The viscoelastic characteristics of solder paste under high frequency oscillatory shear

In this paper, the viscoelastic characteristics of solder paste under high frequency (50 Hz-90 Hz) oscillatory shear were investigated by oscillatory shear measurements. Measurement results show that the dynamic viscosity of solder paste decreases as the oscillatory frequency increases, which means the oscillation can help to enhance solder paste fluidity. This proves that in a stencil printing process, the application of a vibrating squeegee can help the paste to fill the apertures. Within the oscillatory frequency range, results show that the solder paste loss modulus is much higher than its storage modulus. With shear stress of 500 Pa, the phase shift (ζ) is around 700, and with shear stress of 1000 Pa it is around 750, so the behaviour of solder paste under oscillatory shear is more like a viscous liquid than a solid. The storage modulus with low shear stress, corresponding to small shear strain, is higher than that with high shear stress, corresponding to large shear strain. This means that the solder paste storage modulus is not only dependent on frequency but is also dependent on shear strain. High oscillatory shear strain can reduce the solder paste deformation recoverability. It suggests that when passing over the apertures, the vibrating squeegee can help to obtain uniform and high solder particle packing density inside the apertures