A COMPARATIVE ANALYSIS OF PTH HOLEFILL PERFORMANCE BETWEEN SnPb AND Pb-FREE USING DESIGN OF EXPERIMENTS ON COMPLEX PCBAs

摘要

Demand for product performance and functionality has resulted in increased complexity of Printed Circuit Board Assemblies (PCBAs) used in them. The complexity is in the form of increased routing density and multiple power sources and grounding requirements within the PCBA. These PCBA enhancements, although meeting the functionality and performance requirements, present challenges in both Surface Mount (SMT) and Plated Through Hole (PTH) assembly processes.Achieving acceptable solder fill levels per IPC standards has been a challenge using SnPb alloys for high mass and thick PCBAs. In comparison with SnPb, Pb-free solder alloys have higher melt points, a disproportional increase in wave solder pot temperature and reduced wetting characteristics. It is therefore important for us to understand the impact of change in alloy on our PCBAs to wave soldering.The objectives behind this study were two fold, the first to quantify the potential reduction in holefill achieved when changing from SnPb to Pb-free alloys. Secondly, it was essential to identify critical design factors that improve holefill and create optimal design rules for PCB design when soldering with Pb-free.This research details the results and statistical analysis of a Pb-free wave solder study conducted on a test vehicle comprised of varied physical attributes and PTH design elements. The test vehicles werefabricated in three (3) board thicknesses, five different lead to hole ratios (holes sizes) and with four different PCB plane connection schemes. Components with two different lead diameters were designed onto the test boards.The test vehicle was wave soldered at multiple contract manufacturer sites using both SnPb and Pb-free alloys. Each contract manufacturer processed the test boards with and without selective wave pallets using their respective wave soldering best practices. Each contract manufacturer best practice was predicated on their wave soldering equipment, flux chemistry used, flux application method, flux volume applied, preheat temperature, belt speed, solder pot temperature and card immersion depth. Holefill measured via X-Ray was the primary responsevariable. A total of 9000 data points for SnPb and 3000 data points were measured for Pb-free.The results were analyzed using statistical analysis tools to identify primary and secondary interactions between the various factors studied in this research.Numerous interaction plots and interaction profiles were generated for both SnPb and Pb-free that numerically and graphically display solder results and provided basis of the comparative analysis.The results indicated that while PTH solder fill results varied by contract manufacturer site, the overall trends and interactions tracked very well. The interactions were similar between various factors for both SnPb and Pb-free alloys, though holefill with Pb free alloy was reduced.As improvements to performance and function evolve, ever increasing PCB complexify changed to meet the demand. Typical in that evolution has increased routing density, increased layer count and incorporated multi power sources within the circuit board. These PCB enhancements, although meeting the functionality and performance requirements, has present challenges in both SMT and PTH process. Increased functionality has resulted in greater intermixing of large and small packages having both higher and low I/O counts, density and pitch. Increased routing density and multi voltage application has resulted in increase PCB routing layer and power and ground planes all of which increase over card layer count and card thickness. These high mass circuit boards in conjunction with the high mix of small and larger (size and mass) SMT components has reduced the SnPb SMT process windows and its impact on SnPb plated thru hole (PTH) wave soldering has been shown to be most impacted. Wave soldering may be soon be approaching its limits in achieve acceptable solder fill levels per existing industry standard. PCBA process challenges can only be further challenged in Pb-free assembly. Pb-free wave soldering high mass thick PCB cards using solder alloys with higher melt points, reduced wetting characteristic and a disproportional increase in wave solder pot temperature to that if its SnPb counter part may well render high complexity PCB unable to meet present PTH solder fill specifications. This report focuses on PTH wave solder. Results of two experiments will be presented.