Hot stuff

Aluminium Silicon Carbide's unique properties and low cost of fabrication make it the perfect material for thermal management heat spreading solutions for microprocessor and flip chip applications, write Mark Occhionero and Richard Adams of CPS Corp.

As microprocessors have become ever faster and smaller, the need for effective thermal management solutions to rapidly remove the heat from the chip has grown in importance. This is especially true of flip-chip applications.

These solutions are usually provided in the form of a lid or cap that is integrated into the ball grid array (BGA) flip chip or controlled collapse chip connection (C4) assembly.

Aluminium Silicon Carbide (AlSiC) - a composite of aluminium (Al) and Silicon Carbide (SiC) - has emerged as one of the hottest materials for providing integrated thermal management solutions for a growing number of microprocessor and flip chip applications (Figure 1).

For microprocessor applications, the integrated heat-spreading lid material must provide thermal management and a coefficient of thermal expansion (CTE) solution for the whole microprocessor assembly (chip, BGA, interposer, board) as well as supporting functional designs.

A balanced assembly CTE increases device reliability by reducing thermally induced stresses that can cause delamination or cracking failures in the device, substrates or interconnections.

Since different microprocessor manufactures have different assemblies and designs, the CTE requirement for the AlSiC integrated lid must be tailored to the specific application.

Fortunately, AlSiC can easily achieve different CTE values by changing the ratio of Al to SiC in the composite (Figure 2).

This method can be used not only to match AlSiC's CTE value to that of individual components but also to provide a CTE value that would moderate the overall CTE of an assembly. The latter is particularly useful for an integrated thermal management lid.

The most common composites used for lid applications are AlSiC-9, AlSiC-10 and AlSiC-12 (with an average CTE value of 9, 10 and 12ppm/¡C respectively over a temperature range of 25¡-150¡C).

The thermal conductivity of AlSiC remains relatively constant at around 200W/mK irrespective of its composition.

AlSiC can also support numerous surface finishes depending on what is required by the application. The "as fabricated" AlSiC product surface is similar to aluminium, with a cosmetic appearance that supports laser marking, screen printing and painting. These surfaces can be chemically treated in a similar way to aluminium: nickel or nickel-gold plating to support solder and braze attachment schemes; or anodised and chromic conversion for corrosion or cosmetic reasons.

As illustrated in Figure 3, there are many material systems that contribute to a flip chip's overall thermal performance and the overall thermally induced stresses associated with differential CTE values.

The device materials, Si or GaAs, have low CTE values (4.2 and 6.5ppm/¡C respectively). These materials are attached (bumped or soldered) to higher-CTE-value metallic (high temp solder, Cu and Au) I/O BGAs, which are then attached to a ceramic substrate (alumina with CTE value of 6.7ppm/¡C) or a PCB (12-15ppm/¡C).

Often a filled epoxy underfill material is added to compensate for differential CTE thermally induced stresses between the device and ceramic substrate or PCB. These assemblies often require a lid for heat spreading and device protection in subsequent end-user assembly operations (heatsink application or fanned heatsink attachment).

Traditional lid materials like Cu and Al have high CTE values of 17 and 23ppm/¡C. These lid materials are thermally interfaced to the device through thermal grease to avoid introducing a large thermally induced stress associated with direct attachment. The lids have to be taken into account when calculating the thermal stress equation of the total microprocessor assembly, since they are attached to the PCB or substrate by epoxies or solder.

The thermal stress equation for microprocessor assemblies is complex, depending on the geometry and the CTE behaviours of many different materials and requiring complex thermal modelling evaluation. In general, component materials that have CTE values more closely matched, like the AlSiC lid materials, minimise the thermally induced stresses of the overall assembly.

However, if the lid is attached to the substrate or PCB, a close CTE match to these materials is critical. AlSiC-12 with a CTE value of 12.4ppm/¡C is appropriate for PCB board attachment; AlSiC-9 with a CTE value of 8.3ppm/¡C is suitable for mounting to ceramic substrates.

The primary design aim of an integrated heat-spreading lid is to minimise the gap between the lid and the top of the device and to decrease the bondline length to reduce device lid/interface thermal resistance. This is necessary since thermal grease materials have low thermal conductivity values of 1-2W/mK.

Minimising this gap requires close control of dimensions and tolerance of the lid cavity depth and the height of the lid walls. In addition, the flatness and parallelism of the lid are also important in reducing the bondline length of this interface. Generally, the AlSiC lid forming process can provide cavity wall height dimensional tolerance of ±0.054 mm and meets a flatness of 0.04mm/25.4mm (Figure 4).

The lid must also provide mechanical protection of the device from the end-user's heatsink attachment. Maintaining clearance between the cavity and device assembly can provide this protection. However, the clearance increases the bondline length and, thus, thermal resistance as discussed above. The amount of clearance can be minimised by choosing a lid material with a higher stiffness value (reducing deflection of the lid during subsequent heatsink attachment).

AlSiC materials can have stiffness values that are 30% greater than Copper lids and three times greater than Aluminium lids. Automated assembly process yields have also increased by switching to the lighter weight AlSiC material. The lower inertial response in high-speed assembly reduces stresses between the die and solder balls, and solder balls to circuit assembly.

The AlSiC forming process can also provide integration of advanced thermal dissipation material inserts during fabrication. This allows the direct integration of high heat dissipation materials for improved thermal interfaces compared to brazed, soldered or epoxied assemblies.

Materials like Thermal Pyrolytic Graphite1 (TPG) and CVD Diamond substrates can be directly integrated into the AlSiC composite (Figure 5). These materials are relatively expensive and are often difficult to integrate into an assembly because of their fragility or the need to treat surfaces for attachment.

AlSiC adds functionality to these materials by providing a method of direct integration. These materials can be more economically used, since the AlSiC integration can locate high heat dissipation materials in the area of most need.

In the past, these applications have been limited to high performance and military systems. Currently, AlSiC flip chip heat spreaders are being evaluated for commercial applications.

AlSiC microprocessor and flip chip lids are currently being manufactured at production rates of 50,000-100,000 a week.