An Electron Microscopy Study on Materials in Electronic Packaging

Doctoral thesis, 2002

This thesis is concerned with an investigation of the microstructure on materials in electronic packaging. The materials include Ag-filled conductive adhesives, Sn-Ag-based lead free solders and metal-oxide semiconductors. In order to characterize the microstructure, several transmission electron microscopy techniques were applied: imaging, selected area electron diffraction, energy dispersive x-ray spectroscopy, convergent beam electron diffraction and high-resolution electron microscopy. Some other techniques were also utilized to assist the analysis, scanning electron microscopy, secondary ion mass spectrometry and x-ray diffraction. The direct observation of the distribution of micro-sized, nano-sized, micro/nano-sized mixed Ag-particles and Ag flakes in epoxy resin was carried out by TEM. The perfect continuous linkage of particles is hard to find and the chance of contact and subsequent contact area becomes less with increasing amount of nano-sized particles. So the conductivity in micro-sized particle fillers is dominated by the constriction resistance, while with increasing content of nano-sized fillers, the conductivity is controlled by thermionic emission. The microstructure of Ag3Sn and Sn becomes finer and more uniform in Sn-3.5Ag-0.5Cu-0.5B solder. It is suggested that the dispersed boron particles provide heterogeneous nucleation sites on the Ag3Sn particle and Sn matrix during solidification. The Cu6Sn5 phase keeps its high-temperature crystal structure after soldering. Two types of modulation structures were observed by electron diffraction patterns after the samples were heat treated at 100oC for 1000h. Both of the modulation structures are caused by the ordering of extra Cu atoms in trigonal bipyramidal sites in a NiAs-type hexagonal structure. Two crystal structure models were constructed for the three-time modulation structure, and according to the arrangement of extra Cu atoms, the composition for this modulation structure is Cu7Sn6. Besides the modulation structure in Cu7Sn6, satellites and diffuse scattering co-exist in the diffraction patterns. They correspond to short-range order of Cu atoms. The thermal cycling tests on solder joints show that the lead-free solder has lower microstructural coarsening effect compared with tin-lead solders. However, the dissolution of Pb from component leads decreased the melting temperature and caused obvious grain growth in Pb-containing phases. Two phases, (Ni,Cu)3Sn4 and (Cu,Ni)6Sn5, were found at the interface between Sn-3.5Ag-0.5Cu and Au/Ni/Cu metallized substrates. The content of Cu in the (Ni,Cu)3Sn4 phase is as high as 21 at.%, but the crystal structure is still the same as that of Ni3Sn4. A P-rich area was found in the Ni layer due to the electroless plating, and two metastable phases were formed after heat treatment. The Si/SiO2/PolySi1-xGex interface was characterized by high-resolution electron microscopy, and the strain fields at the SiO2/Si interface were revealed by large angle convergent beam electron diffraction and two-dimensional reciprocal space mapping.