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50 The PCB Magazine • May 2016 enough to cause a defect. In addition to hair, there are skin flakes, dander and fibers from gar- ments—all much larger than typical particles in ambient air. In order to prevent air particles, hair, skin, fibers and large flakes of dust from affecting fab- rication yields on high density circuits, critical processes are performed within a cleanroom en- vironment. A cleanroom for electronics manufacturing may have the following features and more: • Special high performance air filtration systems • Non-shedding surfaces on walls, floors and ceilings • Special clothing or gowns for operators • Unidirectional airflow (from ceiling to floor) • Cleanroom protocols • Highly controlled entrance and exits Cleanroom environments are commonly designated by classes. Typically the class desig- nations are: • Class 1 • Class 10 • Class 100 • Class 1000 • Class 10,000 • Class 100,000 The smaller the class number, the lower the maximum particle count in the room. The class designation numbers actually correspond to the number of 5-micron particles or greater per cu- bic foot. So a class 100,000 cleanroom would al- low 100,000 particles of 5 microns or greater in a cubic foot. That sounds like a lot, but remem- ber ambient air is starting at over 200 times that density. Air filtration, room construction, lay- out and operation protocol will differ from one class to another. The lower the class number, the higher the construction and operation costs of the cleanroom. Each manufacturer needs to determine a cleanliness strategy that will best serve its needs. In addition to high capital and operating costs, cleanrooms can throttle agile manufacturing ef- forts and compromise flexibility in process flow and facility utilization. Manufacturers often adopt a strategy of using portable "soft wall" cleanrooms combined with clean workstations. A clean workstation may be a single machine or small area that is contained in a plastic cur- tain with a unidirectional HEPA air shower. For example, a large area might be controlled to a class 100,000 level, but within that large area isolated workstations are controlled to a class 10,000 level. Operators may be required to wear hairnets and cleanroom smocks to enter the class 100,000 areas, but may be required to put on a coverall, shoe covers, gloves and facemask to work in the class 10,000 area. Circuitry Fabrication Cleanliness Considerations • Work in process (WIP) needs to be kept clean during processing within cleanroom sta- tions. Taking product out of a cleanroom and then bringing it back in should be minimized to avoid contamination. Clean pass-throughs and clean storage cabinets are frequently utilized to avoid bringing material out of a cleanroom. When material is taken out of a cleanroom, the material and container should be cleaned be- fore returning. • Imaging operations require the highest level of cleanliness. This includes resist coating, laser direct imaging (LDI), photo exposing and screen printing. • Coverlay assembly is a critical area and is best performed in a highly controlled clean environment. Along with contamination in- troduced into a cleanroom with product trans- portation, the biggest potential issue is operator hair or clothing fiber shedding onto the cover- lay material. • Drilling operations, including laser drill- ing, are done in a non-cleanroom area, but in most cases the material goes through wet pro- cess after drilling so it will get cleaned before imaging. • Wet process areas are generally not per- formed in a cleanroom. These operations in- clude etching, micro-etching, developing, strip- ping and plating. The final stage of these chemi- cal operations includes a rinse and dry which will result in very clean material. Another focus flexible CirCuit fabriCation and Cleanroom manufaCturinG