Experiments on Mechanical Behavior and Electrical Conductivity of Au/Ni-Coated PMMA-Core Composite Particle During Micro Compression Testing

In this paper, the mechanical behavior and electrical conductivity of the metal-coated conductive balls in the ACF (Anisotropic Conductive Film) material after being compressed and deformed by a flat die are analyzed experimentally. The method used is to take five Au/Ni-coated PMMA-core composite balls with an average diameter of about 5.65 microns in the conductive balls, and perform the compression behavior of loading and unloading with a single ball to obtain the data of compression strength, deformation, electrical resistance, compression rate, and recovery rate in the compression process. The relationship between the contact radius and the contact area with the compression rate is further calculated through theoretical assumption. For the assumptions and calculations of the contact area, the differences in the average stress-strain and the nominal stress-strain can be evaluated and used to explain the three stages in the deformation process of the conductive ball. The three main stages are: the initial deformation stage (Cr: 0-0.2) of the conductive ball is in the small deformation elastic compression zone, the middle stage (Cr: 0.2-0.5) is the large deformation plastic compression zone with part of the metal shell rupture, and the final stage (0.5-0.8) of the conductive ball is severely delaminated and crushed. The results show that the conductive balls can maintain a stable resistance of about 10 ohms at a range of compression ratio (Cr < 0.5). Such electrical resistance (~10 Ohms) can maintain stable current transmission and such average compression stress (~300 - ~750 MPa ) can make the effective rebound force (15-17 mN) of the metal-coated plastic ball. In order to effectively maintain power supply stability and mechanical structure reliability, it is suggested that the compression ratio in the ACF packaging must be effectively controlled to the value (Cr in this study: 0.3-0.5) before the occurrence of the compression strength.