QUALITY CONTROL
Quality assurance can tackle the intricacies of flexible circuits Here’s how advances in testing can improve feature inspection.
F
Timothy Weber | Buehler |
lexible circuits may seem to be a recent development, but they date back to German inventor Albert Hanson in 1898. He described the production of flat conductors on a paraffin-coated sheet to improve telephone exchange boards. What flexibility the waxed paper provided, it lacked in durability. Flexible circuit creators today sandwich them between a polymer cover layer and base film of either polyimide or polyester. Designers favor polyimide where soldering of the assembly is required, and it is the material of choice for nearly all chip-scale packages and flex ball grid arrays. The wider application of flexible circuits took the recent development of new materials and manufacturing techniques, enabling the design to recently become one of the fastestgrowing segments of the global printed circuit industry. New applications, usually
involving smaller, more mobile devices, have pushed this expanded use along, particularly in the medical field. Circuitry in medical devices — a tight squeeze Medical device product specifications can limit space available for circuitry. A multilayer flexible circuit board can handle all the control functions of a complex medical device and provide greater functionality. Evolving design can pose manufacturing and testing challenges. PCB manufacturers are finding that thinner material leads to smaller vias, a reduced aspect ratio of vias, smaller lines and spaces, less copper plating and better fill grades, but it can also increase handling complexity. Flexible circuits’ quality assurance needs differ from rigid PCBs’. In response, testing systems manufacturers are developing new products and techniques to inspect features such as vias and through-holes. These approaches meet the unique needs of increasingly smaller and lighter flexible circuits demanded by the medical market. Metallography plays an integral role in analyzing and understanding the effects of these improvements on the PCB internal structure. Quality control benefits from evaluating the product at various stages of a new production process. Crosssectioning samples can reveal vital details, including cracks, voids, conducting layers and interconnections.
Flexible medical circuit Images courtesy of Buehler
30
Medical Design & Outsourcing
