Examination of the parameters affecting tack strength as measured in the IPC tack test

Solder paste is used in the electronics industry to connect surface mount circuit components to printed wiring boards. A printed wire board (PWB) is populated with components by applying solder paste on the PWB, placing components and finally reflowing the solder paste in an oven. Between the time the component is placed on the PWB and the solder paste is reflowed, the components are to be held in position. This is helped by tackiness of the paste. The present paper describes an experimental investigation of the tack strength of three solder pastes with probe size, probe surface roughness, drying time, and surrounding pressure as Design of Experiment variables. The results indicate that the probe size has a significant and dominant effect on all the pastes examined. A mechanistic view of the possible mechanism for this phenomenon is also presented. Measured tack strength is shown not to be simple adhesion, but to include a probe size effect which may likely be due to vacuum under the test probe such that tack strength (force per unit area) increases as probe size increases.