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Although I am not a designer by trade, I want to share my thoughts on what additive manufacturing means for designers, especially how it relates to solder mask. I feel the following topics are the most important to address.
1. Definition of Solder Mask
By its nature, the definition provided by any EDA tool is a negative one; the CAM vector files specify what goes away from an assumed continuous surface. Until now, solder mask has always presented itself as subtractive. SUSS MicroTec developed a front end, JETxSMFE, that can operate at the CAM station to smooth out manufacturing. The software understands all of this, managing the details correctly on the incoming files.
2. The Advantage of Inkjet Solder Mask
One advantage of inkjet solder mask is to avoid any filling of holes or vias. Another way, maybe an annoyance to some, is to say that “tenting” is not possible. Still, the consensus is that solder mask-free vias improve a board’s reliability (Figure 1). The JETxSMFE removes solder mask at declared holes. Undeclared drills, which might still exist depending on the manufacturing convention on the production floor, will result in ink on the print table. This is not a big issue as a scraper easily removes excess solder mask, and alternatively, replacing the table or its liner (if present) gets the job done. However, both solutions lead to a small downtime. Long story short, if you want friends on the shop floor, be sure to have all the drills in your design.
3. The Dam Dogma
These small solder mask traces on laminate are supposed to separate two nearby copper pads. Such a pattern indicates the non-solder mask defined (NSMD) pad design choice. However, this choice involves several constraints, and these create an artificial need for narrow dams. From my earlier example, this time with numbers: Imagine two pads, 200 µm apart, nothing extreme. What is the maximum size of a dam between these two? Assuming that LDI technology is used, the dam size results from applying the state-of-the-art constraint of the technology. This 200 µm pitch decreases by the laser beam width plus the registration accuracy twice—one for each pad. Therefore, it quickly comes down to 100 µm. Any more challenging a pad distance will also make it harder to define a dam.
From this reasoning, the many requests to create 50 µm dams or less with traditional technology seem legitimate. So, what do we do with inkjet printing? Does it join the race to the last micrometer?
To find out the answers, continue reading this article in the April 2022 issue of Design007 Magazine.
