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62 PCB007 MAGAZINE I MARCH 2020 masks also require a thermal bake after print- ing and UV curing to achieve the best possible final cure. Identifying and Sourcing Suitable Candidate Raw Materials Once the formulation requirements and con- straints have been identified and taken into ac- count, suitable raw materials are then identi- fied and sourced. As already mentioned earlier, the choice of resins and monomers is restricted to those with a low or very low intrinsic vis- cosity to enable a low viscosity product to be produced. Any solid or higher viscosity resins can only be present in small amounts. The selection of the correct photoinitiator is crucial to obtaining good performance. The photoinitiator must combine a rapid cure— to minimize the UV energy required for pin- curing and thus avoid restrictions on print speed—with a good through-cure to avoid "wrinkling" in thicker areas (such as along track edges) and when building up thicker coats. Once again, toxicity can be a concern with several excellent initiators ruled out be- cause of health concerns. Pigments are usually pre-dispersed in a suitable medium before incorporating them into the ink. It is essential that any dispers- ing agents used are compatible with the other components of the solder mask fluid, the pig- ments are stable during storage in both pre- dispersion and solder mask, and they do not settle or separate out. The pigments them- selves must be able to withstand the process- ing and service conditions of the solder mask and must take account of any low-/no-halogen content requirements. Other solder mask components may include flow agents, debubbling additives, surface property modifiers, thermal curing agents, and many others, depending on the final properties desired. Formulating Test Products The protocol used for formulating and de- veloping an inkjet solder mask is similar to that used when formulating standard solder masks. Screen Formulations for Basic Solder Mask Properties A number of trial formulations are mixed in the laboratory. These can be quickly coated us- ing non-jetting methods such as a wire-wound rod (K-bar) to deposit a known thickness onto a test substrate. The coatings are cured using different UV light sources to identify the most efficient curing method, and then baked as re- quired for final cure. Basic solder mask properties (e.g., solvent/ chemical resistance solder resistance, hardness, etc.) are then evaluated. Pigment stability and hotbox stability of the test formulations can also be assessed at an early stage of the development. Modify Formulations as Required Following the assessment of the test formula- tions, poor-performing versions are eliminated or modified as appropriate and re-evaluated. Select Formulations for Jetting Tests Once a number of suitable candidates are identified, these are then jetted using an inkjet printer. Electra Polymers uses the LP50 labo- ratory inkjet printer from Meyer Berger. This printer has an integral, water-cooled UV LED for pin-curing and permits a number of print settings to be modified for different formula- tions (Figure 7). Evaluate Jetting Performance of Selected Formulations The jetting performance of the test formula- tions is assessed using the built-in drop watch- er and by examining the resultant prints for image quality, coating smoothness, and a lack of "striping" and "stitching." The effects of Figure 7: Meyer Berger DixPro LP50.