Investigation of WLCSP corrosion induced reliability failure on halogens environment for wearable electronics

Continuous miniaturization of microelectronic devices is enabling more portable and wearable electronic products such as thinner smart phones and smart watches. WLCSP (Wafer Level Chip Size Package) has been widely adopted for portable and wearable electronic products due to its compactness. Microelectronic devices in wearable products may be exposed to chlorine and fluorine because they come in direct contact with the human body under wearing condition. Products of chlorine corrosion have been found in WLCSP product evaluations, thus it is necessary to investigate WLCSP corrosion resistance and the factors affecting its corrosion reliability. This study examines the effect of laser saw power on the corrosion resistance of WLCSP packages. To simulate potential chlorine and fluorine rich environments, a chemical humidity reliability test was developed by preparing chemical solutions containing NaCl, HCl and HF in different volume percentages and performing experiments by dipping specimens in the chemical solutions and then placing them in a chamber with high humidity at 110 degrees C for 24hrs. The effectiveness of the test was checked by reproducing corrosion effects seen in real failure samples. WLCSP packages prepared with different laser saw recipes were evaluated with the chemical humidity reliability test. Preliminary experimental results showed laser grooving power having significant impact on WLCSP corrosion resistance and reliability performance. The split with laser power > 2W showed metal corrosion and delamination of IC inter-metal-dielectric layer after chemical humidity reliability test. For the split with low laser power < 1W, slight corrosion was found. IC seal ring structure also affected WLCSP corrosion resistance. It is believed that more severe heating due to operation of laser grooving at higher power settings induced IC inter-metal-dielectric layer interface damage and accelerated the corrosion process during chemical humidity test. Optimizing laser grooving power improved WLCSP corrosion resistance. The results of the study can serve as good reference for WLCSP enhancement to prevent potential corrosion induced defect of wearable electronic products.