Assessment of lead-free solder pastes with different type of powder for fine-pitch application

Stencil printing is a firs step in surface mount assembly process. For small apertures, solder paste consistency and volume are critical to solder joint reliability. The pastes which are most often used in surface mount technology contain SnAgCu alloy with type 3 or 4 powder size. It seems that for small apertures the paste with type 5 powder particle size will have better release from stencil apertures than pastes with larger particle size. On the other hand reduction in the particle size diameter of the solder powder leads to an increase in the metal oxide content and consequently to the formation of solder balls during reflow. In the paper the results of assessment of lead free solder pastes with SnAgCu alloy and three powder particle sizes printed by the small stencil apertures are presented. The pastes with particle size type 3, 4 and 5 easy of attainment on the market were chosen to investigate. The rheological and technological properties of those pastes were investigated. There were made: wetting test, viscosity test, slump test, copper corrosion test and reflow process tests. All of the investigated solder pastes pass the slump test. However, results of wetting test show that solder paste with powder particle size type 4 and 5 is more adequate for fine- pitch assembly. But it should be notices that the solder pastes type 5 are more prone to oxidation and are more expensive.