Solder pastes are complex products which are designed to meet the conflicting requirements of printability, slump resistance, good tack and a range of reflow conditions. This paper describes how the metal content and solder particle size distributions in solder pastes affect these properties for a typical RMA type product. The major effects are explained by reference to the purely physical effects of alloy density, metal content and solder powder size on the mean particle separation within the paste. Lower alloy density, high metal content and smaller solder powder size all reduce interparticle separations causing viscosity to increase, slump resistance to improve and peak tack force to increase. The paper also discusses the more subtle effects of the chemical interactions between solder powder and flux medium on the same properties. These are illustrated by reference to the changes which take place in performance of a typical solder paste during storage over 12 months. Ageing by solvent loss and rosin drying brought about by the effects of metal soaps formed between solder and activators probably contribute to these changes. Ageing can result in improved performance in tests like slump resistance and consequently solder balling, while in others, such as open time, there is a decrease in performance.
Soldering & Surface Mount Technology – Emerald Publishing
Published: Mar 1, 1991