One of the keys on my Rigol DS1054Z oscilloscope wasn’t working. I opened up the scope and this is what I found. Is that K5 pad corroded? Other than sanding it is there any other way to restore functionality? I tried cleaning it with IPA and then Acetone but that didn’t work.
Clean it as Tony suggests (pink or non-abrasive white eraser) and it looks like you got water into it. Don't do that. Maybe the water got in deeper and damaged something else - you should check that out. IPA alone should have cleaned it (assuming it is proper high purity isopropanol and not drugstore rubbing alcohol which is adulterated with additives).
Do NOT sand it as you will destroy the super-thin layer of gold plating on the contacts and it will never be reliable again.
Generally the silicone pad that has the membrane keys molded into it is designed such that it hugs down close to the PCB and helps to seal the switch contact area. The pad should still be designed with narrow openings between the pockets under each key so that when a key is pressed the air under it can can displace to adjacent pockets. It is possible that the silicon mat did not stay in close contact with PCB in the K5 area and let in the heavy moisture laden air in your environment.
I'll also note some other things as well to ponder on:
- The fiducial mark right next to K5 appears to have the same discoloration as the K5 contacts suggesting that what ever has happened was area oriented rather than just key/button related.
- I see that the PCB has lots of fairly large via holes that are not plugged with solder mask. These holes could tend to defeat the sealing that a silicon membrane pad is meant to achieve.
- It is possible that the PCB itself was compromised during the manufacturing process and some contaminants got on that K5 area before the gold plating process and the thin gold plating did not 100% cover the contacts. I have seen finger print images on PCBs in various areas including in the copper etch, silk screen or solder mask. There is a possibility that you may have to ask the manufacturer for a replacement PCB.
- There is also the possibility that the corrosion of those contacts may be due to some type of contamination on the silicon pad itself. Hard to really know without looking at the membrane very closely.