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C O N N ECT T H E D OTS BY M AT T ST E V E N S O N , AS C S U N STO N E C I RC U I TS Designing for the Future of Manufacturing Reality: Surface Finish W hen designing the complex boards that many electronic devices require to operate, designers should consider manufacturability at every step. This is my last ar- ticle focused on designing for the always-evolving manufacturing reality. The previous installment focused on the solder mask and legend process. The last phase of pro- duction involves applying surface finish to protect copper from oxidation and facilitate soldering components onto the board. Choosing the right surface finish has always been important. If you are creating intricate designs with a wide variety of components, like for an ultra-high density inter- connect (UHDI) board, surface finish is a critical last step. Surface finish for UHDI boards requires extreme flatness and high solderability for fine-pitch com- ponents. Applying surface finish to the solder pads can affect solderability during assembly to a limited extent, but today's surface finishes are designed with solderability in mind and are neces- sary to sustain the productive life of the board. Evaluating Surface Finishes When selecting the right PCB surface finish for their project, designers find themselves weighing a variety of factors. Designers should consider the following when they establish their surface selec- tion criteria: • Cost • Restriction of hazardous substances (RoHS) • Flatness of the board surface • Solder joint integrity • Shelf life • Wire bondable (if applicable) With more boards having to accommodate larger components and more leads over multiple edges or surfaces, flatness is a more critical manufactur- ability consideration and surface finish plays a large role. There are five commonly used surface finishes, plus several more that are used for specialty ap- plications. Hot Air Solder Leveling (HASL) HASL has a long and successful history, and is still relied upon across many industries, most notably defense. It has become associated with the adage, "Nothing sells quite like solder." HASL is cost-effec- tive, durable in harsh environments, and very reli- able. It also has a long shelf life (the time the board can sit in inventory before assembly) and provides excellent solderability. But it has drawbacks. It often leaves a lack of 62 I-CONNECT007 MAGAZINE I APRIL 2026