George A. Riley
Wire bonding has been the traditional way to interconnect die in stacks, but Vertical Circuits, a technology development and licensing company, offers high-speed jetting of conductive polymers to create stack edge bonding, with space, performance, and cost advantages over wire bonding.
Stacked die, perhaps the simplest and most used 3D technology, piles up and connects a stack of die. Edge bonding deposits vertical conductive tracks across the edges of the die stack as an alternative to wire bonding. Edge bonding avoids the complexity of wire bonding and reduces the footprint, while eliminating wire parasitics that limit speed and performance.
The Vertical Circuits process first redistributes bond pads if needed, with either a standard wafer redistribution layer or with conductive polymer.
FIG 1. Die with redistributed pads. (courtesy Vertical Circuits)
Next, die are coated with a thin dielectric layer that insulates the edges and bond pads. The die are prepared for stack interconnection by laser ablation to remove the dielectric from only the bond pads in a column that will be interconnected. Thus each conductive track makes contact only with selected die in the stack.
FIG 2. Die after dielectric coating and ablation. (courtesy Vertical Circuits)
Next, an inter-die adhesive fixes the die in a parallel or offset stack.
FIG 3. parallel stacking, with interposed sheets of adhesive.
(courtesy Vertical Circuits)
Finally, high-speed jetting of silver-filled conductive polymer forms a conductive pillar from each exposed bond pad to the bottom of the stack. The conducting columns extend slightly below the bottom die, creating a complete chip-scale packaged stack, ready for mounting on a board or substrate or in a package. Figure 4 shows a die stack with vertical extended columns, ready for mounting.
FIG 4. Completed die stack. (courtesy Vertical Circuits)
All die preparation is done at the wafer level. All handling and processing beyond wafer level uses gang or array methods for maximum throughput and minimum cost. Only standard facilitation and assembly equipment is needed, keeping the capital investment small.
Prices are competitive with wire-bonded stacks. Unlike wire bonding, interconnection cycle time does not increase with stack height, so edge connecting can be 20% to 30% below wire bond costs.
In summary, Vertical Circuits appears to meet the industry’s common goals, with stacked die packaging that is better, smaller, simpler, and cheaper.
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