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PCB-Mar2017

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12 The PCB Magazine • March 2017 by Dave Becker ALL FLEX FLEXIBLE CIRCUITS Earlier this week, I saw my name in the com- pany newsletter announcing my 10-year anni- versary with All Flex. This wouldn't be so bad if the anniversary wasn't preceded by almost 30 years at my previous employer, who also hap- pened to be a manufacturer of flexible circuit- ry. In other words, I have become an industry "greybeard." If I were honest with myself, I'd admit the nickname was probably appropriate several years ago, but it does allow me member- ship in a dwindling club who have witnessed the dramatic industry changes over the past four decades. But first, what are flex circuits? The industry segments flexible circuits into three classes (per IPC-6013, an industry stan- dard document). 1. Class 1 circuits are used in applications with the fewest reliability requirements and minimum inspection. In reality, almost no one calls out inspection at Class 1. 2. Class 2 circuits have moderate inspection, testing, and performance requirements and are typically found in consumer applications such as cellphones or cameras. 3. Class 3 circuits are found in the highest- reliability applications, require the highest inspection and test standards, and are often specified by in military and crucial medical electronics. Flexible circuits are also categorized in IPC- 6013 by "Types." There are five Types defined but only four (Type 1, 2, 3 and 4) are common. Although the insulating material and copper trace access also defines the circuit type, in gen- eral, Type 1 flex circuits are defined as those with a single conductive layer. These are known as single-sided circuits. Type 2 flex circuitry has two conductive layers and are referred to as double-sided circuits. These parts have plated through-holes connecting copper layers. Type 3 circuits are multilayers and have 3+ conduc- tive layers with plated through-holes. Type 4 are rigid-flex, a combination of rigid and flexible circuit boards characterized by plated intercon- nect holes through both flex and rigid materi- als. Frequently confused with rigid-flex are flex- ible circuits with selective rigid stiffeners. These are more appropriately termed "rigidized flex" and might be any of the first three circuit types. In one sense the flexible circuit industry has remained amazingly stable. Things I learned on my first day at work are still relevant. Polyimide remains the workhorse dielectric film; imaging is done by exposing a pattern on a photosensi- tive film; copper is subtractively removed from T he Wide World of Flex FEATURE

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