Characterization of solder joints made with VPS on DBC substrate

Ensuring the best quality of the solder joint can be crucial in assemblies which operate in different environmental conditions, since the reliability of the whole circuit depends on the quality of the joint. Solder joints on a ceramic substrate can be more prone to cracking because of the higher stress generated in the metal layer than is the case with simple FR4 substrates. In addition soldering methods like vapour phase soldering or convection soldering can have a significant influence on the solder joint's mechanical quality because of the thickness of the intermetallic layer, the number of voids, solved intermetallic compound and precipitations in the joint. Solder joints from a SAC305 (96.5 % Sn3 % Ag0.5 %) alloy were prepared on DCB substrates which have Al2O3 as the base material which is covered by a Cu wiring layer and an Ni/Au solder preservative coating. Three different soldering methods were used, vapour phase soldering with and without the vacuum option with convection reflow soldering as a reference. The solder joints were studied by shear strength measurements, 2D and 3D X-Ray computed tomography to evaluate void formation. The fracture surfaces and cross-sections of the joints were studied by optical and scanning electron microscopy to compare the microstructural differences of the joints. The tests carried out showed the particular relationships between the mechanical strength of the solder joints and the applied soldering technology in the case of DCB substrates.