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44 The PCB Magazine • August 2015 II. Failure Mechanism A. Electrochemistry For electrochemistry to occur, the following factors need to be present: a pathway, an elec- trolyte (i.e., ions in a mobile substance) and a bias voltage [2] . Dendritic growth is a type of electromigra- tion causing metallic deposition from the cath- ode. It is typically associated with surface mi- gration. Use of flux in assembly can contami- nate the surface and increase the probability of dendritic growth. Surface properties have been the main focus of specifications for PCB cleanli- ness and for surface insulation resistance (SIR) tests. Conductive anodic filament (CAF) growth is a specific type of electrochemistry causing cop- per salts to deposit from the anode along the glass-resin interface. CAF is typically associated with subsurface electrochemistry. Thermal stress deteriorates the glass-resin interface which in- creases the susceptibility to CAF. Hollow glass fibers provide another pathway for CAF. B. Discontinuities in Laminate Any kind of inhomogeneity within the di- electric material can provide a pathway for elec- tromigration. Bromine flame retardants have a limited solubility in polyimide resin and may separate during high-pressure lamination. Con- glomerations can be formed by the flame retar- dant or, likewise, by ceramic fillers, as typically used in epoxy materials with high glass tran- sition temperature. Manufacturing problems causing voids or delamination can degrade the insulating properties. A brittle resin system prone to cracking has previously been subject of ESA Alert EA-2010-MAT-12-B after critical short circuit failure in a power application during ground based testing. In addition to the above, PCB laminate can be contaminated by foreign material, such as chemical residues from the production process, epoxy resin dust in polyimide laminate, metal- lic debris, dust or fibers. Contamination of PCB laminate by foreign fibers can provide a path- way for electromigration if fibers bridge the clearance between nets of different potential. Mobility within the PCB laminate can be pro- vided by humidity. PCBs for space applications are typically made from polyimide laminates, which are thermally stable materials. However, polyimide resin is hygroscopic. Small amounts of chemicals can provide mobile ions creating a conductive electrolyte, even when cleanliness requirements for PCBs are met. Another problem associated with contami- nation is loss of insulation caused by metallic particles or by carbonization of organic con- taminants during lamination or during assem- bly. These failure mechanisms occur earlier and Figure 4: in-plane section of a via showing radial resin cracks and a dendritic structure on a delaminated flex layer. Figure 5: in-plane section of a via without capture pad showing cracks and copper ingress caused by drilling in brittle resin. LATENT SHoRT CIRCUIT FAILURE IN HIGH-REL PCBS continues FeAture