Small form factor WLP (or WLCSP) type component has been increasingly applied to portable electronic devices. However, wafer level package type active components are more vulnerable to the failure than the passive or conventional BGA or QFP type active components. WLP constitute a neck point in terms of PCB assembly reliability. Therefore, it is very important to optimize the structure and material to secure high reliability of PCB assembly.Three variables are chosen for WLP reliability evaluation. They are solder ball size, PCB pad size, and underfill (use or non-use). DOE (Design of Experiment) technique is adopted to analyze variables effects on WLP solder joint lifetime. Board assembly level thermal cycling, high temperature and high humidity, and 4-point bending tests are employed as output parameters. The TC test results are compared with the finite element analysis.Solder joint cracks were mainly initiated from the triple point in IC side where solder, redistribution layer, and UBM meet together. WLP ball size was the major factor to determine solder joint lifetime whereas PCB pad size was not a critical parameter in all three reliability tests. Ironically, for the small ball sized WLPs, underfill decreased thermal fatigue lifetime. This is quite a different result compared with other previous reports. This discrepancy is presumably due to improper combination of underfill material property with high stress induced WLP passivation material, BCB (Benzocyclo Buten).

Aerodynamic flow control phenomena were investigated with a low-current DC surface discharge plasma actuator. The plasma actuator was found to operate in three different discharge modes with similar discharge currents of about 1 mA or less. Stable continuous DC discharge without audible noise was obtained at higher ballast resistances and lower discharge currents. However, even with continuous DC power input, a low-frequency self-pulsed discharge was obtained at lower ballast resistances, and a high-frequency self-pulsed discharge was obtained at higher set-point currents and higher ballast resistances, both with audible noise. The Schlieren image reveals that the low-frequency self-pulsed mode produces a synthetic jet-like flow implying that a gas heating effect plays a role, even though the discharge current is small. The highfrequency self-pulsed mode produces pulsed jets in a tangent direction, and the continuous DC mode produces a steady straight pressure wave. Particle image velocimetry (PIV) images reveal that the induced flow field by the low-frequency selfpulsed mode has flow propagating in the radial direction and centered between the electrodes. The high-frequency selfpulsed mode and continuous DC mode produce flow from the anode to the cathode. The perturbed region downstream of the cathode is larger in the high-frequency self-pulsed mode with similar maximum speeds.