Selective Plating on Flexible Glass and Polymer Substrates with Laser-Drilled Holes
Authors: Norelli Kevin M , Dubin Val M 
Published: February 2020

The simultaneous advancement of Moore’s law along with the miniaturization of interconnects towards so-called HDI (high-density interconnect) design rules has led to a significant revitalization of the printed circuit board industry. As semiconductor wafers become increasingly densely patterned, interconnects have shrunk to keep pace. Shorter interconnect distances translate to shorter rise times and decreased voltage drops, such that a synergy has developed between the shrinking of both integrated circuits and their associated circuit boards.

Lasers have largely supplanted mechanical drills to produce microvias (via diameters < 150 µm, often <50 µm) because mechanical bits are prone to catastrophic failure when they become sufficiently thin. Specifically, the recent reduction in cost of ultraviolet lasers has seen an upward trend in adoption among high-end PCB and flex circuit manufacturers. Meanwhile, the technology used to pattern metallic traces and plate microvias has not advanced significantly since the 1970s.

There are two competing techniques for PCB metallization: the first is a fully subtractive process whereby a copper clad PCB substrate is lithographically masked and then wet-etched, or laser ablated. The second approach is to utilize a semi-additive process whereby a mask is lithographically defined on the substrate and used to restrict metallization as needed.

We report a novel technology to perform electroless metal photo-patterning without the need for traditional lithography. A thin-film precursor solution is applied to a flexible non-conductive substrate, dried, exposed to UV light, and developed in an aqueous solution. Only the irradiated areas remain with catalytically active platinum group metals exposed at the surface. The same ultraviolet lasers used to drill vias can now be utilized for laser direct imaging (LDI), eliminating the need for a separate lithography station. This dense, inorganic polymer is on the order of ten nanometers thick, and promotes adhesion of subsequent electroless metal plating while exhibiting excellent thermal stability.

A variety of plating chemistries have been applied, including nickel/tungsten alloys, cobalt/tungsten alloys, and copper. This electroless metal photo-patterning technique not only streamlines PCB fabrication, but is separately a core enabling technology for the low-cost and scalable manufacture of virtual/augmented reality displays – which rely heavily on micro-LED devices to shrink pixel dimensions.

NANO3D Systems has demonstrated uniform, well defined metal structures on a variety of flexible non-conductive substrates. Adhesion of nickel alloys to glass substrates was measured by pull test to be >65 MPa. Sheet resistance of as-deposited electroless nickel/tungsten alloy was found to be 18.5 Ω/□ and 157 µΩ/cm for an 85 nm thick film. Once electroplated with 1.1 µm of copper, resistances dropped to 31.7 mΩ/□ and 3.4 µΩ/cm respectively. Chemical composition was confirmed via EDX and WDX. Deposit stress of the Ni/Cu stack was measured to be <13 MPa. Non-uniformity of film thickness by optical profilometry was 1.7% for the Ni/Cu stack.

Key challenges in plating chemistry, surface functionalization, and exposure processes have been addressed and are ripe for technology transfer to adjacent applications.

Figure 1