Mechanical Properties Of Cured Isotropic Conductive Adhesive Investigated By Micro-indentation

Since the lead-free development of electronic packaging, lead-free solderand conductive adhesive have been the major interconnected materials.Compared with solder alloy, conductive adhesive has the advantage such aslow technological temperature, ultrafine line printing, being easy to connect, etc.Research on the new-type products and mechanical properties of conductiveadhesive have caused extensive concern in this field. With the miniaturizationand multi-functionalization of electronic products, the size of interconnectedmaterials in electronic packaging is becoming smaller (micron level). Therefore,developing the measurement method with high resolution in small scale andcharacterizing the mechanical properties of cured conductive adhesive from themesoscopic view have an important operation significance in establishing themechanical properties database and industry design and production ofconductive adhesive. In this paper, the effect of loading strain rate, temperatureand humidity on the mechanical properties of cured isotropic conductiveadhesive (ICA) were systematically investigated by micro-indentation, andbased on numerical simulation the thermal mismatch stress of cured ICA were analyzed. The related research contents and conclusions are as follows:1. The strain rate sensitivity of cured ICA with different Ag content (50wt.%,60wt.%and70wt.%) were investigated under different strain rates (P P=0.005s-1,0.010s-1,0.050s-1and0.100s-1) by micro-indentation. Results indicatethat the stiffness, elastic modulus, shear modulus and hardness increase with theincreasing strain rate, showing the strain rate effect. Due to that the silverparticles restrict the deformation and movement of the molecular chains ofmatrix resin, the stiffness and yield strength of cured ICA were enhanced tosome extent with the increasing Ag content. The mechanical properties of curedICA are strongly dependent on the strain rate.2. According to the JEDEC standard hygrothermal aging condition (85℃/85%RH), the effect of hygrothermal aging on the mechanical properties ofcured ICA were investigated by micro-indentation. Due to the moistureexpansion and molecule degradation of matrix resin occurred underhygrothermal condition, the stiffness, yield strength and creep deformationresistance of cured ICA decrease. Based on the semi-empirical method fromgeneralized Kelvin model the creep behavior of cured ICA was characterized.With the increasing aging time, the proportion of instantaneous elasticdisplacementhe in total displacement during loading and constant0related toviscosity coefficient decrease, but the creep compliance increase, indicatingmore obvious relaxation phenomenon. Under a high degree of hygrothermalaging, the strain rate sensitivity exponent m of cured ICA is relatively large, showing low creep resistance. Meanwhile, the strain rate sensitivity exponentsunder different aging time are0m1, indicating inhomogeneousnon-newtonian flow. On the whole, the cured ICA with high Ag content is moresensitive to hygrothermal aging.3. Measured by thermal mechanical analyzer, the thermal expansioncoefficient (CTE) of cured ICA with high Ag content is low but glass transitiontemperature (Tg) is high. The high-temperature mechanical properties of curedICA were investigated by micro-indentation with the high-temperature testmodule (G-Series Hot Stage Frame Stiffness Cal) and results show that with theincreasing temperature the elastic modulus, shear modulus and hardness ofcured ICA decline from3000~7000MPa,1000~2500MPa and100~300MPa inthe glassy state to6~200MPa,2~70MPa and1~10MPa in the rubbery state,respectively. Compared with the glassy state, the proportion of instantaneouselastic displacementhe in total displacement during loading in the rubbery stateis relatively large but less than25%overall. Meanwhile, the smaller constant0related to viscosity coefficient and larger creep compliance in the rubbery stateindicate the stronger viscous flow and more obvious relaxation phenomenonrespectively. The effect of raising the temperature and decreasing the strain rateon the mechanical properties of cured ICA are the same, showing thetime-temperature equivalence.4. Based on the high-temperature measuring result and finite elementanalysis software ANSYS12.1, the thermal mismatch stress of cured ICA with different Ag content and thickness (20μm,40μm and60μm) in flip chippackaging were analyzed under the plane strain condition. On the whole, withthe increasing temperature the maximum of thermal stress component(X-stress,Y-stress, Z-stress and XY-Shear stress) increase, then decline rapidly, anddecrease slowly, showing a “tadpole-shaped” or “semi-tadpole-shaped”. Thethinner of cured ICA the smaller of shear thermal stress occurred at the interface.To raise the glass transition temperature (Tg) and lower the thermal expansioncoefficient (CTE) of cured ICA can avoid its yield behavior induced by thermalmismatch stress and improve the service reliability at high temperature.