Mechanical Characterization and Constitutive Description of Thin-Film Polymeric Materials

Abstract

AbstractMechanical properties of thin-film polymers often dictate the mechanical integrity and performance of microelectronic assemblies. For example, excessive interfacial stress between the polymer and signal via induced by thermal mismatch will lead to delamination of the interface or via cracking. This type of problem becomes more aggravated as dimensions of assemblies continue to shrink. In order to define the optimal design variables and process windows, the mechanical properties of polymer thin films throughout the complete process cycle must be carefully characterized. This paper will first review the experimental techniques commonly adopted in the mechanical measurements of thin-film polymers. Then, it will focus on the most difficult part of the mechanical characterization which is along the out-of-plane (Z) direction. Experimental set-up using the capacitance gauge will be introduced. Critical mechanical properties of polymers will be discussed. For the purpose of illustration, two polymers are chosen as examples. A BPDA-PDA polyimide is used to demonstrate mechanical characterization before melting whereas a PTFE-SiO2 composite is chosen to demonstrate the rheological characterization after melting. Before melting, mechanical behaviors such as the stress-strain curve, creep, relaxation, coefficient of thermal expansion, strain-rate dependence, and temperature dependence are discussed. After melting, the rheological behavior and the effect of aspect ratio measured by a squeezing flow experiment are addressed. Constitutive descriptions are proposed. The functional dependency between stress (pressure), strain (displacement), time, and temperature are characterized. A comparison between the numerical simulations and the experimental measurements is also presented.