The subject of today's post is an unfortunate ASUS R510L notebook. This one was in pretty bad shape, as it was the victim of a liquid spill. The residue from the spill caused a short circuit which overheated and burned up one of the input MOSFETs. It also caused a pretty big crater at the site of the short and the failure of the EC and LAN chips, but today I want to concentrate on just the input MOSFET and how I repaired it.
Since the failed MOSFET also left a pretty good crater, that left me with nowhere to attach its replacement. It was also necessary to replace two nearby capacitors which burned up along with it. There are a couple ways I could have gone about this (a piece of FR4 to mount it on, for example), but I came up with a technique that worked well and was actually quite fun to do. I decided to etch some new pads by utilizing the existing copper on the board. I went with etching because cutting the copper runs the risk of exposing lower layers (e.g. a ground plane). That's a disaster waiting to happen once you add solder to the mix.
This is what I had to work with...
Okay let's clean this up a bit...
Yikes! Looks like the MOSFET split right in two. I removed it and also removed every bit of the charred PCB underneath. That stuff is conductive and had to go...
Welp, I guess we know where the replacement MOSFET isn't going to go. Time to make some new pads. I scraped off the solder mask over the copper I wished to remove. I needed the pads to be big enough to solder the wires and capacitors to later...
The pictures get a little distorted around the edges at this point since I had to resort to holding a magnifying glass up to my phone. Anyway.. now comes the really fun part. The etching...
I used a distilled white vinegar/hydrogen peroxide/salt mixture to do the etching and created a "bowl" for it to sit in using some hot glue. Hydrogen peroxide can come in various concentrations, so I found it easiest to just start with a 50/50 mixture and then tweak it until I got a nice reaction with lots of bubbles. Enough salt should be used so that some remains undissolved in the mixture. It's also a good idea to cover any metal other than the copper you are etching with some hot glue. The mixture may react very mildly with those other metals, but not nearly as much as it does with solid copper.
After the etching was complete (I believe it took about an hour), I soaked up the etching liquid and then used rubbing alcohol to easily remove the hot glue.
The next step was to expose the copper where the new MOSFET would go...
I also exposed a little bit of copper for one of the capacitors, but that's not shown in the picture. After soldering on two new MOSFETs (I wasn't going to trust the second one just because it wasn't blackened), the two capacitors, and wires to replace the missing traces, I covered everything with an MG Chemicals black overcoat pen. It's not the best thing in the world since it wipes away too easily, but it's better than nothing...
The entire image album can be found here:
http://imgur.com/a/FJUz8A video of the etching mixture in action can be found here:
I just wanted to document this experience in the hopes that it would give others some ideas to utilize in repairs of their own.
That's all for today. Take care everyone.