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58 The PCB Magazine • August 2015 cause investigation when a final PCB fails elec- trical testing. The results obtained from these in- spections are presented in the previous chapter and show varying level of quality of base mate- rials. In some cases, contamination is identified that exceeds the requirements of IPC-4101 [8] . In other cases, contamination is identified that is within the requirements of IPC-4101 but is still proven to be unsuitable for PCB manufacture in high-reliability applications. B. IPC-4101 Specification for base materials The concerns regarding lack of cleanliness on base materials have been addressed to the IPC subcommittee 3-11, which is responsible of updating the IPC-4101 to revision D. This standard currently has non-stringent require- ments for the sample size of screening and for acceptance criteria of subsurface imperfections. Although the sample screening on a batch of laminate may be statistically inefficient, it does provide a simple method to quantify cleanli- ness level, against which base material suppli- ers can be held accountable. One remarkably weak requirement of IPC-4101 is to permit fiber contamination up to 13 mm length. This is in contrast with IPC-6012 that does not allow for- eign inclusions that reduce dielectric spacing to below the minimum requirement for class 3/A PCBs for space application. As a comparison, common PCB de- sign for space may use in-plane insu- lation in the order of 0.5 mm, with a minimum as-manufactured of 0.15 mm for 100 V, as specified in ECSS- Q-ST-70-12. A proposal with tightened re- quirements for a new class of clean- er base materials has been drafted [7] and presented to the IPC subcom- mittee. This proposal requires 100% visual inspection on prepreg prior to copper cladding for the manu- facture of laminate, as well as on prepreg to be provided as B-stage cured sheets to PCB manufacturers. Furthermore, the proposal requires fibers to be evaluated as opaque for- eign matter limiting its size to 0.5 mm. A sample inspection of 2% is required for batch acceptance of laminates. Some base material suppliers report good experience with high-pot testing on thin laminates. This is specified as an optional test method to be agreed between supplier and customer. C. Procurement Specification to Base Material Suppliers The proposal described above cannot be seen as separate from the other risk mitigations. The proposed requirements can still cause latent short circuits when contaminants are situated in a critical area with minimum spacing. It has been seen, however, that base material suppliers on occasion struggle to achieve the current re- quirements on IPC-4101. Implementing a more stringent specification is, therefore, expected to necessitate better cleanliness control. Moreover, the required inspections for batch release pro- vide accountability. The proposed specification does not de- scribe an ideal material. It is rather deemed to be a compromise that is realistically achievable in an attempt to define a new class of cleaner base materials. It is not the intention to specify requirements on a unilateral basis that cannot be achieved by base material suppliers. This is for instance the case for the copper foil class D FeAture LATENT SHoRT CIRCUIT FAILURE IN HIGH-REL PCBS continues Figure 14: in-plane section of particulate contamination with metallic constituents embedded in base laminate causing high-ohmic short.