Studies on interfacial reactions of lead-free solder joints and electroless deposited diffusion barriers in the Bi2Te3-based thermoelectric system

• Main Authors: Chun-Wei Chiu, 邱俊暐
• Other Authors: Chao-Hong Wang
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/632yph

碩士 === 國立中正大學 === 化學工程研究所 === 102 === Thermoelectric (TE) devices consist of many pairs of p- and n-type semiconductor elements, which are interconnected electrically by soldering technology. Bi2Te3-based alloys are the most popular thermoelectric materials. Sb and Se are usually alloyed to Bi2Te3-based alloys for p- and n-type, respectively, i.e., Bi0.5Sb1.5Te3 and Bi2Te2.7Se0.3. Ni is frequently used as a diffusion barrier to prevent the fast IMC growth. In this research, we studied the solid/solid and liquid/solid reactions between Sn-based solders (Sn or Sn-58Bi) and Bi2Te3-based thermoelectric materials. The SnTe growth rate of p-type couples were larger than the n-type. For the liquid-state aging, Sn reacted with TE substrates with two different directions (⊥c-axis or //c-axis), we found that the IMC growth of the Sn/p-type substrate reaction couple was anisotropic. The growth kinetics were also investigated. For the solid-state reactions, the growth is parabolic for p- and n-type reactions. For the liquid-state reactions, the growth of the p-type reaction is linear, and the n-type case followed a parabolic law. This study also simulated the solder joints of the thermoelectric device, using the electroless deposition of Ni-P or Co-P as a diffusion barrier between Sn and TE substrates. In the Ni-P system, the reaction was inhibited significantly between solder and p-type material. For the n-type cases, Sn diffused through the Ni-Sn-P layer when the Ni-P barrier was depleted, the NiTe phase occurred phase transformation in substrate side, and the SnTe and BiTe phases were formed. During the liquid reaction, the Ni-P layer was peeled off from the interface due to thermal expansion stress, resulting in the fast IMC formation. For the Co-P cases, Sn fast reacted with the Co-P layer and formed the CoSn4 phase. After an aging period of the p-type reaction, the metastable CoSn4 phase was transformed to Co(Sn,Sb)3 phases and the SnTe was formed in substrate side. In the solid/solid reaction of n-type, the CoSn4/Co-Sn-P/SnTe/BiTe structure was found. Both the p- and n-type liquid/solid reactions, the CoSn4 and Co-Sn-P phases were formed, and massive spallation of the CoSn4 phase occurred. In addition, the electroless Co-W-P was deposited on Cu substrate, and reacted separately with Sn or SAC305. The layer-structured CoSn3 phase and Co-Sn-P IMC were formed at the interface between Sn and the Co-W-P layer, while the (Cu,Co)6Sn5/CoSn3/Co-Sn-P structure was observed for using the SAC305 solder. In the liquid-state reactions, the Co-Sn and Co-Sn-P phases were formed in the initial stage. With the reaction proceeding, the Co-Sn phase was spalled into the liquid-state solder. When the Co-W-P was depleted, Cu fast diffused into the solder and the Co-Sn spalling phase transformed into (Cu,Co)6Sn5. The Co-Sn-P ternary phase grew thicker obviously, and a thick porous-structured Cu6Sn5 phase was formed at the interface. Finally, the Co-Sn-P layer was peeled off and the Cu6Sn5 thickness grew significantly.