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60 PCB007 MAGAZINE I MARCH 2020 head integrator in partnership with the ink supplier. The objective of calibration is to achieve the perfect ink droplet. This means the droplet breaks cleanly from the nozzle plate when it is ejected with a nice clean tail, which is ab- sorbed into the main droplet before it hits the substrate surface. Thus, you avoid "satellites," which are small droplets deposited in non-im- age areas (Figure 2). Heads can be either non-recirculating or re- circulating types and often include integral heaters to reduce the jetting viscosity of the ink. With the former, ink is fed to the head from a tank or sump, sometimes via an inter- mediate tank held just above the head to en- sure a positive pressure to the head. A vacuum is used to prevent the ink from dripping out of the head when not printing. With a recirculating head, as the name sug- gests, ink is pumped continuously through the head, making for a somewhat more complex system. The benefits claimed for recirculating systems are easier priming, improved degas- sing, temperature control, and prevention of sedimentation of pigment. The choice of print- head also determines the droplet size that is jetted, which influences the productivity and resolution that can be achieved. Identifying Inkjet Solder Mask Formulation Constraints There are a number of significant constraints on the solder mask formulator when develop- ing solder mask for an inkjet application. Vis- cosity requirements limit the choice of suitable raw materials. In general, only very low viscos- ity resins and monomers can be used. Such ma- terials, however, can be more easily absorbed through the skin. Care must be exercised to en- sure their hazard classification is acceptable. The particle size of any pigments and fillers used is restricted to <200 nm, compared to 5–15 microns for conventional solder masks. The restriction on filler size and content is a considerable hurdle for solder mask formula- tors, as the fillers also contribute to the ther- mal shock and solder resistance and can also reduce the flammability of the material. The ink viscosity requirements depend on the printhead being used, with recirculating heads requiring lower viscosity inks than non- recirculating heads. The surface tension of the ink is also restricted to a fairly narrow range for successful printing. Ink viscosity and sur- face tension—together with the ink density, velocity, and path length—can be used to cal- culate Reynolds (Re), Weber (We), and Ohne- Figure 1: Inkjet nozzle structure. Figure 2: Achieving the perfect droplet.

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