Effect of rheological characterization on the jet printing performance of lead-free solder paste

Solder paste is an integral part of surface mount technology which an adequate understanding of its behavior and properties is essential to comprehend the relationship solder paste has to the overall manufacturing process. The jet printing technology of solder pastes is a new packaging method which provides customized packaging solutions for different requirements with a cost advantage. Jet printing technology solves the problem of control of solder paste's height in screen printing and it is also suitable for soft substrate printing, which may possible make it have wide application in complex integrated circuit. This study concerns the rheological characterization of different solder paste component used for jet printing. A series of rheological tests were carried out (viscosity, oscillation stress sweep, thixotropic loop and creep recovery test) to aid with understanding the characteristic of the pastes in the flowing processing. Besides, the pastes samples were tested on a jet printing machine with piezoelectric ceramics actuator as driving force. The solder pastes were jet printed onto a pad and the imagination of droplets were collected by a 3D microscope. Our results show that the nonlinear flow curve obtained in the viscosity test revealed the “shear thinning” behavior of solder paste samples as typical non Newtonian fluid. The linear viscoelastic region(LVR) of three solder paste samples was obtained by the oscillation stress sweep test curve and the transition point between liquid and solid was determined when the storage modulus (G') equal to the loss modulus (G"). Paste 1 indicated the biggest LVR while Paste 3 occupied the highest value of G' and G". The Paste 3 indicated the largest thixotropic loop area and the least unrecovered stress. The creep recovery properties were further tested according to the LVR under shear stress of 5Pa and 50Pa, respectively. In the recovery test Paste 3 indicated the best recovery and the lowest values of creep and recovery compliance in the LVR (more than 5Pa) while the opposite results at the stress of 50Pa, which proved that the irreversible deformation happened in the internal structure of solder paste. Jet printing results show that Pastel could obtain excellent droplet morphology similar to the perfect hemispherical. While Paste 2 obtained highest points in three samples as the largest ratio of height to diameter. Fracture and collapse arose after the droplets of Paste 3 were printed.