Fan-out Wafer Level packaging (FOWLP) is one of the fastest growing advanced packaging segments today. Its growth was ignited when the iPhone 7 A10 processor incorporated the fan-out techniques for their high-end mobile application processors. According to Yole Research, FOWLP is expected to grow to $2.5B by 2022, up from $320 million in 2016.

FOWLP has introduced a significant shift in wafer level test requirement.

  1. Due to the ability of the fan-out nature, the native pitch for wafer test can potentially be reduced significantly to well below today’s 100um-ish pitch in production, down to 30-40um pitch range, for wafer testing prior to die singulation and fan-out.
  2. Higher quality of electrical test is demanded at wafer level prior to singulation to ensure only know-good-die go through the fan-out back-end processes. This has required higher level power integrity (PI) and signal integrity (SI) at wafer level.
  3. FOWLP process has also introduced new test insertion possibilities, where probing on Cu material directly is desired. Probing on Cu has been widely perceived to be challenging due to concerns with Cres stability due to rapid oxidation of Cu in the air, especially at elevated temperature. In addition, Cu pads are nearly double the hardness of AI pads.

With this shift, comes requirements for Cu pad probing, including:

  • An adequate non-oxidizing probe metallurgy to scrub through the oxide layer so that the probe does not stick on the pad
  • Probe geometry and tip shape that allows penetration through the oxide layer to have adequate scrub at the optimal overtravel
  • Stable and low contact resistance through an optimized cleaning recipe

Samsung and FormFactor have collaborated in developing a new vertical MEMS technology to break the myth of “probing on Cu”. We have evaluated a new type of MEMS probe tip material that’s optimized for Cu probing. Earlier this year, we presented our evaluation process and the promising results from the Cu probing process for FOWLP, as well as capabilities to support known-good-die testing and pitch reduction to sub-40um. You can download this presentation and see the results, which are very promising, here: