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48 The PCB Magazine • May 2016 photo artwork, or is deposited directly on the photo-sensitive resist during image exposure. With circuitry at ultra-high density, particles in ambient air might cause issues, however, par- ticle size is not the only factor. Quantity and/or density of particles is also a consideration. Am- bient air contains 35 million particles per cu- bic meter, so in an uncontrolled environment, thousands of particles might easily be deposited in a small area. And even if particles don't settle on material, they can refract collimated light sources that are often used during exposing for creating the circuit image. These particles also accumulate on work surfaces and are a risk to contaminate in-process materials or operators. It is certainly fair to say the biggest source of contamination is not from ambient air, but the human operator. Human hair is approximate- ly 60 microns in diameter and can be several millimeters in length or longer. A human hair landing on the artwork or coated resist is sure to obstruct the light required for proper imaging Facility cleanliness is a vital part of process control for flexible circuit fabricators. As higher density requirements continue a relentless drive toward finer traces and spaces, particles and for- eign material can cause problems in a number of operations. A particle can cause conductor width and spacing issues at imaging. Conduc- tive material can cause current leaking when trapped under a coverlay or embedded in solder mask. Dirt, dust, or other unintended material may reduce layer-to-layer adhesion. Engineer- ing models suggest hair and fibers can affect in- sulation resistance and impedance. How big does a particle need to be to cause issues? Consider that typical outdoor ambient air has a normal distribution of particles that can be as small as 0.5 microns and as large as 16 microns. An ultra-high-density flexible cir- cuit might get to 25 micron spacing. Roughly speaking, one 7−8 micron particle could cause a conductor width and spacing quality criteria violation if it landed on coated resist, or on the by Dave Becker All Flex Flexible ciRcuits llc Flexible Circuit Fabrication and Cleanroom Manufacturing feature Column: all about flex 48 The PCB Magazine • May 2016

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