Popcorn phenomena in a ball grid array package

For the purpose of studying popcorn phenomena, plastic ball grid array packages with 119 I/O's were tested under the pre-conditioning test conditions. Observations using scanning acoustic tomography and optical microscopy were carried out to investigate the existence of delaminations and cracks in the package, and the cracking patterns after IR reflow. Package deformations and thermo-mechanical stress distributions in the package were calculated by the finite element method. Three types of substrates were tried to prove that open thermal viaholes under die pad could prevent popcorn cracking during IR reflow. From the experiments and the observations, it was concluded that package cracking, which was caused by the expansion of moisture concentrated at the die adhesive layer, could be prevented using open thermal viaholes under die pad. The open thermal viaholes acted as vent holes, through which the expanded water vapor could go outside, not causing popcorn cracking. The die-attach process using U.V. tape was effective in the assembly of the packages with open thermal viaholes.<
>