More Than a Word: Solder Mask

Estimated reading time: 4 minutes

Do you spend time, as I do, musing on the language of PCBs? We have developed our own lexicon to convey as much by picture, as by word, what exactly we mean. I’m talking about words like micro-etch (hyphenated or not) and stripper (a whole new meaning), metalization and metalize (one “l” or two?); terms like mouse bites, measles, voids, vias, pits, etc. And that’s not mentioning the myriad acronyms, from PCB right through to ENIPIG (we don’t stop at just three letters, oh no).

Which brings me to solder mask (one word or two?). We have chosen to make it two words in this issue, and plan to keep it that way, regardless what some of our contributors say. But, why do we say “mask” at all? What exactly does that mean? I know some say solder resist, which is more appropos, as this material does indeed more or less repel or resist the solder. The mask part pertains to the underlying circuitry that is masked or protected from the solder. But, as one of our experts this month has pointed out, sometimes there will be no solder, in which case the solder mask is protecting against nickel-gold or immersion tin or one of the other final finishes. We can go on and on… But with all the things that can now affect the solder mask—finer features, higher-temperature solders, final finishes, direct imaging, inkjet—we felt it was high time to give it a little more attention. As one of the last processes a PCB sees, you want it to do its job well and precisely; now is not the time to be scrapping your expensive multilayer—or any board for that matter. With that, let’s get to our lineup this month and start learning.

We first wanted to learn what was new in the solder mask area, so we met with Electra Polymers’ Shaun Tibbals and Antony Earl. We discussed the difficulties of clearing small holes of solder mask and the problems with plugging holes—opposite requirements that are often specified. The conversation ranged further into direct imaging of fine features and the projected use of inkjet for solder mask—there are advantages and disadvantages.  One area of concern with solder mask that is covered (ooh, a pun) by Atotech’s Rick Nichols et al., is the effect of a solder mask and its residues on the subsequent final finish process. The focus here is on identifying possible markers that could be detrimental to both the final finish chemistry and the solderability of the board during assembly.

While it was good to talk with the chemistry people, we wanted to hear from the equipment folks, too. Circuit Automation has been making automated screen printing equipment for a very long time, so we tapped Tom Meeker, Larry Lindland and Yuki Kojima. As is often the case, we ended up learning about much more than just screen-printing equipment, including how different formulations require different set-up parameters, the difficulty of plugging a variety of hole sizes on one board, the importance of a uniform coating, handling flex—and the list goes on.

Lackwerke Peters’ Sven Kramer wrote a great article on the thermal capabilities of solder masks. Not only must they survive hightemperature solders and rather aggressive final finishes, but they are then exposed to considerable thermal stress and high humidity conditions in the field—think automotive applications—all while reliability demands have become more stringent. Kramer presents a primer on solder mask formulation, aging test data with illustrations, and then a fine discussion on white LPI solder masks.

As you can see, there are many areas and much to learn and think about with solder masks, but let’s move on to our columnists. This month, Mike Carano, RBP Chemical Technology, discusses flexible metalization, specifically possible problems associated with adhesion of electroless copper to both substrate and copper layers. Needless to say, the process is significantly different from that used for FR-4 and other common substrate materials.

Bringing up the rear is IPC’s John Mitchell and his column about automotive electronics. After reviewing some of the major changes that have occurred in the past few years, he discusses the automotive addendums that are under development through IPC and asks for your input and expertise to help with that.

Well, it’s July and 2018 is already half gone. As always, we ask, how did that happen? But of course, we are not nearly done with providing interesting and informative coverage on a wide range of topics in the upcoming issues. Next month the topic is reliability and then we plan to focus on mSAP and SLP in September. I sure hope you are a subscriber by now!

Patricia Goldman is managing editor of PCB007 Magazine. To contact Goldman, click here.

This article originally appeared in the July 2018 issue of PCB007 Magazine, click here.