CCGA solder column ? Reliable solution for absorbing large CTE mismatch

Solder balls encounter large stresses due to the inherent mismatch of the coefficient of thermal expansion (CTE) between a large ceramic ball grid array (CBGA) and a glass-epoxy printed circuit board (PCB). The array of solder balls are stretched and squeezed with considerable strain from corner to corner across a large square CBGA module, as the temperature swings plus and minus 100o C from hot to cold in repeated cycles, typically encountered in harsh operating environments of aerospace and defence. While the amount of travel - about the thickness of a single sheet of paper - seems small, such movement ultimately leads to catastrophic failure as solder balls delaminate from the PCB board and/or the ceramic CBGA module. Overcoming CTE mismatch can be accomplished by replacing spherically shaped solder balls with cylindrically shaped solder columns, making a device known as a Ceramic Column Grid array (CCGA). CCGA Solder columns are compliant and offer improved absorption from CTE stresses caused by stretching and pulling in applications involving high vibration, shock and wide temperature variations. This presentation covers several types of solder columns that are commercially-off-the-shelf (COTS) available such as plain Pb90/Sn10 columns, copper wrapped Pb80/Sn20 columns and Micro-coil Springs. This paper provides "safe-harbour" recommendations when it is safe to use CBGA with solder balls, and when it is best to use CCGA with solder columns.