Mold Compound Properties for Low Warpage Array Packages

The size reduction of array packages presents a number of assembly and reliability challenges. These are associated with increasingly thinner and more fragile dies, complex substrates, reduction in pitch and line spacing and decreased die attach bond line as well as mold cap thicknesses. In addition to minimization of various parts such as substrate, die, mold cap thickness and package-to-package gap, the device warpage is of prime importance. Excessive package warpage during the reflow process can lead to undesirable issues such as solder ball bridging, die stress and/or no connection between solder joints. Fortunately, device warpage can be controlled by proper material selection; especially, the mold compound. Physical properties of the mold compound are commonly optimized to control the warpage characteristics of an array package. These optimizations include the use of epoxy resins cross-linked with flexible and functional hardeners, higher filler loadings and an appropriate choice of the catalyst system. This work describes the dependence of warpage on the choice of the epoxy type ("flexible" versus "hybrid") and their ratio in a formulation. Among other performance parameters the filler level in these systems was also optimized to provide reduction in shrinkage and CTE. A series of mold compounds exhibiting a wide range of glass transition temperatures (Tg), coefficients of thermal expansion (CTE) and total shrinkage properties was prepared. Warpage of the molded block and singulated package was measured at room to solder reflow temperatures by Shadow Moire\´. The warpage was correlated with total shrinkage and thermal expansion data. Correlations were found to be useful in material selection for appropriate package warpage control.