A study of the lead-free hot air solder levelling process

摘要

Purpose - The traditional surface finish for bare boards, eutectic tin-lead solder from the HASL process, is well characterized, but very little is known about the implementation of lead free solder in that application. This paper will present the results of a study that defined process parameters for both horizontal and vertical machines. Design/methodology/approach - A test vehicle containing high aspect ratio PTH technology and fine pitch surface mount pads was used for the testing. Solder samples were taken while copper clad laminates were run through the machine. Test panels were run after each 100 laminates and were assessed for solder coverage, appearance, thickness and distribution. Board cleanliness was measured by SIR and ionic contamination. Findings - Operating parameters for the respective machines were established. Fluxes and oils were found to meet the requirements. Bright solder coatings produced on test panels contained no exposed copper and were found to meet industry standards. SIR results met the requirements of SM-840C. Research limitations/implications - Future research to analyze and replenish the lead free solder pot with tin and nickel is needed to assist board shops with the production use of the process over a longer period of time. The work of this paper covered a few days of simulated production, depending on the daily throughput. Practical implications - Work has been done and reported to demonstrate that the Lead Free HASL is a viable process ready for commercial implementation. Originality/value - The work of this paper will allow a circuit board fabricator to have a starting point to implement the lead free HASL process. Copper and nickel tracking data are helpful to set up a solder dilution schedule to maintain their concentrations within operating limits.