The size, aspect ratio and direction angle of the Si particles in the stir zone were investigated for the ADC12 FSW joints to analyze the material flow in the stir zone by quantification of three elements (size, aspect ratio and direction) under various welding conditions. The number of finer Si particles increases with the increasing welding speed, whereas, the change in the number of finer Si particles is not simple when the rotation speed is varied. The stirring action during the FSW is appraised by the size of the Si particles. The aspect ratio of the particles increases with the increasing size of the particles. The welding conditions also influence the direction angle of the Si particles. Under the proper conditions, the flow directions of the Si particles in the top and bottom are horizontally aligned, while they are longitudinally aligned on the retreating side and advancing side. However, under insufficient heat input conditions, the material flow becomes random in the bottom and retreating side regions. Under abnormal stirring conditions, on the other hand, it is random on the advancing side and top regions, even though a defect forms on the advancing side in both cases. This kind of identification of the material flow might be very useful to determine the optimal welding conditions by avoiding any defect formation.
The trend towards miniaturization of electronic devices has pushed the micro-electronics packaging industry towards multi-die packaging where both the package footprint and the height are required to be as small as possible. This paper presents a folded stacked package that has many advantages including an extremely thin profile (four memory dies were successfully stacked with an overall height of only 1.2 mm), the ability to stack identical die, and a manufacturing process based on that of the familiar lead bonded pBGA@ package. These stacked packages can also be made with wire bonded structure, but with some increased package height and overall dimensions. An additional required step, the substrate folding process, must be throughly understood in order to ensure high reliability. As the pBGA package has high reliability, the key to ensuring high reliability of the folded stacked package is to minimize any interaction due to folding between individual dies. A folding design rule has been developed for this purpose. Fast-cure adhesive materials were also evaluated to minimize the folding process time. To minimize overall package height, a 140 pm ball height was used and a compatible test socket contact mechanism was identified. Also, electrical and thermal performance of the package were determined to assess the suitability of this package for various applications.
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