Module camber effect on card assembly and reliability for large flip chip BGA organic packages

Component manufacturers and card assemblers are regularly confronted by the specification limits for the coplanarity of electronic components, and the effects of component warpage over the temperature range encountered during reflow processes used during card assembly. Card level assembly defects are often imputable to incompatible planarity levels of raw cards and their associated components, but the resultant impact of coplanarity mismatch is not well understood from the perspective of primary assembly yields at card joining and reliability. The present work studies the effect of module level camber on assembly yields and the reliability of a 55mm x 55mm, 2809 I/O FCPBGA component with lead free second level solder joints. The coplanarity of the components was modulated by applying different capping strategies, such as variation in heat spreader thickness, design, and attach process to achieve a range of coplanarties, including some well above current specifications, as measured at room temperature. Component coplanarity was measured for each component, and the warpage behaviour of each component variation throughout a reflow profile with a peak temperature of 240 degrees C was also characterized. Module and card assembly limitations and the resultant solder joint geometries are further detailed in this work. Following assembly, the test vehicles were subjected to accelerated thermal cycling. The failures observed in the assemblies as a result of thermal cycling occurred in the second level ball grid array joints, located mostly in the center area of the package around the die shadow. The current study enabled the identification of the principal factors affecting the failure rates of the solder ball grid array, which include the board thickness, component design parameters such as heat spreader configuration and die size, and the shape of the solder balls, which is a function of the difference in height resulting from PCB and module warpage at the solidification temperature. Interestingly, room temperature coplanarity was not a strong indicator of reliability in this study. The work emphasizes the importance of achieving module to card coplanarity near solder reflow temperatures.