PCB007 Magazine

PCB-Aug2015

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42 The PCB Magazine • August 2015 failure. However, a working hypothesis has been developed indicating that fiber contamination may have caused a latent short circuit. This hy- pothesis was further substantiated by reports on contamination issues in base materials and by a test method that demonstrated the breach of insulation due to fiber contamination. At the time of the observed failures at equip- ment level, PCB manufacturers reported poor cleanliness levels of base laminate materials, causing poor yield. It is not possible to screen in an efficient manner for contamination in copper-clad laminate, since visual inspection requires stripping of the copper. Several in- spection methods show the lack of cleanliness of base materials, which is specified in IPC- 4101 [8] . This paper identifies a major gap be- tween the requirements specified on base ma- terials and the requirements on manufactured PCBs and presents a proposal for a new class of cleaner base materials for the manufacture of high-rel PCBs. The processes of PCB manufacturing are an additional contributing factor to pos- sible contamination in the dielectric insula- tion. PCB manufacturers are audited for their cleaning methods on etched innerlayers and on prepreg prior to stack-up for lamination. Recommendations have been issued [1] for a cleanliness control plan, for in-process in- spections and for electrical testing on final PCBs [5] . Concerns with cleanliness of PCBs ini- tiated the formation of a dedicated working group comprising of space agencies, original equipment manufacturers, satellite integrators and PCB manufacturers. The objectives of the working group are to inventorise the sources of contamination and to define risk mitigations, as described in the present paper. LATENT SHoRT CIRCUIT FAILURE IN HIGH-REL PCBS continues Figure 1: cross-section of a failed PcB showing extensive damage due to electrical overstress and a dendritic structure. Figure 2: in-plane section of a failed PcB showing a trace of electrical discharge along glass fibers in the clearance of a via through a plane. Figure 3: in-plane section of a failed PcB showing fiber contamination in a clearance of a plated through-hole. FeAture

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