Wetting Characteristics of Solder Reflow on Copper Traces without Solder Mask

In electronic packaging industry, solder is a very important material which mainly provides the electrical connections between two different components. Also it gives mechanical support between the package and the substrate. In SMT process, the self alignment capability of solder during the reflow process pulls the initially misaligned components back to the desired position. This capability benefits many high-end products such as micro-opto-electro-mechanical systems (MOEMS). Those sophisticated devices usually required very precise alignment between components in order to have optimized performance. In order not to use active alignment, self alignment of solder joints provides a low cost packaging solution. However, in some situations, a thick copper film is used in MOEMS device in order to provide mechanical rigidity. Thick copper film can be easily obtained by electroplating. It can be used as the electrical connection trace and bond pad. Conversely, it is very difficult to deposit passivation layer. Without the passivation layer, the bond pad location may not be well defined and the solder may wet along the copper trace. This causes the wetting of solder during reflow unfavorable. In this paper, a parametric study is conducted. The objective is to analyze the wetting behavior of both lead tin eutectic and lead free solder during reflow on thick copper traces. A testing vehicle is designed so that the effect of the copper trace width on the solder flow can be examined. Two sizes of solder balls, 300 mum, and 750 mum, are included in the study. The experimental results show that the copper trace width controls the solder flow. Also the pretreatment of the sample plays a very important role in the wetting process. The results are useful in predicting solder flow during device design