Study of Au Ball Bond Mechanism and Reliability on Pd/Ni/Cu Substrate

Ball Bonding;Reliability;Mechanical Engineering

Huang, Yan

University of Waterloo

关键词: Ball Bonding;    Reliability;    Mechanical Engineering;   

Others  :  https://uwspace.uwaterloo.ca/bitstream/10012/4694/1/AsReceivedByDepartment_Thesis_YanHuang_MASC_20090515.pdf

瑞士|英语

来源: UWSPACE Waterloo Institutional Repository

【 摘 要 】

Microelectronic wire bonding is a manufacturing process used to electrically connect integratedcircuits with circuit boards or other substrates. Conventionally, balls are molten at the end of a Aubonding wire and subsequently bonded on Al metallization of a integrated circuit. However,Pd/Ni metallization has recently been used for its improved mechanical properties.The bondability, bonding mechanism, and reliability of Au ball bonds on Pd are studied in thisthesis. The substrates were produced in this project using three different materials. The base materialis polished Cu in the shape of a coupon (1.0 cm × 1.0 cm × 0.5 mm). Cu coupons are platedwith Ni (1.0 μm) using an electroless process, followed by electrolytic plating of a layer of Pd(0.3 μm), resulting in an arithmetic mean roughness of the surface of 0.08 μm (baseline sample,sample 0). Higher roughness values of 0.2, 0.4, and 0.5 μm are artificially produced by rolling(sample 1), sanding (sample 2), and sandblasting (sample 3), respectively, on the Cu surfacebefore plating Ni and Pd.A 25 μm diameter Au wire is used for bonding on the polished and roughened substrates with aprocess temperature of T = 220 °C, and it was found that ≈ 4 % to ≈ 18 % less ultrasonic amplitudewas required for successful bonding on the roughened substrates compared to the polishedsubstrate. Bondability is measured by shear testing the ball bonds. An average ball bond strengthachieved on the polished substrate is 130 MPa. This value is lower on the roughened substratewith the exception of the sandblasted substrate.Long-term thermal aging at 250 °C was performed with ball bonds on samples 0-3 for durationsof ≈ 300 h. The reliability of the bonds is characterized by non-destructive contact resistance analysisduring aging and destructive cross section analysis after aging. Contact resistance values forthe ball bonds range from 1.6 to 3.5 mΩ at 20 °C before aging, and does not correlate with roughness.For the baseline sample, contact resistance of the ball bonds decreases during aging by -6 %(median value), which indicates electrical integrity of the interconnections at high temperature.This decrease possibly is due to interfacial gap filling by Au or Pd diffusion. In contrast, the contactresistance increases for the roughened samples 1-3 and changes are 0.4, 5, and 14 %, respectively(median values). A conclusive explanation for this increase has not yet been found. After250 h of aging, a TEM analysis showed Au to Pd diffusion in the baseline sample with a diffusiondepth of ≈ 0.1 μm Au. No intermetallics, voids, or contamination is found on the interfaces afteraging according to nanohardness, SEM, and TEM analyses. No bond lift-offs or electrical openswere found for the aging temperature and durations chosen. No conclusive evidence for the presenceof Au-Pd intermetallics or voids is found.