Leaving the photoresist on after etching

[akis]

I noticed that if I do not strip off the photoresist after etching, it provides nice copper protection. But is there a way to remove it just on the pads so I can solder the components? I have noticed that it is possible to apply solder straight on over the photoresist, but it takes time and is much harder than trying to solder on clean copper. Any ideas?

[Balaur]

I mainly use Bungard pre-sensitized boards and they describe a technique to use the photoresist as protection/solder mask with an extra expose/develop step.

Extract from this manual:

Bungard photoresist as soldermask

The fact that the photoresist can be exposed and developed several times, can also be used for selectively removing the resist from solder pads. This way you combine perfect solderability on the pads with protection of the copper by the remaining photoresist on the conductor tracks.

Step 1: Etch your ORIGINAL BUNGARD PCB as usual.

Step 2: Expose and develop the positive photoresist after etching again, this time using a negative film leaving only your solder pads free.

Step 3: Now tin the open solder pads with BUNGARD SUR-TIN (chemical tin). The photoresist remains on all conductor tracks and protects them. It also serves as a thin solder mask.

This approach is not widely known, but also leads to excellent results at no extra cost!

[akis]

I had no idea that the photoresist can be developed again (or at least the Bungard one). Would that mean that you'd have to develop and then etch the board in a dark room so as to be able to expose the board once more?

[Balaur]

... a dark room ...

Typical photoresist is most sensitive at 400nm. Direct sunlight is obviously not recommended.

The following picture from this site would suggest that LED and incandescent bulbs could be OK. CFL are problematic?

[akis]

I was told by Mega to be quick when working with the protective film removed, as sunlight even through windows is enough to compromise the photo resist. I am not sure about fluorescent tubes though and other sources of room lighting. Developing and etching are not very difficult processes so I suppose I could have a yellow or red or whatever low wattage lamp just for this purpose. I will try the Mega photosensitive boards tomorrow to see if they can be redeveloped.

[nctnico]

I strip the photoresist by exposing the PCB again (for a couple of minutes) and then use the same developer to strip the resist. Much nicer than using ethanol or acetone. After etching I spray the PCB with SK10 (from Kontakt Chemie) which is a flux and protective coating in one product.

[akis]

This SK10 looks good.

[hlavac]

Note that sides of the traces will not be protected and solder can still get to the traces, so its not much of a solder mask....

[akis]

Note that sides of the traces will not be protected and solder can still get to the traces, so its not much of a solder mask....

Which one the SK10 or the leftover photoresist?

[nctnico]

The leftover photoresist!

[madires]

I strip the photoresist by exposing the PCB again (for a couple of minutes) and then use the same developer to strip the resist. Much nicer than using ethanol or acetone. After etching I spray the PCB with SK10 (from Kontakt Chemie) which is a flux and protective coating in one product.

SK10 works great. I'm using it for over 20 years and my old PCBs still look fine, no visible corrosion.

[akis]

I strip the photoresist by exposing the PCB again (for a couple of minutes) and then use the same developer to strip the resist. Much nicer than using ethanol or acetone. After etching I spray the PCB with SK10 (from Kontakt Chemie) which is a flux and protective coating in one product.

Why? Dipping it in the stripper takes like 10 seconds and it's all done, why bother re-exposing (ie reusing the UV light box) and then weakening the developer? I reserve the developer for "surgical" operations, and the stripper for "mass effect"