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32 The PCB Magazine • March 2014 This work also required that we redefine our criteria for failure. In most compliant sys- tems where "strain events" are recorded, failed samples inspected after penetrating dye has been applied exhibit complete separation of the BGA structure from the board. The crack path is very consistent; it involves a cone-shaped fail- ure in the "butter coat" of the laminate and follows the top surface of the glass bundles in the first reinforcing layer. We have yet to see any work in the literature th at investigates the crack path. We normally assume that cracks ini- tiate at the surface but it has been postulated and subsequently demonstrated that there could be internal separation and coalescence of micro cracks below the surface before this catastrophic failure occurs. As result of our analysis we have defined two distinct portions of the pad crater crack, which represent two different levels of failure categorized as cohesive and adhesive separation. SPHERICAL BEND TESTING continues • Cohesive Separation: The crack initiates at the surface of the laminate in close proximity to the solder pad and travels at approximately 45 degrees to the first level of reinforcement. Testing identified that this portion of the dam- age can be generated without any progression under test conditions to the next phase • Adhesive Separation: The crack subse- quently follows the top surface of the first rein- forcement layer until it finds a short path back to the surface inboard of the solder pad We understand now that in these stiffer sys- tems the cohesive portion of the crack occurs without generating any discernible disturbance in the strain profile, but we believe that it rep- resents a significant risk to product shipping into service environments. To summarize, any damage to the laminate resin system discernible by dye penetration and optical microscopy is considered to be a failure. Figure 7: mixed response zone.