After removing the 3 electrolytic smd capacitors from a car ECU, I found some leakage, although not sure if this is the main culprit of the problem in my ECU, but I'd like to clean them. I tried 99% isopropyl alcohol but didn't remove the leakage, can I use sanding to remove the leakage from the board?
Others have suggested it may be glue. It's not. That is damage from leaked electrolyte. I've replaced plenty of leaking capacitors, that's exactly what it looks like when the electrolyte starts to eat away at the board.
The little dots look like it's gotten into some small vias that were under the caps, hard to say for sure without a really close look at it. If they're just going between ground planes for heat dissipation or lower impedance that might not be a big problem, but if it gets into a via that carries a signal, especially on a multi-layer board where connections to inner layers could be lost, it can be trouble.
I would not use sandpaper to clean it. What I've always done is clean it with IPA and an acid brush with the bristles cut real short so it's got some scrubbing power. I also use a small screwdriver with a sharpened blade and some picks to carefully scrape away anything scrubbing doesn't get off such as blackened solder mask, and a pencil eraser. I use a soldering iron to tin any copper that gets exposed in the process, but you have to be careful not to overheat it to the point where the traces delaminate.
At first I thought those might be glue (used in production to keep the components in place before soldering). If they were I'd expect to see them applied as a single or double "dot" instead of a random scattering. Anyway. Let's assume they are not glue for now.
Even if it's glue or leak you don't have to remove it.
... can I use sanding to remove the leakage from the board?
Personally, I wouldn't. Judging from the picture, it looks like there's either a chemical or a thin layer of golden plating on top of the copper to prevent corrosion. If you use sanding there's a risk of removing the protective layer and therefore a risk of exposing bare copper to possibly humid air. This will lead to corrosion.
If you really want to get rid of them you can start with thin sandpapers (e.g. 1000-1200 grid). As I said, I wouldn't. You can also try flux cleaners. We are using these to remove lots of different types of unwanted things (permanent pen, solder flux, etc).
