Viscoelastic Effect of FR-4 Material on Packaging Stress Development

In the microelectronics device, FR-4 material is one of the key component materials in the packaging structure. The structure of FR-4 is featured as a typical orthotropic polymer composite material reinforced with glass fiber. Although the material may exhibit viscoelastic behavior, the material is commonly regarded as temperature-dependent elastic one in the microelectronics reliability analysis. The purpose of this study is to investigate the material property modeling effect of FR-4 on the reliability prediction. To obtain the FR-4 material properties, a series of stress relaxation test was performed. Based on the data, the behavior of the FR-4 was modeled as orthotropic temperature-dependent elastic and viscoelastic. An analytical method was employed to calculate the engineering constants required for the calculations. The elastic calculation was performed by ANSYS, and the viscoelastic calculation by a FORTRAN program, respectively. The FORTRAN program was written separately, and capable of calculating the viscoealstic calculations when the material is orthotropic. Board on chip (BOC) packaging structure, a new type of the first level packaging for memory chip, was considered for the stress development analysis. The results were compared to examine the difference based on the FR-4 material property modeling.