Repairing a .5mm trace on a 4.5x35mm board, successfully.

Thread Starter

Ya’akov

Joined Jan 27, 2019
6,851
This isn't earth shattering but I am so pleased with my success I wanted to share it. This is long, and potentially boring but I'd read it if someone else had written it so I though someone else might read it if I did.

I bought some LiPo protection boards, and they are quite small (4.5x35mm as it says in the title).

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The little board, with a transistor for scale.

The vendor goes out of their way to warn:

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Which I took to mean, "We shouldn't have to tell you this but if you hook the battery to the output you will destroy the board, and, no, your board is not broken and that's the reason your LiPo with less that 2.4V charge won't cause it to output anything, charge your battery please."

Well I took due note of these two common (user induced) failure modes and set my mind to not making those mistakes. I almost managed to do it, too. Unfortunately, the LiPo cell I was working with had a tab spacing a little to narrow and so the width of the tabs put them dangerously close to the two, inner P+/- pads. I was working out the alignment when... it slipped and something contacted something else (sorry to be vague, I didn't see it I smelled it) and I knew I had not acted "strictly according to the diagram".

Inspecting the three chips (a DW01 protection chip and two 8205A dual MOSFETS) on the board I couldn't find any cracks or burns. The passives also looked fine. So, I tested it and it had become a small resistor with a ~.2V drop and no other functions. The boards were cheap and so I chalked it up to experience and set it aside.

I was cleaning up the bench and noticed the defective board and decided to give it a once over under the microscope. I looked carefully at the silicon but suddenly noticed that a tiny .5mm trace from pin 8 one one of the MOSFETs near as I could tell (a common drain, along with pint 1) to pin 1 of the DW01 (Vss) was well melted and partly just gone.

I didn't need to repair it, I had more and it wasn't an expensive board, but it seemed a challenge. So, I switched the Hakko's tip to the tiny T15-C1 and formulated a plan. I decided to try to make a bodge wire out of 32AWG magnetic wire. I suppose it's not really a bodge since it's a repair but it's the same spirit.

I put the board under the scope and cut loose the flapping bit of trace, then cleaned the mask on the on the MOSFET side to make a pad. Then I applied Amtech NC-559 tacky flux to the edge of the board and tinned the make shift pad, it wasn't too hard and nothing lifted from the board. I also added some lead based solder to the DW-01 leg so I wouldn't have trouble with tacking the magnet wire.

Next was somehow getting that (what looked like) huge wire onto the board. I cut a piece about 25% too long and scraped the varnish off one end with an Xacto. Carefully positioning the end flush to the board, I soldered it on. Every tiny twitch of my hand was like an earthquake and I needed the wire to stay in place. Somehow, it worked. The rest was relatively easy. Once the end was held down I could route it and older onto my new pad almost as easily as soldering a normal SMD package.

I trimmed off the excess with my Xuron 9130 snips which are super sharp, super point, with a scissor action. It's almost like having metal cutting iris scissors. The, using Loctite AA-349 UV cement, I bonded the wire to the board and put a protective coating over it. For no good reason other than I wanted to, I added a layer of green Mechanic HY-UVH900 UV cured solder mask over the Loctite. And, I even tempted fate by doing all that before testing it!

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The repair is on the red box, inset in blue box for scale.
But, I hooked B+/- to the power supply set to 3.7V and P+/- to the DMM and, upon pressing the on button on channel 1, it worked! 3.7V in 3.69 out. Still needed one more test, so I slowly lowered the voltage and at 2.4V the DMM said 6.99mV and would have said "0" if I had pushed Rel before I started.

Unfortunately, I don't have any before or progress pictures because I didn't intend to write this up. I know the is long and about almost nothing but if you've rad this far I hope you enjoyed it, and if I am not talking to you because you didn't find it interesting, that's OK. I did and maybe someone else will too.
 
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