Lead-Free Soldering Technology Review - Evaluating Solder Pastes and Stencils

Lead-free solders differ from lead-bearing solders in several properties such as surface tension, wetting ability, etc. In paste form they have greater adhesion force, so they can block small stencil apertures more easily. Because of this fact, the printing parameters and stencils design criterias have to be changed. Since lead-free soldering has to be implemented in the electronic industry until 1^st of July 2006, selection of the appropriate pastes and stencils is more important than ever. In our experiment six TSC (Tin-Silver-Copper) solders from different suppliers were evaluated from aspects of wetting properties, solderability and printability alike. Component self alignment was also investigated. In addition three kinds of stencils (lasercut stainless steel, lasercut nickel and electroformed nickel) were investigated as well by measuring transfer efficiency in function of area ratio of stencil-apertures. Results have shown that electroform stencils have outstanding printing properties while lasercut stencils (stainless steel and nickel) have similar capabilities independently of the material, thus it is not sure it is worth using lasercut nickel stencils for double the price. The evaluation of solders has revealed that the new lead-free pastes have equally excellent printing and soldering properties. Lead-free soldering with these pastes will not be problem.