Soldering Woes

[krumpelstiltskin]

Hi folks. I'm devistated.  I've made a few mistakes that could cost me the board that my friend gave me. It holds high sentimental value, so I'm trying everything to save it now.
I desoldered my PCB in order to lubricate all my switches and stabs, and to add a layer of polyethelyn to deaden sound, the J and H keys did not work when I got everything back together.

The solder looked very good so assume it was the switch, and desoldered the two.  (This was my first mistake, as I could have just checked the switched with the multimeter) The switches were fine so I soldered them back on.  Still broken J and H. Desoldered the J again and BAM, the pad came right off. So now I have that do deal with.

It was only sometime after that that I noticed some tiny black thing that was crooked.  I touched it and it came right off in my hand. Here's a picture of the mess:

https://drive.google.com/file/d/1RAHJs2tnIjniwa3Tlm75qSAMPq6clVqA/view?usp=share_link

You can see that D36 has a tiny box bridging two terminals, while D35 and D50 do not have one. So, I'm guessing that this was the problem all along. I must have knocked off the black box bridging D50, before the D35 came off in my hand.

I have a few questions for you folks:

1. What are these little black boxes?

2. Can i just solder these back on (I will have to buy a microscopic soldering iron)?

3. The pad on the J is gone and I need to fix it.  I've seen videos where people scrape off a layer to get to copper trace, but in this case I don't see where that trace would be going.  Is the trace on the other side of the board?

[butre]

the "black boxes" are diodes.  I couldn't tell you what model without a higher detail picture
you can just solder them back on.  YouTube search "SMD soldering" for a tutorial.  do note that they are directional.
you should have access to the trace from the other side of the board.  I doubt it's a multilayer PCB.  you most likely need a jumper wire

[Sup]

You can solder those SMD switches back on. You need some flux, tweezers and a soldering iron where you can adjust the temperature to 240C to avoid burning the pad. Since you lifted the pad J i think you use too much heat. Check the orientation of the other SMD diodes there is a line on it that should face the same way as the other diodes.

You don't need to scrape off a layer to get to fix it. You can also bridge the switch with a wire but, you would need to follow some guides that tell you how to find it or have someone point it out for you. I don't have experience with it.

I don't know who made this PCB but, some of those diodes barely have solder so no surprise that two of them fell off. But, yeah its fixable for sure.

[krumpelstiltskin]

Thank you both very much for the great advice.  I see that diodes are used for keeping the current in one direction.  I'm guessing that this would be to avoid a short if I spill a liquid on the keyboard?

If that is the case, do you recon I could just bridge those terminals with some wire, or with solder, instead of putting diodes back in those places?

I would lose the water proof feature, but at this point not only am I short one diode, but those things are microscopic (much smaller than the tip of the brutish soldering iron I have).  I guess I would have to buy a much more refined soldering station to do this.

I'll check the youtube tutorial!

[krumpelstiltskin]

I'm lucky this time it seems.

I replaced those two diodes with a tiny bit of wire and solder, and everything worked!
So it seems that the J does still have enough connection with the circuit to work.  fhew.

Thanks folks.
I'm extremely happy with the way everything turned out.  Lubing the switches really made a huge difference for feel and sound. Adding that PE layer also helped a lot. Zero resonance or spring ping now (KUL ES-87 with MXclear was so insanely noisy, even though I don't bottom out).  The keyboard sounds and feels extremely solid now. Love the deep pitter patter of typing on this thing.

[Maledicted]

Glad to hear you got it fixed. The diodes are mostly for n-key rollover. I usually bypass them when I'm fixing a board as well, since I think it is a gimmick that really is never needed. The Model M only has 2-key rollover and there are still crazy people that game with them. Gaming is one of the few situations in which I could conceive of where rollover even matters.

I'm lazy, so when I fix dead switches I solder a wire to whichever leg of the switch still has continuity with the rest of the matrix, plug the keyboard in, and touch that wire to the opposite pins of nearby switches until it outputs the right key, desolder my test wire and solder a permanent jumper from the leg with the bad trace to the leg I tested against nearby.

[krumpelstiltskin]

Ahh... that jumper trick is a great idea.

So the diode has nothing to do with waterproofing.  I played around with rollover and see that now if I hold the four keys hjkl down simultaneously it acts as though I've depressed <CTRL>.  I'm fine with this, since I will literally never do that again