No abstract

In this study, the reliability of flip-chip (FC) packages with various underfills using ultrasonic bonding was evaluated in temperature and humidity (TH) tests. Fatigue cracks began at the interface between the Au bumps and glass substrate and then propagated through the interface with increasing dwell time in the TH test. The initial electrical resistance of Au bumps with lower viscosity underfill was lower than that of Au bumps with higher viscosity underfill. Entrapped underfill between the Au bumps and glass substrate or void formation between the Au bumps in FC packages was caused by high viscosity of the underfill. As the dwell time of the TH test increased, the electrical resistance of the FC packages increased. The fatigue life of an FC package with underfill that has a higher glass transition temperature (Tg) and lower coefficient of thermal expansion (CTE) value was higher than that of an FC package with underfill with lower Tg and a higher CTE value. Therefore, the properties of underfill affect the fatigue life of FC packages with underfill using ultrasonic bonding.

This study was focused on the effect of atmospheric pressure plasma treatment conditions on the ultrasonic bonding properties of Au flip-chip bumps on Au-finished glass substrates. The spreading test of de-ionized (DI) water was carried out on the substrate after the treatment. The plasma treatment was performed with three different gases, N2, N2 + O2, and N2 + H2, for various treatment times from 0.3 to 18 s. The surface analysis was carried out after the treatment using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The treatment conditions greatly affected the spreading angle and joint strength. The addition of O2 and H2 gases to N2 gas was effective to improve the wettability of the Au-finished substrate and the bondability between the Au bump and substrate, respectively. The spreading angle rapidly decreased with increasing treatment time from 0.3 to 1 s and then decreased slightly with further increase in the treatment time, whereas the bonding strength peaked after the treatment for 0.5 to 2 s and then decreased.

In this study, the morphological development of electroplated matte Sn and Sn-xBi (x = 0.5 wt.%, 1.0 wt.%, 2.0 wt.%) film surfaces was investigated under diverse testing conditions: 1-year room-temperature storage, high temperature and humidity (HTH), mechanical loading by indentation, and thermal cycling. These small Bi additions prevented Sn whisker formation; no whisker growth was observed on any Sn-xBi surface during either the room-temperature storage or HTH testing. In the indentation loading and thermal cycling tests, short (<5 lm) surface extrusions were occasionally observed, but only on x = 0.5 wt.% and 1.0 wt.% plated samples. In all test cases, Sn-2Bi plated samples exhibited excellent whisker mitigation, while pure Sn samples always generated many whiskers on the surface. We confirmed that the addition of Bi into Sn refined the grain size of the as-plated films and altered the columnar structure to form equiaxed grains. The storage conditions allowed the formation of intermetallic compounds between the plated layer and the substrate regardless of the Bi addition. However, the growth patterns became more uniform with increasing amounts of Bi. These microstructural improvements with Bi addition effectively released the internal stress from Sn plating, thus mitigating whisker formation on the surface under various environments.