Pyoung-Wan Kim

Laser Dicing and Subsequent Die Strength Enhancement Technologies for Ultra-thin Wafer

Current mechanical wafer dicing process adopting diamond grit shows advantages of low cost and high productivity. However, mechanical process for ultra-thin wafers would induce residual stress or mechanical damage, which can lead to wafer broken and die cracking. With the development of laser technology, laser precision micromachining has been employed for thin semiconductor wafer singulation, which shows advantages of no chipping, small kerf width, and high throughput over mechanical blade dicing. However, thermal damage to the chip induced by laser ablation results in die strength degradation. For ultra thin chip, low die strength tends to induce die crack in packaging process. Thus, thermal damage to the chip needs to be studied. In this study, first we made a comparison between mechanical blade sawing and laser ablation processes. Die strength and microstructure changes were studied by means of bending test and transmission electron microscope (TEM) analysis, respectively. Die strength results showed that the die strength obtained by laser dicing was far lower than that obtained by blade sawing. TEM analysis demonstrated that formation of microcracks and porosities in laser diced face, caused the die strength degradation. In addition, significant deviation between frontside and backside die strength was found in the laser micromachinned dies. The reason for this deviation was clarified as the defects density difference existing in top and bottom layer of the chip sidewalk Experiments results showed that the die strength obtained by laser dicing can not meet the demand of the packaging process. It tends to crack or fracture in the die attach or wire bonding process. Thus, it is essential to improve the die strength. Thus, in this investigation, etching processes including wet-etch and dry-etch were attempted to recover the die strength by removing the chip side wall damage. SEM and TEM images indicated that, before etching, the laser diced side walls were with rough surfaces, voids and microcracks. After etching, the surfaces got smooth and most of the voids and microcracks were removed. Chip strength measurement also verified the partial die strength recovery after etching process.

Improvement of drop reliability in OSP/Cu pad finished packages

This study investigated the improvement of drop reliability of OSP (organic solderability preservatives) pad finished packages having half etched solder ball pads. Besides the effect of the Cu pad etching depth, effects of other factors such as solder composition or reflow peak temperature on drop reliability were examined by the bending impact test and drop test. The bending impact test results showed that the increase of etching depth at the solder ball pad increased the drop reliability because of the fracture mode transition from solder/pad interface failure to solder bulk failure, but that the increase of reflow peak temperature decreased the drop reliability. The drop test results showed that the increase of the etching depth at the solder ball pad increased the drop reliability without the fracture mode transition, and that the change of the solder composition from Sn3.0Ag0.5Cu to Sn1.2Ag0.5Cu0.05Ni increased the drop reliability and shifted the fracture mode from interface failure to the bulk failure. The optimal conditions for the drop reliability improvement are presented in terms of the etching depth at the solder ball pad, the reflow peak temperature, and the solder composition.

Effect of Ni Surface Finish on Half Etched Cu on Solder Joint Reliability

Thermal cycling (TC) and drop test results of electronic package are presented in this paper, which have been known as the main reliability test items in electronics. Especially, as the movement of personal computer to handhelds accelerates, this paper focused on concurrent requirements for TC and drop reliability. The research was carried out by the modification of current solder pad finish and the comparative test results were represented. Considered that OSP on half etched Cu was normally used in mobile package, Ni/Au finish on half etched Cu was applied in this work. SnAgCu solder ball was used, and Ni/Sn on half etched Cu was also evaluated as Sn plating is used in some applications. Test results showed that Ni/Au finish on half etched Cu can be one of solutions to satisfy TC and drop reliability simultaneously.