Viscoelastic properties of solder pastes and isotropic conductive adhesives used for flip-chip assembly

The printing of pastes (solder pastes and isotropic conductive adhesives) through very small stencil apertures required for flip-chip pitch sizes is expected to result in increased stencil clogging and incomplete transfer of paste to the printed circuit board pads. There is wide agreement in industry that the paste printing process accounts for the majority of assembly defects, and most defects originate from poor understanding of the effect of printing process parameters on printing performance. The paste printing process depends on conditions such as good paste roll, complete aperture filling and paste withdrawal. The paste rheology is important in the printing process because the viscosity of the paste must be low enough for the squeegee to force the paste through the stencil apertures, but high enough to recover to its required shape and not flow beyond its stencilled area. The rheological measurements can be used as a tool to study the deformation or flow experienced by the pastes during the stencil printing process. Rheological measurements for pastes are understood through parameters such as viscosity, storage modulus (G'), loss modulus (G") and are measured through flow curves such as shear stress vs. strain, creep-recovery and visco-elastic (oscillation) tests. The main objective in studying the rheology of pastes is to understand and anticipate the forces needed to cause deformation and flow during solder paste printing process, such as paste roll, aperture filling and emptying. The paper focuses on the extent that the oscillatory stress sweep can be used to study the solid characteristic and cohesiveness of the paste materials formulated specifically for flip chip assembly. The pastes will have viscous (fluid) and elastic (solid) like components. These properties changes whilst the paste experiences changes in the applied stress/strain. The oscillatory test can characterise the paste's visco-elastic behaviour and is used to understand the changes occurring in its structure during the stencil printing processes. In the test an increasing sinusoidal stress is applied to the sample and the measured parameters are the storage modulus (G') and loss modulus (G"), which are indicators for elastic and viscous properties respectively. The oscillation stress sweep can also identify the material's linear visco-elastic region (LVR). The LVR can be defined as the region, in stress or strain, where the material's structure is undisturbed, i.e. where G' and G" are unaffected by changes in stress/strain. Beyond this region can be identified as the point where the material's structure begins to break down. The results from the study show that the linear visco-elastic region for the pastes lies in a very small stress range. In addition, the stress at G'=G" can be used as an indicator of the paste cohesiveness. The oscillatory stress test can be used to study the structure of various paste formulations design for flip chip assembly.