SMT Study for Exposed-Pad Type of Electronic Package

摘要

The exposed-pad concept has been implemented in leadframe type of electronic packaging to enhance its thermal/electrical performance. The integrated circuit die is attached to the leadframe die pad using a thermally conductive epoxy. The package is molded so that the leadframe die pad is exposed at the surface of the package. This provides an extremely low thermal resistance (θ_(jc)) path between the IC junction and the exterior of the case. Because the external surface of the leadframe die pad is on the PCB side of the package, the external surface of die pad can be attached to the board by solder paste when the package is mounted to PCB. The efficient attachment forms an accessible path for heat conduction and permits board structures to be used as heat sinks for the IC. In addition, it also provides an excellent electrical grounding effect to the PCB. For the reason of superior thermal dissipation and great electrical performance, the exposed-pad packages are a better solution for many products like high power and high frequency ICs. Therefore, the exposed-pad package has become more and more popular and there are many derivative products such as BCC, QFN, and LQFP/TQFP ..etc. However, there are SMT quality concerns during board assembly process for exposed-pad packages since the mounting area of PCB/lead and PCB/pad are quite different. The surface tension force of solder paste during reflow process would push the exposed-pad package upward to result in the potential leads contact problem. In our study, the capillary theory is utilized in SMT process to prevent the leads contact problem. The solder paste is printed on the backside land of PCB then package is mounted on the topside of board rather than the conventional method with solder paste printed on topside of PCB land. The qualitative and quantitative analyses are used to evaluate the experimental results.