Increase of SMT Process Robustness Through the Right Selection of Solder Pastes
Authors: Jose M. Servin and Cynthia Gómez Company: Continental Date Published: 10/14/2012
Abstract: Due to the growing quality demands by customers, the development of stringer tests for selection of solder pastes should be a requirement to reach a robust process. For that, several tests were developed for the selection of solder pastes in SMT lead-free Processes in order to increase quality and production demands: the parameters were selected to simulate extreme environment or/and process conditions. The test set was divided according to main SMT processes: Printing, Pick and Place, and Reflow. In printing, two stages of tests were performed: Evaluation of Viscosity to measure the performance of solder paste over stencil, and Slump test to check paste capability in fine pitch apertures and special applications in the product; these tests were adjusted for controlled environments and no controlled as well. To obtained the performance in pick and place, we extended the JIS tackiness test for more than 4 hrs but less than 24 hrs in a temperature and humidity chamber; besides, tackiness were measured in hot conditions simulating reflow oven conditions; tackiness is extremely important when we talk of miniaturized technologies (0402 and lower sizes). For reflow process, wetting test in nickel and copper with cleaned and oxidized coupons was included to determine if the flux is capable to clean common and uncommon oxides, especially with the new incorporated lead free alloys.
Besides, solder balling test was also done and this challenges the solder paste to clean even more oxide coming from its own metal, solder powder oxidation should remain at a level to avoid solder beading or shorts. The results showed that many solder pastes are developed to get acceptable performance in standard process conditions; however, they have weaknesses when process variability occurs. Therefore, the solder pastes with the best performance in the extreme lab tests got the best results in normal process performance.