Jim Liu

Complete UV emission of ZnO nanoparticles in a PMMA matrix

Preparation and photoluminescence (PL) properties of zinc oxide (ZnO) nanoparticles embedded in a lipophilic polymethyl methacrylate (PMMA) matrix are reported in detail with an unbalanced sol–gel route. A high-resolution transmission electron microscope (HRTEM) indicates that ZnO particles are highly crystallized, with a size of 5–6 nm and hexagonal wurtzite structure. During the sol–gel reaction, partial ester groups of R-COOCH3 in PMMA are hydrolyzed to form carboxylic ion groups, which chemisorb on the surface of ZnO nanoparticles to eliminate the defects; thus ZnO nanoparticles in the PMMA matrix exhibit complete ultraviolet (UV) emissions, while emissions in the visible region are fully quenched.

Research on failure modes of BGA assemblies with lead-free solder on different PCB materials

Emerging environmental regulations worldwide, most notably in Europe and Japan, have targeted the elimination of Pb usage in electronic assemblies, due to the inherent toxicity of Pb. Similarly the toxicity of halogen based PCBs in landfills has also prompted the industry as well as various governments to work toward the introduction of halogen-free PCBs in electronics. So research on the reliability and failure mode of BGA assembly with lead-free on different PCB materials including traditional FR4 material and new environmental halogen-free materials are very important in order to speed up their application in electronic. In this paper, firstly the mechanical bend fatigue reliability was studied by the Mini-Mechanical Tester (MMT) system. Then the failure mode was studied by cross sectional method. The results show that two failure modes were found for lead-free and halogen-free assembly. One is crack through, halogen free material and the other is crack in lead-free solder. For lead-free and FR4 assembly, two failure modes were also found. One is crack in solder and the other is crack in the copper trace. Considering the mechanical bend fatigue reliability of these two assemblies, when crack initiate from solder materials, the reliability of assembly with lead-free solder on halogen-free shows higher than that of assembly with lead-free solder on FR4. For the assembly with lead-free on halogen-free, the obvious law was found that the halogen-free material failure was the main failure in short life and the lead-free solder failure is the main failure mode in long life area. However there is no obvious law for the assembly with lead-free on FR4.