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APRIL 2022 I DESIGN007 MAGAZINE 29 mask coating are convenience (less developer chemistry use) and the need to support any stencil frames. e first is no longer a problem, since inkjet does not use developing chem- istry; the second has an elegant design solution: meshes or arrays of shapes to provide the same support with significantly less mate- rial coated. When combining the idea of constant coating thickness and the PDSM pro- posed earlier, a novel level of compact- ness can open for ball grid array (BGA) structures. 5. Design Rule Check Here is a mélange of considerations which fall under the category of design rule check, where the CAM soware will eventually implement them. How- ever, their description here might avoid misunderstandings in the future. The designer will have to know that: • Solder mask will have a minimum radius. Such radius depends strongly on the inkjet machine chosen. • e smallest feasible gap between solder mask features is smaller than the smallest feasible printed solder mask feature. • inner solder mask allows for smaller feature size; vice versa, thicker copper or a requirement of thick solder mask will increase the minimum feature size. Figure 4: An example of a solder mask profile independent of the copper traces. (Source: ACB-Atlantec SAS) Figure 5: What about a more relaxed BGA structure? The traces between pads can have a reduced pitch since inkjet will place a defined volume of solder mask.

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