Au Stud Bumping for Wafer Probe Testing

With flip chip emerging as a mainstream technology, demand for a full array of (solder or copper bumped) wafer probing technology has increased along with the challenges associated with it when the pitch becomes narrower. A flip chip substrate implemented for flip chip packaging, typically, is also used to build the full array vertical probe card as an interposer to fan out the tight pitch flip chip array that eventually connects to the PCB, in turn, for tester. However, pad metallization of substrates for flip chip packaging is not always suitable for probing applications due to high contact resistance (Cres) and pad wear-out through its application life.

In this study, it is the first time that the Au stud bump is applied on a pad metallization of the fan-out interposer to improve probe contact performance in its life service, and demonstrated successfully for this application. Due to the variety of incoming pad metallization, a different approach was taken to convert the pad into a thicker Au metallization to accept Au stud bumping. Electroless plating was developed to convert the incoming pad metallization into Ni and thick Au to allow for Au stud bumping. NSOP (non stick on pad) sometimes was observed on the converted pad metallization.

A failure analysis found that uneven Au or Ni plating thicknesses between pads was responsible for NSOP. In other cases, the P distribution in Ni within the pad resulted in an uneven hardness, which also limited bumping integrity. Further mechanical lapping to planarize the bump contact surface could further enhance the contact integrity of vertical probe cards.

A PCA (probe card analyzer) test was used to examine the probe card performances in terms of contact resistance (Cres) and planarity during the mechanical cycling test to emulate its service life. An extremely tight Cres distribution and an excellent planarity demonstrated greater than 1.3 million cycling, using the Au stud bump built probe card.

[1] January Kister, Interface structure for contact probe beams, US06570396B1, May 2003.