What contacts should I clean with ISOPROPYL ALCOHOL?

I will explain what happened with my iPod Touch 3rd Gen.

After one year of use, one day the screen got weird, with white and black strips and some other colors. The iPod did NOT fell into water, the problem started with no apparent reason.

I tried to restart and restore later with no positive results.

After two days I turned it on and, for my surprise, the LCD worked 100%, with no more strips on it, but the digitizer had stopped working...

After some days, I decided to order a new digitizer from eBay (actually I bought two).

When arrived, I used the iFixIt guide to open and replace the digitizer.

RESULT: the first and second digitizer did NOT work.

After read some articles and reviewing some related questions at iFixIt, I saw that some guys were trying to clean the contacts using ISOPROPYL ALCOHOL.

So, my great question is: What contacts should I clean with ISOPROPYL ALCOHOL? All of them?

Hope you guys can assist me on it.

With best regards,

Leonardo Ramos.