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64 PCB007 MAGAZINE I MARCH 2020 relatively small changes in formulation proper- ties, such as viscosity and surface tension, can be easily seen. The UV energy used for pin- curing can also be adjusted and optimized for good print performance and final properties. Evaluate Cured Coating Performance Using the lab printer, suitable test coupons can then be printed, and the cured film proper- ties fully evaluated. As well as the basic solder mask properties assessed at the basic screening stage, coupons for electroless nickel immer- sion gold (ENIG), immersion tin, and immer- sion silver are prepared and tested in different plating solutions. Modify Formulations as Required After the jetting tests and cured coating per- formance results are assessed, further chang- es and modifications may be necessary. If so, these are made, and the effects of the changes reassessed. As can be seen from the steps identified pre- viously, the process of formulation is an itera- tive one, with many loopbacks and incremen- tal changes. Designed experiments can be used where appropriate to speed the optimization of the formulation(s), but much of the work still relies on the skill and knowledge of the formu- lator and—some will say—is neverending. Submit Final Formulation(s) for External Compliance Testing Once a "final" formation has been achieved, test coupons are sent to various external agen- cies for compliance testing to standards such as UL94 and IPC SM-840. Meeting Solder Mask Performance Requirements Throughout the formulation process, the aim is to produce a product that meets all the per- formance requirements of a solder mask when applied on a PCB. These requirements are es- sentially the same for any solder mask, re- gardless of the type and application method— namely, to protect the (mainly) copper circuitry chemically, electrically, and physically from high temperatures, humidity, moisture, corro- sives, dust, dirt, and contamination (Figure 8). Further, Figure 8 showed one example of why solder mask is needed. Solder masks are also required to: • Withstand ENIG, immersion tin, and immersion silver plating • Enable adequate solder dam resolution • Withstand lead-free soldering temperatures and multiple solder reflow cycles • Be compatible with conformal coatings • Contain low/no halogen For a solder mask to be defined as halogen- free, it must contain less than 900 ppm chlo- rine or bromine and <1500 ppm total halogen. The phthalocyanine green pigments commonly used to produce solder mask contain 47–48% chlorine; even when used at typical levels of 1% or less, these contribute some 5000 ppm chlo- rine (Figure 9). For this reason, halogen-free solder masks tend to be either not green or a mixture of blue and yellow pigments to achieve a green color. Ideally, an ink- jet solder mask should also be a universal prod- uct, suitable for use on rigid and flexible, as well as with non-re- circulating and r e c i r c u l a t i n g printheads. Figure 8: Solder mask can protect from real-world conditions, such as this. Figure 9: Phthalocyanine green molecular structure.