Evaluation Of Tab Conductor Materials

Abstract

Various types of wrought and electrodeposited copper foils are used in the fabrication of TAB lead frames, The grain structure and mechanical properties of these foils may vary significantly depending on the type and temper of the copper foils. The purpose of this study was to develop and understanding of how the mechanical properties of various copper samples that are currently used in the TAB lead fabrication correlate with the reliability performance (temperature cycle) of selected ILB leads. Tensile modulus, coefficient of thermal expansion (CTE), ultimate tensile strength (UTS), yield strength (YS) and fatigue ductility of all the TAB foils were studied and compared. The results of this study indicated that indeed there was a one-to-one correlation between thermal stress, UTS, fracture strength and fatigue ductility and the reliability performance. Evaluation of thermomechanical properties (including tensile modulus and coefficient of thermal expansion) was achieved through Dynamic Mechanical Thermal Analysis (DMTA). The yield strength, ultimate tensile strength (UTS) and fracture strength were obtained using PL Thermal Sciences MINIMAT , while the fatigue properties were determined through an Instron tester. Failure modes in reliability tested samples were determined through Scanning Electron Microscopy. As a first order approximation, the product of the modulus and CTE ( over the temperature range of interest) may be used to determine the thermal stress during temperature cycling. Materials with minimum thermal stresses are expected to yield improved reliability performance. Similarly, a direct relation also exists between UTS and fracture strength (area under the stress-strain curve) and the reliability performance. Yet another property, the fatigue ductility, may also be used to predict the reliability performance, but strain rate effects have to be considered before arriving at optimal solutions. ExDerirnental Two types of ED (LF-A and LF-B) and three types of wrought ( LF-C, LF-D and LF-E) copper samples were used in the analysis. The modulus and coefficient of thermal expansion (CTE) were obtained using the DMTA-Tensile head. For the evaluation of thermomechanical properties, the specimens were subjected to a known dynamic force. A static force of 1.0-1.2N was applied to provide a positive tension, to prevent specimen buckling due to thermal expansion. The samples were analyzed at two frequencies, viz., 1Hz and 10% over a temperature range of 30" to 200" C and at a heating rate of 3" C/min. Both modulus and CTE values were computed by continuously monitoring the deflection of the samples during the DMTA run. The yield strength and ultimate tensile strength OJTS) were obtained using MINIMAT material tester. Stress-strain curves were generated by increasing the load on the sample at a rate of OSN/min and measuring the deflection of the sample as in DMTA.