Solder mask, solder block, screen resist

Solder mask, solder block, screen resist

So, why do we need a solder mask? Well, when we first used circuits to interconnect components the majority of the work was hand processed using soldering irons. The heat was applied to local areas and the chance of shorts was small. As volume use increased, the PCB industry moved on to wave soldering and this is when the absolute need for a method of preventing solder drag between the tracks was needed. The idea was to cover all of the copper except where the pads were needed to solder components in place.

Initially, with discrete components, the circular pads with lots of land around them allowed a simple silk screen process. So, a silk screen was made with pads or circles of emulsion over the PCB pads. When the ink, usually UV cured, was screen printed down, it covered everywhere except the pads and then went down a curing tunnel, and there you go. Associated problems included bleed onto the pads where by a slight covering of ink got onto the pads preventing full solder. Misregistration caused by screen stretch and age so the pads may end up like crescents of the moon.

Another big disadvantage is that a screen was required for every mask so often, a PCB would require 2 mask screens and 1 legend, the options were to have a huge store for screens or a full time screen maker.

The mask was applied over reflowed tin lead and generally looked not very good. The next big step being the introduction of hot air solder level which allowed the solder mask to be applied onto bare copper then, using the hasl process, the pads were selectively plated.

PCBs were technically advancing at a great rate with thinner tracks, smaller holes and then surface mount components. This drove the development of photo imageable solder masks.

Photo imageable solder masks

The method of application could be as previous, so using a silk screen but instead of applying an image, the PCBs were totally covered in the resist and and a b stage or partial cure was carried out. The boards were then registered to a black and clear photograph of the pads, and subjected to a blast of UV light. This took the exposed resist to the next level of cure which allowed the still soft b cured to be developed off in spray process equipment. The PCBs were then final cured, usually in a static oven before HASL.

As this is a photographic process, the results are more accurate and repeatable and cosmetically, it looks far better. It also gives a much better resolution so on very fine webs, eg on a quadpack, separation can be achieved between the pads that would be impossible using traditional screen print methods.

Other methods oh application include electrostatic spray, and curtain coat. Both are for large volume but the process remains the same once the PCBs are coated.

Legend or notation is again a predominantly screen print process but the requirement for accuracy is obviously not as critical. There have been some developments quite recently in that there are machines available that work similar to an ink jet and spray on the notation.

The vast majority of component identification is carried out using a traditional screen methods as the barrier to entry for the alternative processes does not warrant the investment.

Again, this is just an appreciation of the processes, if you want to comment or want more in depth details, drop me an email.

Next time under the eyeglass, Profiling PCBs, unless anyone wants to suggest a different topic.