MPU package lids build in thermal management

Aluminium silicon carbide metal matrix composite materials meet the material property, design and pricing demands for MPU assemblies that require integrated heatsink thermal management solutions.

Increased microprocessor speeds, power dissipation and reduction in total device size require integrated thermal management solutions for current and future generations of microprocessors especially those using flip chip packaging.

These solutions are usually provided in the form of a lid or cap that is integrated into the BGA flip chip or C4 assembly.

AlSiC is a composite material of Al metal and SiC particulate that continues to gain acceptance for providing integrated thermal management solutions for a growing number of microprocessor and flip chip applications.

For microprocessor applications the integrated heat-spreading lid material must provide thermal management and a CTE solution for the total 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 of the device, substrates or interconnections.

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

AlSiC material provides the capability of tailoring compatible CTE values by changing the composition (ratio of Al metal to SiC particulate).

AlSiC composite thermal management properties are a result of the combination ratio of the SiC and aluminium material properties.

This is especially true for the composite CTE value that is intermediate to the high CTE value of Al metal at 23ppm/K and the low CTE value of SiC at 4ppm/K.

The AlSiC composite CTE behaviour can be tailored to match the CTE values of components by changing the ratio of SiC and Al metal.

Furthermore the CTE value can be tuned for an integrated lid that will moderate the overall CTE of an assembly system.

AlSiC composites produced for lid applications are AlSiC-9, AlSiC-10 and AlSiC-12 each have average CTE values of 9, 10 and 12ppm/K, respectively, over the temperature range of 25-150C.

The thermal conductivity value of AlSiC is relatively insensitive to the composition with values around 200W/mK.

The AlSiC material can support numerous surface finishes depending on the requirements of the specific 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 similarly to Al metal including: Ni or Ni-Au plating to support solder and braze attachment schemes; or anodised and chromic conversion for corrosion or cosmetic reasons.

There are many material systems that contribute to the flip chip 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/K, respectively.

These devices are attached (bumped or soldered) to higher CTE value metallic (high temp solder, Cu and Au) I/O BGA, which are then attached to a ceramic substrate (alumina with CTE value of 6.7ppm/K) or a PCB that has CTE values between 12 and 15ppm/K.

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/K.

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 add to 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, and can minimise the thermally induced stresses of the overall assembly.

However, if the lid is to be attached to the substrate or the PCB, a close CTE match to these materials is critical.

AlSiC-12 with a CTE value of 12.4ppm/K is appropriate for PCB board attachment; AlSiC-9 with a CTE value of 8.3ppm/K is suitable for mounting to ceramic substrates.

The primary design consideration 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 control of dimensions and tolerance of the lid cavity depth and the height of the lid walls.

In addition, the lid flatness and parallelism are also necessary in minimising the bondline length of this interface.

Generally, the AlSiC lid forming process can provide cavity wall height dimensional tolerance of +/-0.054mm and meets a flatness of 0.04mm per 25.4mm.

The gap between the device and lid interface, thus the bondline length, can be properly reproduced with the tolerance and flatness capability of AlSiC.

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).

The CPS AlSiC materials have stiffness values that are 30% greater than Cu metal lids and three times greater than Al metal lids.

Automated assembly process yields have also increased with some customers as a result of 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 CPS AlSiC forming process can also provide Concurrent Integration of advanced thermal dissipation material inserts during fabrication.

This allows the direct integration of high heat dissipation materials for improved thermal interfaces compared with brazed, soldered or epoxied assemblies.

Materials like thermal pyrolytic graphite (TPG) and CVD diamond substrates can be directly integrated into the AlSiC composite.

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.

The AlSiC material 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 lids are currently being used for integrated heatsinks in microprocessor designs and other flip chip applications.

AlSiC provides the CTE compensation needed to increase device reliability.

Because of the composite nature of AlSiC, CTE values can be tuned for the specific applications, ranging from 8 to 12ppm/K, by changing the Al/SiC ratio.

The 200W/mK thermal conductivity of AlSiC material provides reliable thermal dissipation.

The AlSiC fabrication process also provides lids with tight dimensional tolerances and good flatness, minimising the interfacial bondline length to reduce thermal resistance throughout the assembly.

AlSiC material provides the necessary EMI/RFI shielding required for higher clock speed microprocessors.

AlSiC microprocessor and flip chip lids are currently manufactured at production rates of 50,000 to 100,000 pieces per week.

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