Popcorn phenomena in a ball grid array package

Plastic ball grid array packages can be an alternative to the fine-pitch plastic quad flat package with many I/O's, due to its advantages, such as small footprint and high density interconnect, improved module assembly yield, superior thermal and electrical performances to plastic quad flat packages, etc. In spite of these many advantages, there are several concerns which hinder the plastic ball grid array package from the widespread use. One of them is the package cracking by popcorn phenomena. For the purpose of studying the popcorn phenomena, plastic ball grid array packages with 129 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 thermomechanical stress distributions in the package were calculated by the finite element method (FEM). 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 viahole under die pad. The open thermal viaholes acted as vent holes, through which the expanded water vapor could escape without causing popcorn cracking. The die-attach process using UV tape was effective in the assembly of the packages with open thermal viaholes.<
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