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62 DESIGN007 MAGAZINE I DECEMBER 2021 important to remember best practices for its application. Breaking Down the Solder Mask Process e solder mask process is much like the imaging process. First, a photoimageable poly- mer is applied to the manufacturing panel. ere are several types of solder mask poly- mers, including liquid epoxy and dry film, but the most common in use today is liquid photo- imageable solder mask (LPI). Liquid polymer can be applied through several different tech- niques including spraying, flood coating, and screening. Direct printing is another option that is gaining adoption in the industry, but it follows a different process than the other three methods and should not be considered as part of the process outlined here. Once the panel is coated in the LPI on both sides, it is heated to remove some of the sol- vents, making the deposit illiquid and tacky to the touch, but not cured either. With the semi-hardened liquid polymer coating, the sol- derable surfaces can now be imaged onto the panel. e majority of the LPI is exposed to a high intensity light source, cross-linking the polymer, and chemically changing that por- tion of the solder mask. During this part of the process, solderable surfaces such as pads, through-holes, and con- tact points are masked from the light source— leaving the LPI in its tacky and liquid form. e exposed panels are then "developed" through a chemical process that removes the unexposed LPI from the panels and leaves the parts that were exposed intact, creating all the pads and fiducials. For most PCB designs there is much more solder mask le remaining on the board than the amount removed. is gives the PCB its familiar green coloring. Creative choice of sol- der mask color can also add some personality to your boards. Red, blue, yellow, and even pink are among the options for those who want a PCB to stand apart from the crowd. Aer developing the panels, they are again heated to drive off the remaining solvents from the LPI. is cures the solder mask coating into a hard- ened epoxy that protects the board. Application of the legend or nomenclature to the panels before final thermal curing will enhance the bonding between the two epoxies. Doing so at this point in the process creates a very strong bond between the solder mask and the silkscreened nomenclature. e chemical bond created at this stage makes it much less likely that the nomenclature will wear off or chip away from the solder mask than if it was applied later in the process aer the final ther- mal curing. e silkscreen application process uses a printer to apply the silkscreen ink to a manu- facturing 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 registra- tion and quality while lessening the process's impact on the waste treatment process. More precise application of nomencla- ture can also improve component placement accuracy during automated assembly. With- out nomenclature, machine assembly won't be as reliably accurate. Unfortunately, Gerber files do not tell the assembly machines exactly where each part is supposed to go and what angle and orientation is required. Footprint errors are common, as are components with ambiguous marking. e addition of solder mask and legend to your PCB design will go a long way toward protecting your design from external factors and give your assembly team the best founda- tion to be successful: a clear roadmap of the parts and enough barriers to prevent solder shorting between components. DESIGN007 Matt Stevenson is the VP of sales and marketing at Sunstone Circuits. To read past columns or contact Stevenson, click here.