Challenges of and Solutions for Contacting Wafer-Level Chip Scale Package Devices

  • Features

    • smallest possible X/Y size: same as die for fan-in packages
    • minimal Z height: thickness of wafer + thickness of redistribution layer + ball height
    • lightest-weight package
    • economical testing: final test on wafer eliminates one entire test step
    • most cost-effective packaging
    • best package for electrical performance
  • Contacting Challenges

    • high parallelism
    • high ball counts (due to high parallelism)
    • fine pitches (and getting finer)
    • planarity issues (compared to wafer probe)
    • challenging test requirements (compared to wafer probe)
    • landing two probes for Kelvin applications
    • making good contact without creating solderability issues
  • Probe and Contactor Feature for WLCSP Packages

    • super-sharp tips
    • lower-force probes
    • large compliance windows
    • fine-pitch Kelvin probes
    • dimensionally-stable contactor materials
    • mechanical simulation capabilities

WLCSP Packages

Wafer Level Chip Scale Packages have been in use for over a decade years now, and the number of devices packed this way continues to grow at a significant pace. WLCSPs are primarily choice for mobile devices for this reason.

The other reasons to package at the wafer level are economic: The packaging itself is inexpensive and packaging at the wafer level allows testing at the wafer level. Final testing at the wafer level eliminates a test step compared the traditional wafer probe + final test.

WLCSP Challenges

Taking full advantage of final test at the wafer level involves testing as many devices in parallel as possible. This result in large contactors with the most-distant contact points relatively far apart. Maintaining probe tip positions (X, Y, and Z) is a greater challenge as the contactors grow. WLCSP devices are less planar than wafers and more compliance is required than needed for wafer probe.

Final test also means higher-performance probes, for both DC and RF testing.

Addressing the WLCSP Challenges

Xcerra contactors employ rigid materials with low hygroscopy to maintain their dimensional stability. Many designs also employ a stainless steel frame to further enhance planarity. The spring probes have a high compliance window to accommodate package planarity variations as well as some contactor bowing. Simulation is useful to predict the contactor bowing so it can be alleviated. Xcerra probes can make good contact at lower forces thanks to super-sharp tips.

Spring probes have the electrical performance required for final test, and have been proven by decades of high-volume production package test.

Finer-pitch WLCSPs mean smaller Kelvin probes, and Xcerra has probes to support true Kelvin at 0.4 mm pitch.