Wearout reliability studies of bonding wires used in nano electronic device packaging
Gan, Chong Leong
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Conventional bare Cu bonding wires, in general, are more susceptible to moisture corrosion compared to gold (Au) and Cu wires. There is very limited knowledge based reliability studies which have been carried out on 1st level interconnect (ball bond in this matter) on nano device semiconductor packages. The objective of this project is to evaluate the wearout reliability, apparent activation energy and Intermetallic compound (IMC) thickness growth of Au, Pd-coated Cu wire and Pd-doped Cu wire used in semiconductor packaging. Methodology of this work include investigation on the effects of bonding wires on wearout reliability of flash component, characterization of the apparent activation energy of IMC and HTSL test and formulation of the failure mechanisms in different wires. Wearout reliability of biased Highly Accelerated Temperature and Humidity Stress (HAST), unbiased HAST (UHAST), Temperature Cycling (TC) and High Temperature Storage Life (HTSL) have been characterized. Samples are loaded into each reliability chambers and stressed until wearout open failure. Weibull plot is plotted for each reliability stresses and for three wire types. First failure (tfirst), median-time-to-failure (t50) and characteristic life (t63.2) and weibull slope (β) are calculated accordingly. Next study includes applying thermal storage conditions at 150 °C, 175 °C and 200 °C at various intervals time. The apparent activation energy (Eaa) has been investigated for HTSL and IMC thickness growth of Au, Pd-coated Cu wire and Pd-doped Cu wire. Dispatch oven is used in HTSL test. Results indicated that the obtained weibull slope (β) of three wire types are greater than 1.0 and belong to wearout reliability data point. Pddoped copper wire exhibits larger time-to-failure and cycles-to-failure in HAST, UHAST and TC tests. This proves Palladium (Pd)-doped copper wire has a greater potential and higher reliability margin compared to Au and Pd-coated copper wires. Bare Cu wire is not observed with lowest wearout reliability performance. Intermetallic compound (IMC) diffusion kinetics has been established among the different bonding wires. Eaa obtained of Au ball bonds are ranging from 0.92 ~ 1.10 eV and 0.72 ~ 0.83 eV for Pd-coated Cu ball bonds in HTSL test. For IMC thickness growth study, Eaa obtained for CuAl IMC are 1.08 eV and 1.04 eV respectively with EMC A and EMC B. Eaa obtained are 1.04 eV and 0.98 eV respectively on EMC A and EMC B on AuAl IMC. Wire pull and ball bond shear strengths have been analyzed and we found smaller variation in Pd-doped copper wire compared to Au and Pd-doped copper wire. In conclusion, Au bonds were identified to have faster IMC formation, compared to slower intermetallic compound thickness growth compared to Pd-coated Cu wire and Pd-doped Cu wire.