Diamonds in the Rough

"It's just like the chocolate chips in a chocolate chip cookie," said Bill Pfeiffer, vice president of business development at NanoPierce (Denver), when describing the company's new technology. Just substitute 10-15 µm diamond particles for the chocolate chips and die bonding pads for the cookies.

Chip bonding pads with embedded diamond particles, overcoated with nickel/gold, offer an alternative to the conventional flip-chip attach approaches using solder, stud bumping, conductive adhesive or anisotropically conductive adhesive. Pressing the chip into the substrates' pads creates good contact as the sharp points penetrate and break through any coating or oxide. A permanent bond is accomplished with a non-conductive adhesive. For high-speed assembly, a UV-cured adhesive can be used, and assembly times can be significantly less than 1 sec.

The NanoPierce Connection System (NCS) offers three significant advantages over other approaches: low temperature assembly, no patterning of adhesive on the substrate required, and potentially faster assembly time. The latter two offer the potential for reducing the total cost of the packaging and assembly. Pfeiffer says the initial applications for this technology are cases in which the packaging cost is a significant fraction of the total component cost.

The lowest-cost wafer bumping process in production today is the zincate process, developed at the Fraunhofer Institute (Berlin) and later commercialized by Pac Tech GmbH (Nauen, Germany). In this standard process, which requires no masking steps or sputtering, a wafer is treated with a zincate solution to replace the aluminum oxide on the pads with zinc. It is placed in an electroless nickel bath to plate up nickel over the zinc, and then in an immersion gold bath. The result is a wafer where all the pads have a small, gold-coated nickel bump. At this point, solder can be screened on and reflowed, or the chip can be flip-chip attached with adhesive.

1. Top view of a 100 µm pad with more than 100 diamond particles. (Source: NanoPierce)

A small impeller provides agitation in the bath so the particles are kept in suspension. As the nickel deposits on the pad surfaces, it occludes diamond particles. The agitation is critical. Too little, and not enough particles are in suspension. Too much, and the shear forces rip the particles from the surface before they can be set.

2. Cross section of chip pad with diamond particles piercing into the copper pad of the substrate. (Source: NanoPierce)

In published test reports, NanoPierce states that, providing there are 12 or more diamond particles on a pad, the contact resistance to a copper trace can be <40 mΩ using a UV-cured adhesive for die attach. In the process, the chip is aligned to the pads on the substrate, a drop of adhesive is applied, and the chip is pressed to the surface while a UV lamp irradiates the site. Total assembly time is <1 sec.

The initial applications for this assembly technology — such as RFID tags — are extremely cost-driven. The conventional approach is to use conductive adhesive flip-chip attach to copper or silver antennas printed on thin PET films. The adhesive is either screened on the wafer or the substrate, and the assembled unit is cured at elevated temperature. Two cost savings are possible with the NCS approach. Lower-cost aluminum foil-based antennas can be used, and the assembly time can be reduced with no post-assembly cure required.

The NCS diamond particle-coated pads will never replace all flip-chip applications. However, the process offers an intriguing building block when considering the lowest total cost of ownership for extremely high-volume, cost-driven packaging applications.

For additional information on semiconductor packaging, go to www.semiconductor.net/assembly.