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54 The PCB Magazine • August 2015 FeAture case for polymeric interleavers that are used to separate and protect innerlayers. Prepreg sheets also show a large amount of static charging. Interleavers, transport trolleys, trays and work benches that are in contact with inner- layers and prepreg should be inspected and cleaned regularly. The lay-up area in which etched innerlayers and prepreg are stacked up prior to lamination, should be under con- trol by clean room practices such as air filtra- tion, humidity control, overpressure, regular cleaning, protective clothing, restricted access, smooth unpainted surfaces, etc. Furthermore it is recommended to implement laminar flow benches for lay-up and the use of de-ionisation equipment. These recommendations are speci - fied in [1] and have become subject to audit of PCB manufacturers. D. Cleaning and Inspection of Innerlayers and Prepreg at Lay-up Due to the fact that the environment of PCB manufacturing is relatively uncontrolled with respect to cleanliness and PCB materials, and consumables are prone to static charging, it is essential that a cleaning step be implemented prior to lamination. The best process for last- minute cleaning is the lay-up. The use of so- called tacky rollers, based on Van der Waals attraction of particles without risking contam- ination of the rolled surface with an adhesive residue, is widely endorsed. Cleaning of prepreg sheets, however, is not so widely performed. Verification of the efficiency of the cleaning can be done by visual inspection under bright light and ultraviolet (UV) light. UV flashlights as well as bright lights are readily available in various wavelength ranges. Using UV light in- spection, it was shown that sealed bags of poly- imide prepreg are occasionally contaminated as delivered from the base material supplier. Poly- imide resin dust was shown to not be fluores- cent, whereas epoxy resin as well as dust fibers do show UV fluorescence. Either source of con- tamination is unacceptable in polyimide pro- duction processes at high temperature. One PCB manufacturer performs vacuum cleaning on etched innerlayers as well as on sheets of prepreg. Special non-contaminating brushes are used and the motor and air ex- haust of the vacuum system is placed outside the clean room. Another PCB manufacturer re- cently demonstrated the efficiency of different cleaning methods using rollers and vacuuming. Sample coupons were manufactured using dif- ferent cleaning methods and were submitted to IR testing, as previously described. E. Inspection on External Layers of Final PCB As part of the release procedure for the final PCB, a visual inspection is performed on exter- nal layers to detect defects in the surface finish, circuit or clearances. It is common for PCBs for space applications to be designed with laminate on outer layers, rather than copper foil and pre- preg. Contamination embedded in laminate re- mains covered by copper cladding throughout the production process, until the final tin-lead pattern plating and etching process on the out- er layer. Rejecting a final PCB often affects on- time delivery. In some recent cases, small but critical fiber contamination remained undetected at outgo- ing inspection as well as incoming inspection at the customer, which is described as NC#1 as part of ESA Alert EA-2014-EEE-3-A. Only after assem- bly and conformal coating, it was discovered that fiber contamination was embedded within the outer layer laminate and bridging the gap between pads. Waiving such non-conformance by repair and testing under careful assessment by NRB was performed but requires high effort and cost, in particular when the delivery of the Figure 12: inspection of a sealed bag of polyimide prepreg showing uV fluorescence of particulate contamination. LATENT SHoRT CIRCUIT FAILURE IN HIGH-REL PCBS continues