| dc.description.abstract | New microjoining materials are needed for the soldering of electronic components due to environmental concern, reliability concern, and designing restriction. For example, a new low melting point solder is needed due to the thermal stress becoming more harmful as a result of thinner products. Development of such new microjoining materials is helped by fast methods for the testing of more candidate alloys in shorter time. This thesis describes the development of a real time resistance monitoring method, and the use of it to improve the understanding of microjoining process and reliability with four examples: current stressing of solder joints, solder reflow process, silver sintering process, and power cycling of solder joints, respectively.
In the current stressing of solder joints, a high constant DC current is applied to the Sn3Ag0.5Cu (SAC305) solder samples, and the sample resistance is recorded in real time. Complete resistance vs. time curves are obtained from start to failure. Four stages are identified from the resistance curve, and are explained with intermetallic layer growth, crack formation, crack healing and formation, and final open, as observed from cross sectional studies.
In the solder reflow process, two SnPb solder pastes with different levels of freshness are studied. Sample resistances are recorded during the reflow process. Simultaneous multiple sample monitoring is achieved. Characteristic resistance vs. time curves are obtained for both solders. Other than the initial resistance difference between the two solders, a special event of transient high resistance is observed for the older solder, and is explained with flux segregation between the solder balls and the lead fingers, as observed from cross sections.
In the silver sintering process, two special digital multimeters are integrated to cover a large range of resistance values (10⁻⁴-10¹⁰ Ω). The setup can work in different modes, depending on the requirement of fast sampling rate, large resistance range, or massive data for statistics. The resistance vs. time signal of the silver sintering process is obtained, and typically shows three stages: 1. dropping and rising forming a “V” shape in the range of ~10⁴-10¹⁰ Ω, 2. steady resistance value of ~10¹⁰ Ω followed by an abrupt resistance drop to <1 kΩ, and 3. steady resistance drop to <1 mΩ. Differential scanning calorimetry (DSC) and cross sectioning are used for understanding resistance signals. As a result, the three stages are correlated to: 1. solvent evaporation, 2. capping agent degradation, and 3. silver sintering.
In the power cycling of solder joints, a total of two different PCB designs, and four solder materials are studied, including SAC305 and three solders that contain Bi. From the first PCB design with Sn57.6Bi0.4Ag solder, two failure modes are observed depending on the peak temperature. When the peak temperature is too high, the resistance signals show insignificant change, and the solder joints becomes grainy. When the peak temperature is moderate, resistance signals show rise until open, and cracks are observed from cross sectioning studies of failed samples. From the second PCB design with a larger resistor size, under the same stressing condition, the Sn57.6Bi0.4Ag solder joints show insignificant resistance change and become grainy in appearance, while all other three solders show resistance rise until open and interfacial cracks in cross sectional studies.
In summary, new methods are developed with real time resistance monitoring technology to study solder joints reliability and microjoining processes. More knowledge is learned for the solder joints reliability such as the finding of crack healing events, and for the microjoining processes such as the discovery of flux segregation. | en |
