy according to the manufacturer's recommendations.
  • After this, I didn't do anything more with the board for the next 12 hours or so, because
  • I wanted to make sure the epoxy was fully cured before proceeding with the repair.
  • The next day, I then started off cleaning the traces for these pads.
  • And I then lap soldered the traces of the pads together with the traces on the board.
  • As you can see, a piece of kapton tape is put on the pads here. That is to prevent the
  • solder from reflowing onto the pads themselves. The pads are now electrically connected to
  • the board. So I cleaned off the flux residues and then started to remove the kapton tape,
  • by carefully peeling it off sideways.
  • I then cleaned again so that there wouldn't be any adhesive material from the kapton tape
  • left on the pads.
  • I'm here checking for continuity to verify that the pads are connected to
  • the traces on the board.
  • The lap solder joint now has to be over bonded in order to prevent it from reflowing when
  • the new capacitor is installed. A piece of kapton tape is again placed over
  • the pads so that no epoxy accidently gets on the actual pads.
  • Here I am again using CW2500 but this time for over bonding of the lap solder joints.
  • Excess epoxy is removed.
  • And I'm again being careful when removing the kapton tape, making sure to peel towards the side.
  • Once again, I cured the epoxy according to the manufacturer's recommendations, and then
  • I let the motherboard sit over the night.
  • The pads are now repaired and the board is ready to accept new components again.
  • New components were installed using standard soldering procedures, including cleaning,
  • fluxing and soldering. You might note here that I'm using a very
  • short dwell time when soldering at the site of C303.
  • And also, I have the iron set to a slightly lower temperature than I normally would for
  • this kind of surface mount soldering.
  • By the way, the smaller electrolytic capacitor to the left of the other two, C612, is a common
  • cause of failure on these Amiga 600 motherboards. That is because this capacitor is part of
  • a timing circuit consisting of a 555 timer, and the output of this circuit can pull the
  • system reset line low. If C612 has shorted out, the reset line will
  • constantly be held low and the computer will never start up.
  • Since this is a common problem on Amiga 600 motherboards, and since I don't know when
  • this capacitor was last replaced, I decided to install a new capacitor in there while
  • I was at it. Because after all, it's much better that I
  • do that now, rather than this capacitor failing in a year or so, which then leads to that
  • someone with inadequate soldering skills tries to repair it,
  • only to end up ripping the pads off the board. So therefore, C612 has now also been replaced
  • with a new one, without lifting or damaging any of the pads, of course.
  • Upon inspection, I here realized that I forgot to repair the solder mask on this trace while
  • I was working with the epoxy. But that is no problem, because I can quite
  • easily add some coating onto that trace using the CW3300 overcoat pen.
  • A final continuity check is a standard procedure for this kind of repair.
  • There is absolutely no need to have non-polarized capacitors in there. That is just stupid.
  • There are in fact some downsides to having non-polarized capacitors in there like this,
  • but that topic is obviously outside the scope of this video.
  • Okay, there we go. No burn marks, no lifted pads etc etc etc.
  • The repair of this motherboard is now complete. So, without using any destuctive methods and
  • without damaging the board during the repair process,
  • this board has now been repaired using controlled temperatures and repair techniques.
  • All that is left to do now is to test the board to verify that it works.
  • Well, I actually don't really need to do that, because I already know the board will work
  • fine since I'm the one who repaired it, but I will still test it quickly
  • just as a standard procedure.
  • I would like to conclude this video with a little piece of advice.
  • As you know, I had to go through the trouble of doing a pad repair on these two pads in
  • order to repair this board. However, if the owner of the board hadn't
  • attempted to fix it up by himself, it's very much likely that the whole step of having
  • to repair the pads could have been avoided.
  • This would have meant less work time for me and less expenses for various repair material.
  • So of course, what it all comes down to is that I will now have to ask for more
  • compensation for this work than I probably would have had if the board hadn't undergone
  • such a destructive repair attempt prior to ending up on my bench.
  • So don't overestimate your abilities. If you don't have the experience and skill to perform
  • a repair correctly, leave the board to someone who can.
  • Because if you don't, not only do you risk doing more harm than good, but you might also
  • end up having to pay more than you normally would
  • when hiring someone else to do the job for you.
  • One more thing... The owner of this board told me that he tried to use the infamous
  • technique of using a pair of pliers to forcefully twist and pull the capacitor off the board.
  • Sadly, I know that there are people out there who are big fans of this "spectacular" technique
  • and actually believe that this is a good method for removing electrolytic capacitors from a PCB.
  • And to those people, I only have one thing to say:
  • Get a clue, will ya! Because if you do get a clue, you won't have to use your glue.
  • Alright, so that's gonna do it for now. I hope you will like the video if you liked the video.
  • See you later!

Download subtitle

Description

This is a demonstration of a PCB repair which involves repairing damaged surface mount solder pads as well as cleaning up corrosion from a leaking electrolytic capacitor. The pads had been damaged by the owner of the motherboard as a result of him trying to repair the board on his own.

I hope that this video will give you some ideas on how to perform a proper pad repair using the epoxy method.
During this repair, I used an epoxy that is heat resistant up to 315°C for 60 seconds.
If you ever find any kind of epoxy on the market that is specified to be heat resistant up to more than 315°C, then I would be happy if you could let us know about it in the comment section as it does appear that high temperature epoxies are quite difficult to find.

The following products was used in this video:
CircuitWorks CW2500 - Two component epoxy overcoat
CircuitWorks CW3300 - Overcoat Pen

I have made several videos before where I have had to fix up other people's mistakes that they have made during their repair attempts, and this repair only adds on to the list.
If you are not a repair technician and your motherboard has stopped working, it would be in the best of your own interest not to attempt a repair that you really cannot handle. Because there is a huge risk that you will actually end up doing more harm than good. We are of course mostly talking about board damage in the case of motherboards, but it gets considerably worse when dealing with things like power supplies, which can easily become a great shock hazard to inexperienced technicians and even result in electrocution.

As always, comments and questions regarding this repair are always welcome.

Trending videos