Figure 2: SN100C. Figure 4: Fluidity compared

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1 Negative Effects of Phosphorus in Nickel Doped Solders. Paolo Corviseri, Balver Zinn, Germany Introduction The large-scale changeover to lead-free solder has significantly increased the importance of micro-alloying elements. On the basis of its pioneering role with nickel doped solder SN100C, Balver Zinn has developed a wealth of experience based on many studies and large-scale technical tests. The Effect of Nickel and the Resulting Advantages: The fundamental importance of nickel was first identified by Tetsuro Nishimura at Nihon Superior. He discovered that a nickel content of 0.05 weight-% produced the best results regarding, for example, flow, reduction in copper dissolution and stabilisation of the intermetallic compound. +Ni Figures 1 and 2: Shiny and smooth appearance of SN100C alloy is similar to that of SnPb. Other benefits include process stability and reliability Figure 1: Figure 2: SN100C Benefit with the original SN100C formulation such as process stability, long-term reliability and reduced copper dissolution is due to nickel doping. This also confers improved fluidity and breakage behaviour (Figures 3-5). Figures 3-5: Fluidity of SN100C is similar to that of SnPb. Breakage behaviour is better than with other lead free alloys. Figure 3: Fluidity dependent on Ni content Figure 4: Fluidity compared Figure 5: Breakage behaviour Because nickel acts as a nucleation site, the structure of nickel doped solder becomes very fine-grained. This enables better accommodation of mechanical strain. Figure 10: SnAg3.0Cu0.5 Figure 6: primary tin dendrites with eutectic Figure 7: Ni uniform eutectic microstructure Figure 8: primary tin dendrites with eutectic Figure 9: Ni uniform eutectic microstructure Figure 11: SnAg3.0Cu0.5+Ni

2 The same nucleation mechanism also minimizes inter metallic compound (IMC) growth resulting in a reduction of interfacial stress. OSP substrate Figure 12: IMC after soldering Figure 13: Cu6Sn5 in IMC After 2 x reflow and 500 hours at 125 C Figures 14: IMC after reflow and ageing +NiGe +NiGe Figure 15: IMC after soldering Figure 16: (Cu,Ni)6Sn5 in IMC with ~3% Ni Figure 17: IMC after reflow and ageing In addition to modifying the structure of the IMC, nickel can be found in the Cu 6Sn 5 phase (Cu,Ni) 6Sn 5. This serves as a barrier and slows down further IMC development during ageing and especially in the event of thermal load. IMC Growth on OSP Finish Intermetallic Thickness (microns) SnPb SnAgCu SnCu SnCuNi Initial IMC Thickness Solder Alloy IMC Thickness After Thermal Cycling Figure 18: IMC development after ageing. (4000 cycles C) Figure 19: Establishment of the number and length of cracks after 2 x reflow and 500 hours at 125 C! Nickel doped solder exhibits the smallest cracks in this situation. Figure 20: Number of cracks and length of cracks A similar process also influences the dissolution behaviour of lead-free soldering systems. This simplifies solder bath management. THT Figure 21: indicates less copper dissolution with SN100C, due to the nickel content. SnAg3.8Cu0.7 SnAg3.0Cu0.5 SN100C SMD SnAg3.8Cu0.7 Figure 21: Copper Dissolution SnAg3.0Cu0.5 SN100C

3 Inter Metallic Compound (IMC) There is a good reason why SN100C is one of the most successful, best known and most copied lead-free solders! Balver Zinn has turned the brilliant idea into a unique solder by, for example, only using raw materials of the highest quality. This affects solder properties and a direct impact on both production and end use, for example negating the need for complex cleaning procedures during manufacture. Phosphorus as a Possible Antioxidant All the advantages referred to above are the results of years of research and studies by the inventor, Tetsuro Nishimura of Nihon Superior. He then focussed on anti-oxidizing additives for SN100C. In light of its success with SnPb alloys, phosphorus was the first element selected. However, this exerted a deleterious effect as it counteracted the positive effect of nickel. Below are a number of research results, which indicate why phosphorus was rejected as an antioxidant. Shiny solder Joints due to nickel: Phosphorous counteracts this!! Figure 22: shows that the formerly smooth and shiny solder joints made from SN100C again exhibit a dull appearance due to the addition of phosphorous. Figure 22: Optical appearance of solders made from the SnCu - alloys Figure 23 shows how SN100C smooth solder joints tend to exhibit nucleation holes with Phosphorus! Figure 23: Optical appearance of lead-free solders Improved fluidity due to nickel: Phosphorous counteracts this!! Figure 25 shows the drastic reduction in fluidity due to the addition of phosphorous. The Ragone Test can be used to establish the fluidity column length of molten solder. SN100C has a length of approx. 650 mm in the same test conditions. Figure 24: Improved fluidity behaviour due to nickel Figure 25: Fluidity depending on phosphorus content

4 Figure 26 exhibits worsening breakage behaviour due to the addition of phosphorus and increased risk of bridging! Figure 26: Breakage behaviour Finer structure due to nickel: Phosphorus counteracts this!! SN100C P P P SnCuNi SnCuNi+0.007P SnCuNi+0.02P SnCuNi+0.06P Reversal of the nickel- effect Figure 27: Reversal of the beneficial effect of nickel due to phosphorus Reduced de-alloying behaviour due to nickel: Phosphorus counteracts this!! Figure 28: Even copper dissolution behaviour changes negatively due to the addition of phosphorus! Figure 28: Copper Dissolution Intermediate Compound 2 Many of the positive properties realised by the nickel are changed in a negative sense by the addition of phosphorus. This renders the SN100C solder bath unsuitable for long time usage. The Nihon Superior patent entitled Regulation of the Nickel Content in Lead-Free Solder Baths was based on this research because it was established that, in certain conditions, phosphorus binds nickel and removes it from the solder bath!

5 Germanium as a Reliable Solution for Oxidation Minimisation During the introduction of SN100C, germanium was not mentioned as a fourth alloy component and as an anti-oxidising solder additive. At a level of 55 ppm it is still part of the basic analysis. Stable and very high raw material prices and the constant pressure on prices experienced by customers inspired Balver Zinn to initiate a further study into process optimisation and cost savings. As a result the solder variant SN100CS with 100 ppm of germanium was examined. The germanium content was further optimised, this time to 250 ppm. As with phosphorus in SnPb alloy, germanium was also consumed (albeit much more slowly than phosphorus / SnPb), necessitating periodic measurement and adjustment. Empirical values show that contents of 130 ppm and more reduce the tendency to oxidize and that the content levels off at around 60% of the initial content. Therefore, 250 ppm basic germanium represents a good compromise. SN100CS SN100C Figures 29-31: Machine configuration: Setup three-row turbulent wave: 65% Setup laminar wave: 70% Solder bath temperature: 290 C Both waves in continuous operation: 8 hours Test atmosphere: Air Figure 29: Residual germanium after time Figure 30: Germanium consumption Figure 31: Dross reduction approx. 20% The germanium consumption established during the test and the dross formation may differ depending on the machine configuration, the material throughput and operation conditions. Figures 32-33: Germanium concentrates at the surface within the IMC Figure 32: No germanium in the IMC Figure 33: Germanium concentrates at the surface Summary The addition of nickel as a micro alloying element causes positive effects with regards to the technological properties of alloys. Phosphorus contamination of just mass % (20 ppm) negates these alloying advantages. Since phosphorus is toxic to lead-free, nickel micro-alloyed Ni0.05 alloys, the use of phosphorus tablets as an anti-oxidant is not recommended. For nickel micro-alloyed Ni0.05alloys, germanium is the only, currently known, stable and effective method to minimize dross formation. It also exerts a positive influence on the fluidity of the alloy. Germanium even continues to work after the solder has solidified and prevents the tarnishing of solder joints at high temperatures (e.g. yellowing of HAL surfaces on printed circuit boards). References: 1. Effects of Phosphorus on Microstructure and Fluidity of Ni0.05 Lead-Free Solder. K. Nogita, C.M. Gourlay, J. Read, T. Nishimura, S. Suenaga and A.K. Dahle. 2. Nihon Superior Presentation on SN100C, 5 th November 2009, Keith Sweatman Senior Technical Advisor. 3. EP A1 Patent, Method of regulating nickel concentration in lead-free solder containing nickel, T. Nishimura.