Effects of package and mold cavity structures on the wire sweep behavior during package array transfer molding
In order to maximize product throughput, an increasing number of electronic packages, such as Small Outline Transistor (SOT) package, are manufactured in a package array mold. Due to the complicated structure design of the package and mold cavity, wire sweep is becoming an increasingly severe problem. In this study, the numerical model which had been validated by experimental work was used for wire sweep analysis. Firstly, the leadframe structure was studied. Given the thin thickness of leadframe and the relative position to the bond wire loop, the leadframe structure has very limited influence on the wire sweep value. Besides, for fixed width and length of the die, the wire sweep behavior was simulated to examine the influence of the die height (thickness). The results show that the thinner the die, the smaller the wire sweep. Furthermore, the effect of cavity height on wire sweep value was also estimated. Based on the previous simulation and analyses, the ratio of the die height to that of the cavity was introduced as an index to optimize the flow pattern of epoxy molding compound (EMC) in order to find a guideline for package and cavity design to avoid the wire sweep issue. In addition, the influence of injection velocity and viscosity of EMC on wire sweep during transfer molding process were inspected and discussed according to the simulation results as well. It can be further concluded that the velocity plays the key role in resulting in the severe wire sweep in package array mold during transfer molding process.
Wire sweep study for SOT package array matrix molding with simulation and experimental analysis
Small Outline Transistor (SOT) packages, widely used in consumer electronics, are very small, inexpensive surface-mount plastic molded packages. In order to maximize product throughput, the molded array matrix is employed for products fabrication, which leads to a potential critical wire sweep issue for SOT package during the transfer molding process. The unreasonably large wire sweep will extremely influence the package reliability, and even lead to failure of the product. In this study, both experimental and simulated works were resorted to analyze the wire sweep of SOT package array matrix mold. At the beginning, two different simulation methods, 3D analysis and Dual Domain analysis, were employed to predict the flow pattern and wire sweep during the transfer molding process with a hybrid finite-element/finite-difference method using commercial software Autodesk Moldflow Insight (AMI). It is found that both of the methods can obtain a well matched flow pattern but Dual Domain analysis acquire the more reasonable wire sweep distribution, compared with the experimental observation. Therefore, it can be concluded that the Dual Domain analysis is the more reasonable and recommended method to cope with wire sweep problem. Furthermore, according to the Dual Domain analysis result, it is also found that the sharply increased velocity is the key reason for the wire sweep issue in the vent side of the mold, due to the flow front concentration. In order to address the issue, the transfer mold process optimization with transfer profile was carried out. The simulation results reveal the transfer molding process optimization is an effective way to deal with this problem.
Control of reflective cracking
The first is 'fatigue' cracking of the layer. This type of cracking is the result of the application of loads on the pavement and is controlled by appropriate thickness design which 'ill not be discussed in detail here. Thickness design includes not only the design strength and thickness of the stabilised, but other design elements such as delaying reflection cracking, or limiting stresses in the stabilised layer, by adopting a minimum thickness of overlying material. This may include an unbound layer utilising the 'upside down' pavement approach. The actual thickness required of the overlying material varies but for example PIARC (Ref.1) recently reported when the thickness of coated materials exceeds 20 cm the emergence of shrinkage cracks at the surface is generally uncommon; when it is 15 cm thick, the cracks emerge at the surface, but do not shorten the life of the surface course, especially if they are sealed. This experience has been gained 'with legal axle loads of 130 kN, considerably greater than the Australian legal axle load.