Advanced assessment of thermal stress related failure modes occurring during the assembly of high pin count BGAs on PCBs

When addressing failure modes and reliability problems of high pin count circuits, the coplanarity of the solder balls is of major importance. In fact those components with more than several hundreds of solder balls only operate correctly if each single ball is correctly soldered. On the other hand, those components are in general critical in terms of thermo-mechanical behaviour. They are often used for heavy calculation purposes, and therefore generate a significant amount of heat during operation. Additionally, the mechanical structure is complicated, involving apart the die itself several laminated rooting layers. All those laminated materials (silicon, copper, different kinds of rooting layer substrate, mould compound, etc.) are characterized by different CTEs (coefficient of thermal expansion). Therefore each temperature variation of the component will inevitably generate internal stress and thus deformation of the entire component. An important amount of thermo-mechanical stress, and thus deformation, occurs during the reflow solder process. Due to the process heat up to 260°C the coplanarity failure of the ball array may become such important that a part of the solder balls is not touching at all the PCB at the moment of solder solidification. Or the contact between single balls and the PCB might be weak, and the resulting connection of this ball will cause a permanent reliability risk during the product's life time. Therefore, with increasing dimension and increasing pin count, coplanarity validation of the balls at solder temperature becomes a key issue for high pin count component reliability.