Laptop wine spill - motherboard repair

[Ian.M]

The plated barrel of the hole is copper, but oxidised/corroded copper (or residue from corroded tin plating) wont take solder unless its cleaned to a bright metal surface. Its very thin to start with, and if its gone O/C then a full thickness annular ring has corroded away, so there isn't likely to be much remaining metal left down the hole.

Also, most solders shrink as they cool so will tend to pull away from any tenuous contact that is made with the edge of an inner layer, and there isn't likely to be enough joint area to resist the forces involved.

If the corrosion is localised to the corner between the pad and the barrel as described and any inner layer connection isn't damaged, its salvageable as described by soldering a wire or rivet through the hole.  The same approach works for fully corroded vias if there are no inner layers.   

However, how are you going to connect to an inner layer track without milling the board to expose it when the only good metal left is an annular ring if you are lucky, or just the track end on one side of the hole, recessed into the laminate 100um or more from the hole wall, covered with a layer of corrosion byproducts?

[Lomax]

Basically, this is my mental image of what's going on with the vias. In the second "corroded" case, I would have thought it should be possible to form a solder bridge between the via and the tracks on the top. In the third case, I'm obviously SOL, but it's worth a try, no? Or have I fundamentally misunderstood something?

[Ian.M]

How are you going to clean up case#2 enough to get solder to take on the corroded end of the copper barrel?  Also, it isn't solid and isn't an insert like your diagram so the only area of metal to take solder is the edge of the thin ring of plating partway down the hole wall. As molten solder has a fairly high surface tension it tends to ball up and you are going to find it very difficult to get a very thin edge right down in an inside corner of a hole smaller than your iron bit to even take solder.   Your best bet for case#2 is to clean out the via right through very carefully with a very fine drill bit held in a pin vice using the least possible twisting motion, working from the good side and taking care not to cut into the plated barrel till you can get a fine strand of tinned wire through it and solder the wire to the pads top and bottom without covering the hole.  You should then be able to get the solder to wick through the hole from one side aided by the wire.

BTDTGTTS to fix salt-water damage, and the oldest such repair that I still know the status of has been in working order for nearly eight years since I did it.

[Lomax]

How are you going to clean up case#2 enough to get solder to take on the corroded end of the copper barrel?  Also, it isn't solid and isn't an insert like your diagram so the only area of metal to take solder is the edge of the thin ring of plating partway down the hole wall. As molten solder has a fairly high surface tension it tends to ball up and you are going to find it very difficult to get a very thin edge right down in an inside corner of a hole smaller than your iron bit to even take solder.   Your best bet for case#2 is to clean out the via right through very carefully with a very fine drill bit held in a pin vice using the least possible twisting motion, working from the good side and taking care not to cut into the plated barrel till you can get a fine strand of tinned wire through it and solder the wire to the pads top and bottom without covering the hole.  You should then be able to get the solder to wick through the hole from one side aided by the wire.

BTDTGTTS to fix salt-water damage, and the oldest such repair that I still know the status of has been in working order for nearly eight years since I did it.

Ok, many thanks. Some great tips there. Yeah, I know vias are tubes, and not solid cylinders; I just couldn't be bothered to reflect that fact in the (very quick) sketch. I'll clean things up and then take another look under the microscope - hopefully the corrosion is not very deep. I should be able to locate the via on the reverse so I can measure the link.

Edit: I've updated the sketch to reflect the tubular nature of the vias.

[Lomax]

Ok, so it seems I've completely misunderstood the nature of those two "vias" - after removing the components and carefully cleaning the "holes" with sharpened toothpicks(!) there is nothing at all left in one of them, and just a tiny speck of copper in the center of the other one. Underneath is just the PCB substrate. If these connected with any other layers it must have been a layer so close to the surface that it's basically at the same level as the base of the topmost tracks. Which doesn't seem to make any sense. Just how thin are the layers in a laptop motherboard? I think I'm about ready to give up on this laptop now but thought I'd post these photos in the hope that someone might explain what it is I'm looking at. There certainly are plenty of "normal" vias on the board, which although covered by the solder mask are clearly tubular structures (as confirmed by holding up the board with light from behind). Then there are these other things, which appear like little domes of solder - it is two of these that have corroded away (since they were not protected by the mask). Three uncorroded examples are also present in the pictures. But what are they?

[Ian.M]

Prepreg layers of FR4 (or other material) in a stackup can be very thin, down to    approx. 60µm.  We were hoping you weren't seeing a blind via that only goes down a few layers.
e.g. http://www.multi-circuit-boards.eu/en/pcb-design-aid/design-parameters/blind-via-buried-via.html

However as there is absolutely no evidence of a hole in the laminate surface, its almost certain that they are simply testpoints.  In that case, simply bridging them with a strand of fine tinned wire, bent to shape would be an adequate repair.

I think you've got lucky!

[Lomax]

its almost certain that they are simply testpoints

Thanks, that does sound somewhat promising! But having looked at the board I'm not so sure - there are plenty of exposed testpoints around but these have all been gold-plated (apart from J5 in the photo which I partially tinned). And why would you put a blob of solder on a testpoint? There's something odd about those holes too; if there had been a copper pad underneath the solder I would have expected to see something akin to the other corroded pads - instead there's a perfectly circular hole pit.

Edit: Actually, they do look rather like the other pads.

[Ian.M]

The only circular hole I can see is the hole in the solder-mask for the possible testpoint pads.  Why some are solder covered and others are gold plated, is anyone's guess.  The one by the D15 legend is clearly *NOT* a via. Scrape the little bit of copper remaining in the one on the track that goes under the J5 legend.  I suspect it will come off clean leaving continuous laminate surface under it which will prove that's not a via either.

Then its just a case of guessing the value of the caps, replacing the components, patching the tracks, and hoping there isn't any unseen electrical damage. 

[Lomax]

Thank you, that's very helpful! I've never done this kind of work on a motherboard before - I've replaced leaky electrolytics, and I SATA modded my Thinkpad T43p, but this is pretty extreme for me. Which is another reason why I'm so keen despite the unremarkable machine it's for. I'm learning here!

Why some are solder covered and others are gold plated, is anyone's guess.

Could it be that the tinned ones are test points for board continuity and trace resistance/capacitance used during board assembly, and the gold plated ones are for testing populated boards? That would explain why they are tinned; they would have served their purpose at soldering, and their somewhat peculiar locations. I confess I know nothing about the manufacture of multi-layered boards, except that they have multiple layers - sometimes connected with "vias" I didn't even know it was possible to create "blind" vias - an interesting engineering problem! I'm guessing such vias would be inserted when only some board layers have been added?

Then its just a case of guessing the value of the caps, replacing the components, patching the tracks, and hoping there isn't any unseen electrical damage.

I've already identified the two components in area "A": a dual schottky diode and a 100k 0402 resistor. The capacitors in area "B" I'm going to desolder, and see if I can measure the two that still have somewhat intact terminals (by tacking some Kynar wire onto what's left). If not, I'm going to go with the suggested 100nf & 1uF.

Edit: I don't know what type of solder they've used but I had to set hot air temperature to 500°C to get it to melt! Seemed unusually stubborn, even for a lead free composition.

[Lomax]

I have managed to measure two of the capacitors now; the larger one (0603) measures 4.8uF and the smaller (0402) one with parts of the terminals intact measures 55nF. So I'm going to assume that I need one 4.7uF 0603, one 56nF 0402, and one 100nf 0402. Although it is impossible to measure the second smaller cap, their position on the board (both going between the same two tracks) strongly implies a decoupling/bypass role, in which case two caps of the same value would make no sense. Sound reasonable?

[Lomax]

I think I'll let the pictures speak for themselves here... 

[vze1lryy]

Good work man! Amazing stuff. You killed it.

These boards are designed to deal with burning hot chips that don't belong in laptops in them, so with a regular hot air station you will need a lot of heat to remove anything. I use 375c on mine for most work and go up to 400c for large ICs that have a bunch of corrosion on them on the JBC JT-A.

The same temperatures and airflow I use to do standard work on these boards, will leave the solder molten for over 20 seconds on a regular board. 

[Ian.M]

+1 for good work.   If Louis says you done good, you can take that to the bank.

[Lomax]

Good work man! Amazing stuff. You killed it.

These boards are designed to deal with burning hot chips that don't belong in laptops in them, so with a regular hot air station you will need a lot of heat to remove anything. I use 375c on mine for most work and go up to 400c for large ICs that have a bunch of corrosion on them on the JBC JT-A.

The same temperatures and airflow I use to do standard work on these boards, will leave the solder molten for over 20 seconds on a regular board.

Why, thank you! I am flattered It was only after watching some of your videos on YT that I felt confident enough to have a stab. Thank you for sharing your knowledge with us amateurs.

I have my hot air tool set @ 380c, and use leaded solder paste - this has worked well in the limited amount of SMD soldering I've done so far, apart from on this laptop where I struggled to desolder until I raised it to 500c. I'm thinking it might be the presence of a thick tin oxide layer that made it more difficult to melt. Re-soldering was done at a lower temp. The lead-free solder didn't end up as shiny as it normally does, probably due to residue on the board, but the joins look healthy under the scope. By far the most problematic step was repairing the track by "J5": it was next to impossible to position and hold the wire (0.1mm dia) in place, and get the solder to form a good bond with the track. I suspect there must be a better way! I know it looks blobby, but he amount of solder used is minute - I had to use the tip of a pin to apply it (seen in photo on the left)...

Five days later and the laptop is still working just fine

[Lomax]

+1 for good work. If Louis says you done good, you can take that to the bank.

Cheers!