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MARCH 2020 I PCB007 MAGAZINE 61 sorge (Oh) numbers, as well as the Fromm Z parameter. (Figure 3). These are dimensionless parameters used to calculate an operating region for good droplet formation (Figures 4 and 5). Low viscosity materials are also more prone to pigment sedimentation, and care must be taken to ensure a stable dispersion has been formed. The resulting contact angle of wet ink on either substrate or cured ink can be a useful predictor of coating performance. The choice of curing mechanism must also be taken into account. Choices include UV, thermal, or some combination of both. UV cur- ing is usually preferred, as it permits "pin-cur- ing" of jetted solder mask, which helps achieve good image definition (Figure 6). The curing mechanism can be free radical using acrylate type monomers and resins, cationic using ep- oxy resins, or—once again—a hybrid of both. The choice of photoinitiator used to generate the "polymerisation-initiating species" will re- flect the type of polymerisation chemistry be- ing used (i.e., acid type for cationic systems, free radical for acrylate systems). Most solder Figure 3: Standard formulas for droplet formation. Figure 4: Physics of drop formation and ejection [1] . Figure 5: Physics of drop formation and ejection [2] . Figure 6: The "pin-curing" process.