Resistance between two unconnected tracks on PCB [in Mega Ohms]

[PsychedelicBreakfast]

I have a PCB that I was about to assemble. In fact, I've already soldered one part onto it (the AT Mega 128L). I was just curious and decided to check what the resistance was between my 1.8V rail and ground and I found an interesting result.

My meter gives me a reading in the mega ohms range. It varies between 0.5M to 40M and it doesn't really stabilize. These tracks are unconnected at this stage. The MCU is only connected to 3.3V and GND and the 1.8V rail does not come near it. I would have expected the 3.3V to Gnd resistance to be in kOhms range and the resistance between 1.8V to GND to be pretty much infinite. The 3.3V to Gnd resistance is as expected in kOhms but the other result is bewildering.

Some (but not all. Most give 0L as the meter isn't able to measure it) other tracks also give me this reading. I'm not sure what to think. I don't have another PCB on which I could test this on but from my limited knowledge, I feel that isn't this entirely normal and could cause my circuit to fail.

Any advice regarding this would be appreciated. Here's a link to my PCB:

Apologies for the not-so-good layout. It was really only my 3rd or 4th one and the most complicated that I've done. I measured the resistance between 1.8V rail and GND at P1 and the pads at C103 and C102 - all unconnected as I mentioned. The PCB is unassembled with the exception of U3 (Mega 128L).

[Jimmy]

All will be good what voltage did you test the resistance at.

[ejeffrey]

Do you have an empty board to test, or was this home built?

Try cleaning off any flux residue and measuring again.

[Jimmy]

0.5M = 500000 ohms

[PsychedelicBreakfast]

Yeah, I actually realized that flux could be the issue as soon as I hit "Submit" on my original post. I'm using a Flux Pen and that somehow got to the center of the board where the 1.8V rail goes to.

I then did a experiment - I had some AT Mega 128 breakout boards and measured the resistance between any two pins without any flux or anything. This was a clean, brand new board and as expected the resistance was 0L. I then applied some flux on two pads and measured it. The resistance shot up to mega ohms. I then used some thinner and applied that and that got rid of the conductivity.

I then did the same on my PCB and it worked. Unfortunately, it sorta smudged my silk screen but that's not a big issue as this is just a prototype. So I feel I've fixed the problem but I have some concerns:

1) What's the best way to clean the board after soldering? Unfortunately, I can't get specialized chemicals as I'm in Pakistan and it's very difficult to import such things. But if a Flux Thinner comes in "pen form", that would be quite convenient.

2) I actually had an identical board that I assembled a week ago. It stopped working out of the blue a couple of days ago. Unfortunately I haven't been able to track down the problem - is it possible that this residual flux could be the issue? I didn't realize that the resistance could be measurable with flux and worked on the board without removing some of it. A hard lesson to learn, but this is something I probably won't forget anytime soon.

[Balaur]

1) What's the best way to clean the board after soldering? Unfortunately, I can't get specialized chemicals as I'm in Pakistan and it's very difficult to import such things. But if a Flux Thinner comes in "pen form", that would be quite convenient.

Isopropyl alcohol is usually the best way of cleaning flux. It's reasonably cheap and mostly available (may be subject to local regulations).

Else, some kind of pure, unflavored alcohol could be also used in a pinch. That's not optimal, but I've successfully used clear vodka (don't judge, OK?) to clean up a board once.

Otherwise, please also consider the use of a no-clean flux.

Cheers,
Dan

[PsychedelicBreakfast]

Thanks Dan. Do I need to mix isopropyl alcohol with water or should I just dip a toothbrush in it and then rub the toothbrush on the board?

Secondly, should I just leave the board to dry after this?

And is damage to components a possibility with this method i.e. should I keep my toothbrush away from components?

[Balaur]

Thanks Dan. Do I need to mix isopropyl alcohol with water or should I just dip a toothbrush in it and then rub the toothbrush on the board?

Secondly, should I just leave the board to dry after this?

And is damage to components a possibility with this method i.e. should I keep my toothbrush away from components?

No, you just use undiluted isopropyl alcohol to avoid any impurities (such as minerals found in the non-distilled water). You should put some isopropyl into a small recipient and dip the toothbrush there.

The alcohol will dry quite quickly without any traces. It doesn't damage any of the normal stuff you will find on a typical board.

Cheers

[FreeThinker]

Thanks Dan. Do I need to mix isopropyl alcohol with water or should I just dip a toothbrush in it and then rub the toothbrush on the board?

Secondly, should I just leave the board to dry after this?

And is damage to components a possibility with this method i.e. should I keep my toothbrush away from components?

Little Tip:-
My Toothbrush + Clear VodKa = OK 
My Toothbrush + isopropal alcohol = BAD 
Use spare one or Wifey's 

[Simon]

Thanks Dan. Do I need to mix isopropyl alcohol with water or should I just dip a toothbrush in it and then rub the toothbrush on the board?

Secondly, should I just leave the board to dry after this?

And is damage to components a possibility with this method i.e. should I keep my toothbrush away from components?

Little Tip:-
My Toothbrush + Clear VodKa = OK 
My Toothbrush + isopropal alcohol = BAD 
Use spare one or Wifey's 

gee can't you afford a spare toothbrush, or don't you change it now and then and use the old one - or do you just want an excuse for vodka 

[Balaur]

My Toothbrush + Clear VodKa = OK 

Come on, cut the middle man and just brush your teeth with vodka! In my home country a popular remedy for a sore tooth was to keep some alcohol in your mouth. Some people were able to keep astonishing amounts of liquor. Not for long though ... traitorous throat!