It's solder mask, put a piece of electrical tape in that spot to keep things from arc / grounding against it and you should be fine.

Personally, I wouldn't be thrilled to purchase a board and find sticky funky tape stuck to it. There's no reason anything would need to be covered in tape.

What would that do? Insulate that one spot from a short; verses the hundreds of pins already there? A properly mounted board shouldn't have any problem staying away from things shorting it.

If it doesn't have any corrosion, you could do a couple options, that I could think of. Just leave it, since it's tinned. Or some people may say put a clear of some type over it.

Personally, If I was worried about it, then I'd probably get a little tube of green solder mask and touch up the board. But in all reality, as long as it wasn't corroded, I myself would probably just leave it be.

The solder mask is of the greatest use when the board is manufactured. It "masks" parts of the board so that solder doesn't stick to those areas.
Nothing to worry about.
Leave it be.
--
Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact
http://www.PinWiki.com - The Place to go for Pinball Repair Info

That is 100% - absolutely NORMAL!

Have a look behind the other boards, chances are they will have similar appearance.

Ignore it.

I believe the solder mask on those types is to primarily help protect against oxidation of the board, as the copper traces under the mask have already been tinned over, from the wave solder, during manufacturing. It is not bare copper under that part of the mask (as you can see). At least for that section.

But yes, solder mask layers are also used to help other traces and things from having solder bridges get across traces, pads, vias, etc. of course.

And obviously top, confomal coats aren't something we would typically see in pinball. So no need to go into that.

If it was bare copper, I may have had more concern to seal it. But since it is a section that is tinned already, I agree that it shouldn't really be a concern, as it is.

Noticing corrosion or oxidation would be a different story, obviously. But that board looks clean to me; from what I can see.

Here's an example of a time where a solder mask would have been GREAT!
It took me 4 hours to find this micro-bridge. Argh.
--
Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact
http://www.PinWiki.com - The Place to go for Pinball Repair Info

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The wrinkling/cracking/flaking on these old boards is normal.
Chris was right that the solder mask intent is to keep "additional" solder from adhering to traces during the fabrication process. This would cause bridges, solder-cicles and other mean and nasty things. Back then the traces already had tin plating on them so there was no oxidation (ok, not 'none' but 'not much').

The reason these boards have solder masks that look so "wrinkly" and often cracked/flaked off was due to how the bare boards were made.
Many years ago - they had a different process for making bare boards.
1 -- etching/drilling ... basic fabrication
2 -- Entire board would be tin plated
3 -- Solder mask would go on
Problem with that order is the tin plating is beneath the solder mask. Many times these boards were 'wave soldered' to attach components to the board. As part of this process - the entire board would get hot enough to melt the tin plating beneath the solder mask. The melted plating beneath the mask would cause the mask to wrinkle and crack.

New boards - they changed the process:
1 -- etching/drilling ... basic fabrication
2 -- Solder mask would go on
3 -- Exposed board would be tin plated
The boards now have 'solder mask over bare copper' (abbreviated on documentation as "SMOBC"). The copper under the masks today is bare copper - the mask is there to seal the copper traces AND to keep additional solder from adhering during soldering process.
Now when heated up, there is no tin plating to melt under the solder mask -- no wrinkling or cracking.

Excellent explanation G-P-E and much appreciated.
--
Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact
http://www.PinWiki.com - The Place to go for Pinball Repair Info