The ability to provide cost effective and reliable interconnects for attachment of die to substrates and printed circuit boards (PCBs) is critical to development of military and commercial electronic systems. Specifically, there is a widespread need to provide these interconnects in communications systems that must be reduced in size and weight for Naval ship and airborne applications. Current trends in designing Monolithic Microwave Integrated Circuits (MMICs) that support the transmit and receive functions for communications antennas involve the combining of functions of several die into a single “System on Chip” (SoC) die. This enables savings in cost and size due to having less die connections and space required, which can also improve performance in speed and transmission. In a current ManTech project at the EMPF, a Silicon Germanium System on Chip (SiGe SoC) is being developed for an Active Electronically Scanned Array (AESA) communication system. This SiGe SoC is a MMIC device that will require flip chip technology to mount the die onto a multilayer organic substrate. This solution provides both logic and RF circuitry in a single die with multiple interconnects. In order to facilitate a low cost attachment to the organic substrate, repeatable and reliable connections must be made for high yield using the latest and best interconnect technologies A novel attachment technique called “Particle Interconnect” (PI) is one of the attachment solutions to be considered that has the ability to provide both connection and rework of the flip chip SiGe SoC, a very desirable attribute for a multi-function die with many connections. This patented attachment technique was developed by Rocky Connections, LLC and can be used in conjunction with, or replace, standard solder ball, stud bump and plated bump flip chip processes. In the case of the aforementioned SiGe SoC, the standoff height required from the PCB will most likely require a standard bump attachment technique to the die, with a PI attachment to the PCB. This effectively replaces the solder step to the PCB with PI, which can be reworked and is equal to or more robust than solder, with potentially much superior Radio Frequency (RF) interconnection properties.

Particle Interconnect utilizes sharp, metallized, diamond particles which have been screened by size. These particles are attached to substrate contact pads, under bump metallization or other conductor surfaces using a standard electroplating process with masks (see Figure 1). These embedded particles are sharp and create a “microbed-of-nails” that makes many parallel electrical contacts. The particles penetrate through oxide or other thin contaminants to make contact without a wiping or scrubbing action that is required for many other conventional contacts. This patented process results in creating a gas-tight electrical contact (see Figure 2).

Particle Interconnect can provide an improved connection to a PCB because it is an environmentally safe, diffusion bonding process requiring a very low contact force, without scrubbing. This technology creates an oxide penetrating, gas tight connection that allows electrical connection in the presence of adhesives or contamination. The thermal stress due to the Coefficient of Thermal Expansion (CTE) mismatch between the SiGe SoC flip chip and the organic substrate can be minimized, since the bonding temperature can be varied from -270° C to > 450° C. In addition, the mono-metallic bonds of PI, unlike the bi-metallic ones of solder, do not decay over time due to temperature. In terms of performance, PI has been shown to provide RF bandwidth greater than 65 GHz, which meets the 20 GHz requirement for the SiGe SOC MMIC AESA application, and a lifetime of greater than 30,000 insertions. This benefits packaging attachment cost by allowing removal of the die as rework, so that non-functioning die can be removed, replaced and retested. This dramatically improves the Known Good Die (KGD) issue for systems requiring many die, such as the 64 die required for one transmit antenna system for the SiGe SOC ManTech project. Thus, die can undergo troubleshooting and be replaced so that a 100% good module or system can be qualified.

The electrical characteristics of PI technology form a very stable contact and performance that can be superior to a standard solder ball connection. Contact resistance is very low, typically measuring 3 - 5 milliohms, and does not significantly change after repeated attachments. The RF performance requirements are also met for the application of PI to the SiGe SOC MMIC. Since the contact joint is made by pressure with no scrubbing action, the connection does not react as an element in the electrical chain, but is instead a matched impedance connection. This is a desirable RF characteristic since there is very little reflected signal due to impedance mismatch. Therefore, faster signal transmission is possible with greater signal fidelity. The physical formation of the electrical contact occurs at the contacting surface with a large number of asperities (very small points on the surface) that concentrate the contact force into a very small area. Application of as little as 10 grams of force per contact will provide the pressure to penetrate oxides and contaminants on the surface and drive the diamond particles into the substrate or bump. Since diamond particles are used in the matrix, these asperities are very sharp and hard due to the nature of the crystal. These diamond points do not dull or wear since the crystal is much harder than the opposing bump or surface metal.

All these parallel connections together form a stable contact with good reliability and durability, a long cycle life, high current capacity, and very low contact resistance. Since the connection formed is gastight, it is also very suitable for performance in harsh environments including acceleration (~65,000g) and shock. Die attached with PI have been reported to pass four successive cycles of MIL-STD-883C (burn-in, thermal shock, pressure, and salt/fog corrosion) without damage. In addition, since there is no scrubbing action required, the connection does not degrade readily and this provides a long mating life. Connection tests in comparison to standard solder balls have reported a life of over 30,000 insertions with no significant change in the contact performance. These attributes will be tested as part of reliability testing using a SiGe SOC daisy-chained bump structure which will be attached to a substrate using PI technology. Thermal shock is one of the planned tests that will be performed in comparison to earlier studies performed at the EMPF on similar attachment assemblies for both shipboard and airborne platforms.

In conclusion, Particle Interconnect is a very promising technology in terms of electrical, mechanical, thermal and reliability characteristics. It may also provide the best price and best performance for the SiGe SOC MMIC attachment as compared to other interconnect technologies. Due to the simplification of process steps and reduced material costs, PI technology could prove to be faster, easier, and require little specialized equipment or skilled labor, which will lead to significant cost savings.

References: http://pitek.us