Understanding the process window for printing lead-free solder pastes

Solder paste is primarily used as a bonding medium for surface mount assemblies (SMA) in the electronics industry. The solder paste is typically deposited using the stencil printing process. The stencil printing of solder paste is a very important and critical stage in the reflow soldering of surface mount devices. A high proportion of all SMA defects is related to the stencil printing process. This is likely to continue with the drive towards the introduction of lead-free solder pastes. Much work is continuing on the metallurgical properties of these lead-free solders, including solder joint strength and material compatibility. However, the initial challenge for the new Pb-free formulations is in achieving repeatable solder deposit from print to print and from pad to pad. To meet this challenge, new formulations in flux medium are now being developed. For a smooth transition to Pb-free soldering formulations, a proper understanding of the printing performance of the new solder pastes is necessary. The key parameters that affect solder paste printing have been identified and the subject of numerous studies. In lead-free solder paste, the replacement of lead with other elements (including Bismuth, Copper) changes the density of this dense suspension (solder paste). In this paper, we investigate the effects of the printer parameters, i.e. squeegee speed and pressure (defined as the process window) on the printing performance of a variety of lead-free solder pastes. A three-level design of experiment on these factors (pressure and speed) was used. Comparisons will be presented with lead-rich solder pastes. The metal content of the lead-free solders had a significant effect of the on the process window.