Solder pad ripped off

Thread Starter

john2k

Joined Nov 14, 2019
207
I'm trying to replace around 17 smd leds on a PCB. I've managed to mess up one of the solder pads where it ripped off on the positive leg of the led. I cannot see any traces at all as the board seems to be blacked out. I used a multimeter to check the resistance between all the led points and they all seem to measure the same amount. So I'm thinking I should be able to patch a positive leg of the led next to it over. Question is, how would I do that?
 

Thread Starter

john2k

Joined Nov 14, 2019
207
Picture attached. As you can see the + pad for LED 8 is torn. LED7 to the left and LED9 to the right if I measure the resistance from negative leg on one side and touch the positive legs of any of those LEDs I get the same resistance. I'm hoping the led8 pad that's torn doesn't have its own individual resistor or something. Could I use a self adhesive copper tape for PCB and just lay it between the positive of LED7 and led8?
 

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Ya’akov

Joined Jan 27, 2019
5,997
If you carefully scrape to the right of the missing pad you should remove the solder mask from the trace the the pad was connected to. At that point you use cyanoacrylate adhesive to glue the LED to the spot, and solder to the existing negative pad, then use a bodge wire from the scraped trace to the other terminal of the LED.
 

Thread Starter

john2k

Joined Nov 14, 2019
207
When the pad ripped off, it appeared it was hanging by a thin wire coming from the top before it ripped off and not from the side. What do i scrape off surface with? i fear what if it damages some other trace on the pcb
 

Ya’akov

Joined Jan 27, 2019
5,997
When the pad ripped off, it appeared it was hanging by a thin wire coming from the top before it ripped off and not from the side. What do i scrape off surface with? i fear what if it damages some other trace on the pcb
If the trace is from the top then that’s where to scrape off the mask. You can use a small knife, a spudger is you have one, even a small screwdriver. You can avoid damage by scraping very lightly first and slowly increasing the force until the mask starts scraping away, then stay at that level. It is a common practice for various troubleshooting and repairs.
 

Ya’akov

Joined Jan 27, 2019
5,997
Further work on your photo seems to suggest that the pad is attached to a via from the back of the board or another layer. The right place to start looking is below indicated by the red arrow.

5991BB9C-7510-4204-8B86-93E40BCC28A5.jpeg
 

Thread Starter

john2k

Joined Nov 14, 2019
207
I will take a close-up photo when i get home to show more detail. by scraping it, what should be uncovered underneath? whatever is underneath should i be able to solder something like the pictured wire below ?

 

Ya’akov

Joined Jan 27, 2019
5,997
I will take a close-up photo when i get home to show more detail. by scraping it, what should be uncovered underneath? whatever is underneath should i be able to solder something like the pictured wire below ?
You should see a copper pad. I am pretty sure you can see the end of the pad that came off with a via in the middle. A semicircle with a hole in the center. Scraping will expose copper, use flux if you have it, and make sure you don't overheat the pad.

First tin the exposed pad. flow a small amount of lead and tin (not lead free) solder onto the pad. Ideally a thin coat. Then make sure the enamel insulation is completely removed from a length of wire that lets you hold it (with pliers) while it is soldered to the revealed pad. Then lay it flat across where is will need to go and cut it with flush cutters to the right length.

Now solder the LED on. Flux will make your life much easier.
 

Ya’akov

Joined Jan 27, 2019
5,997
So I just gently scrape off around that semi-circle red outline area until I can see some copper ? Then apply some flux and solder a bit of wire from there to the led?
Scrape on the inside of the red line, It is the boundary of what appears to be a pad.
 

Thread Starter

john2k

Joined Nov 14, 2019
207
So I gave the area a little scrape. First I scraped too close to the edge and the little bit of copper that was exposed was actually testing back as the negative leg for the switches and LEDs. So then I scrapped a little inwards and right next to that copper there is some more copper with a very very little gap and if I test that copper and the negative pad with multimeter then the other led on the circuit lights up. My concern is that the two copper traces seem so close together that when I go to solder it, I'm worried the solder will somehow bridge both copper pads. Picture attached
 

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Ya’akov

Joined Jan 27, 2019
5,997
There are a couple of ways to deal with that. The best is probably to buy some UV cured solder mask. The most common brand is "Mechanic". It would be a very good thing to use on the finished repair anyway. You can get black though dark green is the most common.

A second is tuse anything that will interfere with the solder wetting the unwanted trace. This could be nail polish, enamel paint, etc. You need a very fine artist's brush. you can put some tape over the trace you want prevent the paint getting on it. Unless you have some thermally resistant pant, a relatively thick cover is a good idea to help it stand up tot he heat.
 

Thread Starter

john2k

Joined Nov 14, 2019
207
I have some black caliper high temperature paint. I could try and dab some of that on. What's the best way to test that even after all that I haven't accidently shorted the two?
 
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