New-Tech Europe Magazine | Q1 2020

to do with conventional PCBs and cables. FPCs are found across the spectrum of electronic and electrical products: automotive, consumer, medical, entertainment, IT and industrial equipment. They are used widely in hand-held devices such as electronic games consoles, laptops, smartphones and cameras. Chest straps and wrist bands for sports and health monitoring devices are obvious wearable application examples. But they are also used in tiny devices such as hearing aids, heart pacemakers and medical pumps. Ultra-thin, flexible substrates have also enabled skin patches for monitoring blood sugar or for dispensing medication. In industry, smart labels incorporate RFID tag circuitry for security, anti- counterfeiting and transport logistics/ tracking. A summary of the benefits of FPCs Space saving: very thin dielectric substrates, some down to 25µm or less, coupled with their planarity, make it possible to bond the circuits to, or within, the structure of a product. Weight saving: the corresponding reduction in weight is amplified as fewer connectors and fixings are required. Smaller conductors and reduced copper content make an additional contribution. Versatility: FPCs are custom- designed to fold, bend and fit into virtually any shaped housing. Ruggedness: robust connections compared to wire harnesses as the flat foil conductors within the FPC can dissipate heat better and carry more current than equivalent round wires. Fewer interface connections

Figure 1: Typical construction of a multilayer FPC

increase reliability. Physically more resistant than rigid PCBs to vibration and shock. Higher operating temperature: thermal stability is better, particularly with polyimide materials, allowing the circuit to withstand more extremes of heat than rigid PCBs. Thermal mismatch is also reduced. Crosstalk and noise: these are more easily controlled using a uniform conductor pattern in the flex circuit. Ground plane options include lightweight cross-hatched, solid copper, aluminium or lightweight shielding films. Stitching vias and internal guard rails can provide 360o shielding, with plated vias along the length of the circuit. Good EMC performance: reduced radiated emissions due to the smaller ground loop created by the guard traces, and better differential-mode transmission loss characteristics. In data bus applications, better- controlled impedance characteristics,

reduced transmission loss and lower radiated field emissions due to shorter current return paths. Simpler, more reliable installation: faster assembly of fewer component parts; improved repeatability as less manual intervention; no need to colour code wiring. Resulting in lower installation costs, reduced risk of rejects during assembly and fewer in- service failures. The IHT manufacturing process and the new applications it enables As mentioned earlier, the length of multilayer FPCs that can physically be manufactured has restricted their adoption in many applications. Length has been limited typically to 610mm, although there are a few manufacturers capable of producing circuits a few metres long. A patented process known as Improved Harness Technology™ (IHT) is now overcoming these limitations.

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