Advanced Materials for Optoelectronic Packaging

Materials impact performance, reliability, weight, manufacturability and cost. A variety of new advanced composite and monolithic materials are now available that provide great advantages over those traditionally used in microelectronic, optoelectronic, micro-opto-electro-mechanical (MOEMS) and micro-electro-mechanical (MEMS) packaging, including:

• Thermal conductivities ranging from extremely high thermal (up to more than 4×that of copper) to extremely low.
• Tailorable coefficients of thermal expansion (CTEs) from -2 to +60 ppm/K.
• Electrical resistivities ranging from very low to very high.
• Extremely high strengths and stiffnesses.
• Low densities.

The payoffs:

• Improved and simplified thermal design, including possible elimination of heat pipes.
• Reduced thermal stresses and warpage.
• Improved fiber alignment.
• Weight savings up to 80 percent.
• Size reductions up to 65 percent.
• Increased reliability.

Advanced packaging materials, which are at various stages of development, fall into five main categories: monolithic carbonaceous materials, metal matrix composites (MMCs), polymer matrix composites (PMCs), carbon/carbon composites (CCCs) and advanced metallic alloys.

Commercial Al/SiC microprocessor lids. (Source: Ceramics Process Systems)

Use of traditional microelectronic packaging materials frequently requires significant compromises, such as the use of underfill with flip chips and thick layers of compliant adhesive to reduce thermal stresses and warping caused by CTE differences. The former increases cost, and the latter reduces thermal performance. Concerns about thermal stresses arising from CTE mismatch often rule out use of solder attachment.

The Table presents properties of traditional and advanced packaging materials, including thermal conductivity, CTE, modulus, specific gravity and specific thermal conductivity, which we define as thermal conductivity divided by specific gravity. This is a useful figure of merit for applications where both high thermal conductivity and low weight are important.

For many applications, we want materials having high thermal conductivities and CTEs that match those of semiconductors, ceramic substrates and optical fibers, which fall in the range of ~0.5-7 ppm/K. Occasionally, we want to match the much higher CTE of PCBs.

Aluminum and copper have relatively high thermal conductivities, but CTEs that are much greater than desired.

The traditional low-CTE packaging materials, such as Invar, Kovar, copper/tungsten and copper/molybdenum, have high densities and thermal conductivities no better than those of aluminum. We note that copper/tungsten and copper/molybdenum are metal-metal composites rather than alloys.

There are numerous advanced monolithic and composite packaging materials at various stages of development. We focus on what are currently the most important commercial materials. However, we emphasize that we are in the early stages of a very dynamic technology. New materials may well emerge that will eclipse the ones considered here.

Injection-molded thermally conductive carbon fiber-reinforced notebook computer heat spreader (Source: 2002 Cool Options Inc.)

Most of the dozens of different types of carbon fibers have relatively low thermal conductivities. However, there are commercial continuous carbon fibers with nominal thermal conductivities as high as 1100 W/mK. Experimental discontinuous fibers reportedly have thermal conductivities of 2000 W/mK. Carbon fibers are used as reinforcements with polymer, metal and carbon matrices.

An important characteristic of fiberreinforced composites is that their properties can be tailored greatly. For example, we can obtain materials with extremely high thermal conductivities in one direction that can compete with heat pipes over short distances. Alternatively, we can achieve high isotropic inplane conductivities to spread heat effectively.

As a result of the relatively high electrical resistivity of PMCs compared with metals, they are being used in heat sinks to reduce electromagnetic radiation.

Most of the advanced materials we present in the Table have high thermal conductivities, because of the importance of this property. However, there are applications for which low thermal conductivities are needed to reduce heat loss and maintain stable temperatures. The Table presents one of several PMCs with low thermal conductivities and tailorable CTEs, aramid fiber-reinforced epoxy.

In addition to diamond, there are a variety of monolithic carbonaceous packaging materials of interest. We focus on two: thermal pyrolytic graphite (TPG) and ThermalGraph, a fibrous panel.

TPG, a highly anisotropic, rather brittle and weak material, has a reported inplane thermal conductivity of 1700 W/mK, and through-thickness conductivity of 10-25 W/mK. The inplane CTE is -1.0 ppm/K, which is lower than desired for most packaging applications (many carbonaceous materials have negative axial or planar CTEs). However, TPG can be encapsulated with materials having a variety of CTEs, which also provide needed strength and stiffness. Encapsulated TPG printed wiring board (PWB) thermal planes (also called thermal cores, heat sinks and cold plates) are being used in spacecraft electronics.

ThermalGraph panels are made of oriented thermally conductive carbon fibers. They have axial thermal conductivities up to 750 W/mK. ThermalGraph can be infiltrated with polymers, aluminum and copper to increase strength and through-thickness thermal conductivity. This material is being used in aerospace packaging applications.

The most widely used MMC packaging material is Al/SiC. The author was the first to use this material in microelectronic and photonic packaging, beginning in the early 1980s. Millions of piece parts are now produced annually. Al/SiC microwave packages and solid and flow-through PWB heat sinks are used in numerous avionic systems. Commercial applications include servers, DSPs, notebook computers, cellular telephone base stations and power supplies for trains, wind turbine generators and hybrid vehicles.

Al/SiC is a family of materials, made by a variety of processes, some of which can produce complex, net shape parts that do not require machining. By appropriate choice of matrix alloy and particle volume fraction, it is possible to tailor the CTE of Al/SiC between 6.2 and 23 ppm/K. Variations of this material reportedly can go as low as 3.8 ppm/K.

The Table presents properties of other key MMCs of interest. Matrices are aluminum, copper and beryllium. Reinforcements include continuous (cont.) and discontinuous (disc.) carbon (carb.) fibers (fib.); discontinuous carbon-graphite (carb.-graph.); and diamond and beryllia particles (part.). Beryllia particle-reinforced beryllium has been used in a limited number of spacecraft applications.

Glass fiber-reinforced polymers, which are PMCs, have been used in electronic packaging for decades. However, these materials have low thermal conductivities and high CTEs.

As discussed earlier, a major breakthrough was the development of thermally conductive carbon fibers. The Table presents properties of polymers reinforced with continuous and discontinuous versions. Continuous thermally conductive carbon fibers can produce PMCs with inplane thermal conductivities 50 percent greater than that of aluminum, combined with high modulus and low density. These materials are being used in a number of aircraft and spacecraft production thermal management applications.

A major advantage of PMCs reinforced with short discontinuous fibers is that they can be formed into complex parts by injection molding.

Although thermal conductivities are lower than those of PMCs using continuous fibers, they are adequate for many applications.

Carbon/carbon composites consist of carbonaceous matrices reinforced with carbon fibers. They are stronger, stiffer and less brittle than monolithic carbon. Some CCCs have high thermal conductivities. The Table presents properties of one formulation.

CCCs have been used in a limited number of production thermal management applications, including launch vehicle PWB cold plates, spacecraft radiator panels, and thermal doublers for PMC spacecraft radiator panels.

The Table presents properties of three other advanced materials of interest for thermal management, silicon/aluminum, beryllium/aluminum and Invar/silver. The former is probably of greatest interest. These materials, which are being used in a number of packaging applications are considered alloys by some and metal-metal composites by others.

As technology matures, materials with better properties and cheaper processes undoubtedly will continue to emerge. The history of advanced materials has shown that, as production volume grows, costs drop, making them increasingly attractive. We have already seen this with Al/SiC. Because of the unique ability of advanced materials, especially composites, to meet future packaging requirements, they increasingly will play a greater role in the 21st century.