Fastening hardware using SMT: why manually install hardware when existing placement lines can be used?

Ed.: The complete article may be viewed at circuitsassembly.com/cms/content/view/5152.

Traditional hardware options for assembling PCBs have always carried potential baggage. The tedious and exacting process of handling and installing loose hardware often drains productivity. Broaching fasteners, while typically a more efficient alternative, can damage fully populated boards if installed improperly. Resulting scrap costs can be excessive.

Recent advances in fastening technology stow this baggage by taking advantage of existing infrastructure. A variety of fastener types have been developed specifically for PCB mounting using the same soldering processes that mount other components. Surface mount fasteners effectively become another board component. Such fasteners when fed on a tape and reel, assembled with pick-and-place equipment, and reflowed with other components can eliminate costs associated with parts handling, contribute to dramatic reductions in quality failures, and accelerate production.

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Many methods exist to fasten hardware to PCBs and all have advantages and disadvantages. For example, loose spacers (serving as plastic or metal columns used to offset two boards) usually require secondary operations: Sometimes they are snapped onto the PCB; in other cases, they may be joined with a screw to another component. The secondary operations, performed manually, rob time.

Loose nuts and screws are handled in much the same way. During assembly, handling can become an even greater issue due to the fine work necessary to align all parts, often on both sides of a PCB. Care must be taken to ensure that any dropped hardware is noticed and removed to prevent rattle or, even worse, shorts and product failures.

Striving to reduce the number of loose parts and simplify the assembly process, broaching spacers or nuts have been popular by virtually removing handling issues altogether. These fasteners broach by pressing the fastener's knurled section into a slightly smaller hole in the board, forcing the knurls to cut into the board, and installing permanently.

But these types are usually installed after boards have been fully populated with electronics components, and the broaching process can subject populated boards to significant amounts of stress and ruin. Micro-cracks may form to sever traces in any of the board's multiple layers, and boards may have to be scrapped--along with any placed components.

Brian G. Bentrim, P.E., is manager of global new product development at PennEngineering (pemnet.com); [e-mail].