Friday, 2 May 2014

Testing homebrew solder resist with enamel paint

UV curable solder mask is great stuff. But when you misplace your tub of it, and a replacement coming from China is going to take anywhere between 15 and 30 days, and you've a project which - for once - actually really needs solder resist to be reliable, it's time to get creative.

Our laser-shades project (ok, it doesn't use lasers, more LEDs, and they're not really shades, more like blanked out 3d glasses) has some really fine 0.25mm traces, squeezed between the pads of a WS2812 6-pin LED. Now we've had experience of working with fine pitched, laser-etched PCBs in the past, and it wasn't pretty. It's one thing to successfully etch a PCB with fine traces - and quite another to successfully solder it up!

So this time, we figured, before we made a mess of the project with our soldering irons, it'd be worthwhile making a solder mask. We've also had some success in the past, using nail varnish as a solder resist (however clumsily applied). This time, rather than using nail varnish, we thought we'd give Humbrol enamel paint a try. Not only does it usually give a nice, solid coat (the nail varnish looks a little translucent at best) but the local hobby store sells a massive range of colours. We went for the most lurid, garish ones he sold (the orange and dark green are actually fluorescent, which doesn't come out too well in the photo below)

The idea is to apply some peelable solder mask (Chemask from Farnell) over the pads, then paint the entire board in enamel paint. Then the peelable mask can be pulled away from the pads, leaving the shiny copper behind, while the rest of the board will be coated in enamel paint (a cheap solder resist).

To apply the dots of mask on each of the pads required a stencil. We used some plastic-laminated thick paper and laser cut the pads, once each had been resized to about 75% (to allow for a bit of "spread" during laser cutting, and to make sure that the mask didn't extend beyond the outline of each pad).

Lining up the stencil with the actual pads was a bit tricky at first, but after applying some double-sided tape to the back, and some careful re-alignment, we finally got a half-decent match

Aside from a bit of smearing as the stencil was lifted off, the result was pretty good. As it wasn't critical that the pads affected on the right-hand side of the board were well masked (it's mostly the bottom-left and middle-left pads that really needed the solder resist to avoid bridging during soldering) we decided this was good enough!

Then we hit our first disappointment (and, dear reader, so might you, as we forgot to take photos of the disaster!). The enamel paint remains sticky for a very long time, taking 6 hours or more to dry fully. It is also quite tacky during application. The very real danger with this method - which we only just realised - was that as the enamel paint is applied, is starts to lift the latex-based mask on the pads, and seeps underneath. Not good!

So Steve did some tests with acrylic car paint. Unconvinced that it would be heat resistant, he tried soldering a couple of spots on some painted copper board. Surprisingly, the acrylic held firm and acted as a solder mask on the un-exposed areas.

When we tested nail varnish as solder resist, we deliberately tried to burn the varnish away with the iron, to see how it holds up under extreme heat. When soldering using paste and SMT components, we tend to apply the iron only for as long is necessary - so the acrylic paint doesn't need to be as robust as enamel-based paint, just good enough to withstand hot solder paste (not the iron directly) for a short period of time.

Inspired by this, we re-applied the latex mask and sprayed the board with Halfords acrylic car paint. Then we a second, and similar, disaster

As the latex mask was lifted off the pads, it ripped the acrylic paint off in chunks, exposing the copper traces we were so keen to keep covered over. A new approach was needed - it was nearing midnight (so the deadline for success was looming over us!). The simplest - and most obvious - method worked quite well:

The entire board was sprayed with acrylic car paint, and under a bright light (it really hurts your eyes to do this under ambient light) the raised copper pads were scraped clear using the tip of a large drawing pin. While the final result is not particularly neat, it does look serviceable:

That was it for another BuildBrighton session. We've now got one PCB we can have a got at soldering up. And a number of methods not to use for making homebrew solder-resist on the cheap!

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