Thermal-mechanical strain characterization for printed wiring boards

Multilayer printed wiring boards are widely used in electronic packaging assemblies. One critical reliability concern is the thermal-mechanical strains induced by temperature change. For example, the in-plane strain affects the thermal fatigue life of surface mount solder interconnections, while the out-of-plane strain affects the mechanical integrity of the plated-through holes of the printed wiring boards. For this paper, a systematic study of the thermal-mechanical strain of epoxy-glass printed wiring boards, below and above the glass transition temperatures of the epoxies, has been carried out. The study includes measurements of properties of basic constituent materials (epoxy, glass fabric, copper), of intermediate building blocks in the fabrication process, and of final products. The study has led to a quantitative engineering model that predicts the average in-plane thermal-mechanical strain for use in modeling surface mount components on a printed wiring board, as well as the average out-of-plane thermal-mechanical strain for determining plated-through-hole reliability in thermal shock processes. The model was verified by two experimental techniques (measurement by thermomechanical analyzer, and moiré interferometry) applied to two epoxy resins and three glass fabrics, with and without copper planes. For thermal shock below the glass transition temperatures of the epoxy resins, the in-plane and out-of-plane strains are described by a modified rule-of-mixtures theory and a biaxial plane stress model, respectively. For temperatures above the glass transition temperatures, the in-plane strains are governed by the copper and glass fabric, whereas the out-of-plane strains are dominated by the incompressible fluid behavior of the epoxy resins. The nonuniform pattern of thermal expansion in regions populated with plated-through holes was examined. The reliability of surface mount solder interconnections and plated-through holes is discussed.