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Green asGrass Soldermask!
- by Matt Stevenson

Soldermask, also known as the stuff that makes PCBs green, is a vital part of PCBs in today’s day and age. Not only does it prevent bridging between features during wave reflow assembly, but it also prevents oxidation of the copper traces, acts as a barrier to the environment (dust, humidity, etc.), limits external conductive influences, and also voltage spikes. All in all, soldermask is a very useful part of the PCB. But how does it get onto the PCB?  

The soldermask process is much like our imaging process.  First, a photo imageable polymer is applied to the manufacturing panel.  There are several types of soldermask polymers, but the most common in use today is Liquid Photo Imageable (LPI) soldermask. This liquid polymer can be applied through several different techniques, spraying, flood coating, screening, and direct printing (this follows a different process than this discussion).  Once the panel is coated in the LPI on both sides it is heated to remove some of the solvents making the deposit tacky to the touch; no longer liquid, but not cured either.  

With the semi-hardened LPI coating the panels, the solderable surfaces can now be imaged onto the panel.   Using a high-intensity light source, the majority of the LPI is exposed to the light source, cross-linking the polymer, chemically changing this portion of the soldermask.  The solderable surfaces (pads, through holes, contact points, etc.) are masked from the light source, leaving the LPI unchanged.  With the panels exposed, they are “Developed” through a chemical process that removes the unexposed LPI for the panels and leaving intact the parts that were exposed.  

For most PCB designs there is typically much more soldermask remaining on the board than is removed, giving the PCB its green coloring. After developing, the panels are heated again, driving off the remaining solvents from the LPI and curing the soldermask coating into a hardened epoxy like coating.

Soldermask doesn’t just come in green, it is available in a variety of colors (red, blue, black, white, and clear) to allow you to represent your University, your personality, or follow company guidelines for the design phase.
Once the panels have cooled it is time to apply the silkscreen image onto the panels.  Doing so at this point in the process creates a very strong bond between the soldermask and the silkscreen that makes it much more difficult to wear or chip away the silkscreen than it does to apply it later in the process.  

Our process for silk screen application uses a printer to apply the silkscreen to a manufacturing panel in much the same way that an inkjet printer applies ink to paper.  It comes directly from the digital data and is aligned directly to the panel image, improving registration, quality, and lessens its impact on the waste treatment process. Typically silkscreen is white, but it does come in yellow and black as well.  

Hope you enjoyed getting a little deeper into the manufacturing steps of PCBs. If you have any questions please feel free to let me know! In a future Tech Drill Down, we will cover the topic of Design Tips for Soldermask so stay tuned.