This is the second in a series of papers which use a new technique for ranking the Lead Free surface finishes, SMT paste alloys and quantifying mechanical performance for each combination. The first paper in this series was presented at INDIANA CHAPTER MEETING SMTA February 23, 2006.
Thomson has developed a test vehicle and protocol to enable our engineers and management decision makers to easily compare performance differences and measure, often elusive, interactions between surface finish, paste flux and solder alloy. The HALT based technique, and its novel PCB and component design has provided useful data and helped eliminate the haze surrounding these complex relationships and their trade-offs. The thermo-mechanical test method measures the average energy required to fracture SMT solder joints under 9 different strain conditions. The ease of replication and simple test geometry allows practical multifactor experiments to be conducted on paste alloys and their flux vehicle. The method permits measurement of thermal stability, packaging and shelf life studies on PCB finishes and allows measurement of their complex interaction with paste alloys, i.e. wetting, fillet formation, intermetallic phases distribution, alloy dilution.
The elegance of this method is that the result is both fast and definitive. Rapid temperature cycling and vibration step stress tests are completed within 8 hours. The accumulated damage required to cause failure is measured as an electrical open circuit, and is not subject to tedious optical inspection, crack measurement nor arbitrary visual rating system. Furthermore, it allows a single PCB supplier to produce all PCB and test components out of identical materials which can then be distributed to a coating supplier for unified coating application. Coated samples are returned to a single assembly test site for Surface mount reflow with the paste and flux required for the analysis.
The first paper in this series discussed the test development and showed interesting performance differences between Lead and New SAC alloys. It showed general performance ranking of multiple surface finishes and demonstrated sensitivity to defective, oxidized and tarnished coatings. Data presented in this paper has expanded upon our earlier work now including evaluation of multiple suppliers of Immersion silver & tin coatings, OSP, ENIG and Lead free HASL. Additional reliability data on 3% & 4% SAC alloy will be presented along with comparisons to new alternate paste alloys; SnCuNi and SnZn.