In order to investigate the use of Sn-8Zn-3Bi solder as a potential substitute for Sn-Pb solder, which has a lower melting point than Sn-Ag family solders for CSP assembly, we studied mechanical properties of CSP joints plated with varying thicknesses of Au and Ni on Cu pad. Joint strength and other mechanical properties were evaluated in relation to reflow peak temperature. The combination of 0.05 mm Au plating thickness and reflow peak temperature of 498 K resulted in the best joint reliability in the as reflowed condition and also after aging treatment. The joint was founded to have thin Ni 3 Sn 4 type interfacial reaction layer that included Cu and Zn between the solder and the Ni plating. This interfacial structure was shown to improve the joint strength.
The lead-free soldering technology has been devoloped all over the world while IEEE ReHS prohibits the use of lead contained solder in 2006. Sn-Ag-Cu solder of which melting point is 219C has the highest solder joints reliability of leadfree solder materials. This melting point is much higher than that of the conventional solder of 183 C. So reflow process for low heat-resistant components on Print Circuit Board needs lower melting point solder. Sn-Zn-Bi solder with low melting point of 197 C has a high barrier to apply to electric products due to lower reliability at high tempereture, in high humidity and after reheating a joint comparing to conventional solder. Adding both bismuth and indium into Sn-Ag solder alloy is effective especially for decrease of melting point of Sn-Ag solder and Sn-Ag-Bi-In solder which had a melting point of 206 C was devoloped. In this paper, we mentioned design of the solder alloy and soldering properties of Sn-Ag-Bi-In to the point of apearance and microsructure of solder joints concerning about the influences of temperature, humidity and heat story of joint surface after 1000 cycles at À40 C/125 C and after 1000 hours of 85 C/85%RH. But the solder joint strength of Sn-Ag-Bi-In in comparable to that of Sn-Pb eutectic solder in each test. And no significant deterioration of Sn-Ag-Bi-In solder had the same reliability as conventional solder and could be useful to expand the practical use of lead-free solder for a lot kinds of products.
A method for designing lead-free solder was studied using the liquidus temperature of a 2-component eutectic solder in order to develop a high temperature lead-free solder. In this work, tests were done to obtain a targeted liquidus temperature of a 4-component lead-free solder based on the weight percentage of two sets of 2-component eutectic solders. This 2-component eutectic solder was assumed to be thermodynamically stable in itself. The weight percentages of five respective types of Bi-Cu, Ag-Cu etc. eutectic solders were varied and the design temperature was predicted and compared against the measured liquidus temperature. In doing this, it was learned that the measured apparent liquidus temperatures were liniear with respect to the design temperatures. Similar studies were conducted into a medium temperature material of SnAg-Cu and a low temperature material of Sn-Bi-Ag. The measured temperatures of these solders also corresponded to the design temperatures.
Sn-Bi alloy is one of the representative low temperature type lead-free solders. However, the bonding properties of the Sn-Bi solder are not good. The reason for such properties is related to Bi microcrystallines that segregate at the interface between the solder and a Cu substrate. We found that ultrasound improves the bonding strength for the Sn-Bi alloy system solders by dispersing and miniaturizing the Bi crystals. To achieve such dispersion, we invented a novel ultrasonic soldering technique. By using this technique, ultrasound can be applied to printed wiring boards (PWB). Besides the improved bonding strength, we found that the temperature of a PWB is increased by the application of ultrasound to the PWB. Sn-58 mass%Bi solder is melted by the vibrational energy of ultrasound without other heating methods. Moreover, the interfacial layer between the Sn-58Bi solder and the Cu land is homogenized by ultrasound. Also, the interfacial layer between the Sn-8Zn-3Bi solder and a Cu land becomes thinner by ultrasound. We believe that these changes in the interfacial structure improve the mechanical properties of the solders. Therefore, ultrasonic soldering technique will improve the usability and reliability of Sn-Bi alloy system solders.
We studied the effects of adding various amounts of Sb to Sn-3.5Ag-0.5Bi-6.0In-0.8Cu (SABIC) solder in terms of the phase transformation temperature for the b-Sn and g(InSn 4 ) transformation, mechanical properties such as strength and ductility, and thermal fatigue properties under thermal cycling (-40°C/175°C). With increasing amounts of added Sb, the temperature at which the phase transformation occurred increased. At 150 and 175°C, breaking elongation values of solders as a measure of ductility became the highest for an Sb content of 0.5 wt.%. As the phase transformation temperature increased, deformations of the solder joints after the thermal fatigue test were suppressed. The thermal fatigue properties were best at an Sb content of 1.0 wt.%, at which the phase transformation temperature was greater than 175°C.It is considered that raising the phase transformation temperature to a temperature higher than the maximum operating temperature of electronic devices should be the topmost priority to improve crack extension behavior.
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