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60 The PCB Magazine • August 2015 surface roughness specified in IPC-4101, which appears to be unavailable from most suppliers. Therefore, the proposed specification has re- cently been provided to key suppliers of base materials for review and for their Statements of Compliance. The cost of investments may be difficult to carry by the base material suppliers since the European space industry is a niche market. This may have a commercial impact that remains to be assessed by the entire supply chain. However, the recent feedback from key suppliers has been positive. vI. Conclusions Latent short circuit failure of power PCBs has been associated with fiber contamination of the dielectric material. Tests and inspections have shown various sources of contamination in laminate, prepreg and on etched innerlayers. It has been demonstrated by IR tests that con- tamination can cause breach of insulation due to electromigration. Risk mitigations have been specified in PCB design, PCB manufacture and base material sup- ply. These include conservative design of insula- tion, inspection on base laminates and prepreg, in-process inspections on etched innerlayers, clean room practices in manufacture, cleaning of innerlayers and electrical testing on final PCBs. IPC-4101 specifies a cleanliness level for base materials that is in conflict with high-rel PCB manufacture and requirements for insula- tion specified for IPC class 3/A. A specification for a new class of cleaner base materials is being issued to key base material suppliers and is pro- posed to be included in a revision of IPC-4101. Acknowledgement This paper summarises collected results ob- tained within the PCB-SMT working group of the Component Technology Board, a supporting unit of the European Space Component Coordi- nation. This working group includes representa- tives of the space agencies CNES and ESA, satel- lite integrators Airbus, Thales Alenia Space and RUAG and qualified PCB manufacturers TESAT Spacecom, PrinctaGRAPHIC, Systronic, and In- votec. Moreover, improvements in PCB design have been the subject of the working group on the ECSS-Q-ST-70-12. Working group members contributed extensively to the common know- how presented in this paper. Moreover, key base material suppliers have supported the working group in the understanding of cleanliness issues in critical processes and in defining risk mitiga- tions. Without their commitment to the high- reliability space market it would not be possible to improve on quality. Meanwhile, the proposal (available on www.escies.org/pcb/ as memo QT/2013/378/ SH) for an Appendix to IPC-4101 with tighter cleanliness requirements has been presented to the 3-11 subcommittee at IPC APEX/Expo 2015. The subcommittee has accepted to draft the Ap- pendix as an amendment 2 of revision D of IPC- 4101 and circulate for review and approval. PCB References 1. QT-2013-730-SH, "Cleanliness control for PCB manufacturers." ESA memo, Dec 2013 www.escies.org/pcb/. 2. IPC-TR-476A "Electrochemical migration: electrically induced failures in printed wiring assemblies," May 1997. 3. ECSS-Q-ST-70-12C, "Space product assur- ance—Design rules for printed circuit boards," www.ecss.nl. 4. C. M. Mc Brien, S. Heltzel, "Insulation re- sistance of dielectric materials under environ- mental testing", Feb 2013. 5. QT-2013-681-SH "High resistance electri- cal test for PCBs", ESA memo, Dec 2013. 6. ECSS-Q-ST-70-10C "Space product assur- ance—Qualification of printed circuit boards," www.ecss.nl, Nov 2008. 7. QT-2013-378-SH, "Proposal for appendix to IPC4101," ESA memo, drafted Apr 2013. 8. IPC-4101C "Specifications for base mate- rials for rigid and multilayer printed boards," Aug 2009. Originally presented at 2015 IPC APEX EXPO and published in the proceedings. FeAture LATENT SHoRT CIRCUIT FAILURE IN HIGH-REL PCBS continues Stan Heltzel is a materials engi- neer at eSa, in the components Technology and Space materials Division, noordwijk, netherlands. To contact the author, click here.

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