Mask Musings.
Soldermask (SM) and its functions have changed and evolved over the years. As an ex-solder mask applicator, I can remember screening Colonial High Gloss Epoxy soldermask on boards back in the early 80's, ending up with a thickness that could at times seem as thick as the board itself, especially before we found out about reducers which were used to think it before screening!!!!
As stated earlier, the main function of SM is to protect the copper traces and any other feature you cover up with it. These days however, SM is being used to do a lot of other functions like shaping pad or providing barriers to keep liquid solder from migrating to areas where you don't want it during assembly. There are even terms like SMD-LLP, Solder Mask Defined Leadless Leadframe Package, or, NSMD-LLP, Non Soldermask Defined Leadless Leadframe Packages.
Liquid solder mask thickness is something that is hard to predict from vendor to vendor as different application methods produce different results. Dry film mask is the same thickness no matter who applies it but liquid, as Dave stated, can be screened, curtain coated, spray coated etc. Even different operators can cause variances in the thickness as squeegee pressure and speed produce different results. Bobby the body builder may push harder on the squeegee than Barbie the ballerina. Both cover the boards but the extra pressure that is exerted by one will deposit thinner solder mask than less pressure from the other.
Our experience is that most vendors when asked about thickness will specify a window that gives them maximum manufacturing tolerances such as, .0008 - .0012 finished thickness as the standard for liquid mask. Because it is liquid, it is very difficult to guarantee even thickness across the whole surface no matter what they tell you. Edges typically are different than the center etc.
Dry film is great for even thickness but it is becoming harder and harder to find shops that still apply it. Also, when you get into the fine features of today's technology, even the vendors that apply it may balk at using it below say .025 (.5mm) pitch sizes due to adhesion challenges etc. Liquid photo imagable (LPI) is better for fine pitch but is definitely thinner when applied per manufacturers recommendations.
Our company uses soldermask to shape solid solder deposit pads. We specify soldermask thickness as .002 + .002, -.000 and leave it up to the vendor to figure out how to achive it. Most of our vendors have developed methods to supply this thickness however, there was some grumbling from a few at first.
So, in conclusion, soldermask is not just for breakfast any more. It is an important part of a succerssful assembly and can be applied thickler if needed. You just have to twist some arms now and then to get what you want.
mk
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