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PCB-Mar2017

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14 The PCB Magazine • March 2017 THE WIDE WORLD OF FLEX a patterned substrate; and acrylic adhesive is commonly used to laminate polyimide coverlay as an insulation. But there have also been some rather dramatic changes… So Big! Flexible circuitry was launched in the Unit- ed States as a commercial product about 50 (!) years ago. Minnesota remains a focal point for the technology, along with New England and California. The Minnesota nexus occurred as G.T. Schjeldal (the entrepreneur who founded Sheldahl Inc.) built equipment to handle thin flexible films in continuous reels. Automated equipment could cut and heat-seal polyester and was used to fabricate plastic bags. This technology foundation evolved and was adopt- ed to produce electrical laminates. Value was then added through roll-to-roll electroplating, imaging, and etching continuous reels as part of the process of fabricating printed circuits. It was a very high-volume process targeting the automotive and communications markets. This technical heritage expanded regionally as engi- neers spawned new, related businesses. In this time frame, the 1980s, the flexible circuit market size was estimated at less than $200 million as Asia had yet to emerge as an in- dustrial player (that seems like a long time ago). Today the North American flex circuit market is estimated at about $350 million but is dwarfed by manufacturers in Japan, Korea, China and Taiwan. Worldwide market size estimates vary considerably and are complicated since few cus- tomers buy a simple "flex circuit." Added value is a common offering as circuits are frequently sold with components as an interconnect so- lution. A worldwide market size of $7−11 bil- lion seems to be a common range. Almost any electronic application is a candidate for a flex circuit with cellphones, cameras, displays, and consumer electronics frequent adopters in Asia. The North American markets are characterized by medical, military, industrial and instrumen- tation applications. Make Mine Smaller! One technology trend has been a vector and can be summed up by the word "smaller." Plat- ed through-holes used to be considered small at .015" (15 mils). Today they are created with lasers at sizes of .001−.002" and are often plated shut. Conductor traces and spaces struggled to get below .010" then; now, designs with .002" conductors are common. Complementing this trend has been improved soldermask place- ment accuracy with a locational tolerance of +.002", base film thickness common at .0005", and 01005 SMT components (with dimensions of .0157" X .0079") that are machine-placed at rates of multiple parts per second. Two technical capabilities driving a signifi- cant portion of the improvements are optics and lasers. Using cameras for optical registration has significantly reduced or eliminated the use of punched or drilled tooling holes. Cameras find fiducials and product alignment is factored to create a best fit registration between sequential processes. Lasers are used extensively by laser direct imaging (LDI) equipment with software created by CAD files defining the artwork image during photolithographic exposing. No more 10:1 hand-laid artwork tapes (I won't mention how long ago that was being practiced)! Lasers are also used extensively to excise circuits from panels and cut film coverlay. Capability is en- hanced as laser cutting can be accomplished without traditional restrictions imposed by drill and routing tools. Get the lead out! On July 1, 2006, the Restriction of Hazardous Substances (RoHS) mandate was implemented by the European Union, requiring manufacturers of electronic and electrical equipment to reduce/ eliminate six hazardous materials. Lead was included in this restriction which meant new methods for circuit board surface treatment and tin/lead soldering had to be invented. Certain dopants used to make printed circuits

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