Abstract:
This thesis was aimed to investigate the soldering between Sn-Cu lead-free solders and copper substrate. The investigation was focused on the wettability, formation and growth of intermetallic phase in the soldered joint. Effects of grain size, grain orientation and residual strain of the substrate on the wettability, formation and growth of intermetallic phase were studied in this research. The Sn-Cu solders used in this thesis included Sn-0.7Cu, Sn-1.0Cu and Sn-3.0Cu. Reflow soldering was utilized in the investigation. The results showed that ƞ-Cu6Sn5 intermetallic phase was the first phase formed at the interface and ε-Cu3Sn was formed when soldered with a longer time. Among the three solders, Sn-3.0Cu provided the thickest intermetallic layer when soldered under the same condition and it was consequently easy to observe. Sn-3.0Cu was therefore selected to study on the wettability on copper substrate. The wettability of the solder was increased with the increase of soldering time. When annealed at different annealing temperatures of 300, 600 and 900 ๐C for 1 hr, the copper substrate had different grain size, grain orientation and residual strain. By using electron backscatter diffraction technique, the substrate annealed at 300 oC possessed the highest misorientation angle and residual strain while the 900 ๐C annealed substrate had the lowest values. The grain size, grain orientation and residual strain of the substrate had no significant effect on the wettability. However, the thickness of intermetallic layer was proportional to misorientation angle and residual strain of the substrate. After thermal aging, ε-Cu3Sn appeared between ƞ-Cu6Sn5 layer and the copper substrate when thermally aged at
75 oC for 1,000 hr. The formation of ε-Cu3Sn was shorter when thermally aged at 100 and 125 oC. The growth of the intermetallic compounds was chemical reaction and diffusion controlled. The copper substrate without annealing provided the lowest growth rate of the intermetallic compounds.