The objective of all thermal control programs in electronic packaging is the efficient removal of heat from the semiconductor junction to the ambient environment. Thermal management can be separated into three major phases:
The first phase is usually beyond the control of the packaging engineer because heat transfer characteristics are determined by the manufacturer of the device. To optimize heat transfer in the second and third phases, an understanding of the fundamental heat transfer processes and a knowledge of material properties is essential.
BASIC THEORY
The rate at which heat flows through a material is proportional to the area normal to the heat flow and to the temperature gradient along the flow path. For one-dimensional, steady-state heat flow the rate can be expressed by Fourier's Law:
k = (q/A) x (d/deltaT)
where:
k = thermal conductivity
q/A = heat flux (watts per unit area)
d = length of the heat flow path
deltaT = temperature gradient
Thermal conductivity, k, is an intrinsic property of a homogeneous material that describes the ability of that material to conduct heat. A higher value means that the material can conduct a greater heat flux for a given temperature gradient.
Inspection of Fourier's Law leads to another thermal property, thermal impedance - the temperature gradient caused by a unit rate of heat flow through a material of a given size. Thermal impedance is equal to:
ø = DeltaT/(q/A)
Thermal impedance is related to thermal conductivity by the rearrangement of Fourier's Law:
k = d/ø
Unlike thermal conductivity, thermal impedance is proportional to the distance the heat must travel.
Fourier's Law describes heat flow within a solid. Suppose two solids are brought into contact and heat is conducted from one solid into the other. In addition to the normal temperature gradients within the solids, a significant temperature gradient is observed due to the inter-face between the two solids. This is referred to as thermal interfacial impedance, or thermal contact resistance.
Contact resistance is caused by inherent irregularities of the contacting surfaces. Each surface, no matter how well polished, consists of peaks and valleys. The actual metal-to-metal contact area is a small fraction of the total contact area. Voids formed by valleys are filled with air and contribute little to the conduction of heat. The majority of the heat flow is constricted to the small areas of metal-to-metal contact. This accounts for the observed temperature gradient across the interface.
THERMAL INTERFACE MATERIALS
Heat generated by a semiconductor device must be removed to the ambient environment to ensure the device'reliable operation. Unless space is available to provide sufficient forced convection cooling, this requires a series of physical interfaces to provide a thermally conductive path. Not only must these interfaces offer minimum resistance to heat flow, but they often must also provide electrical isolation. Such requirements can be met by using conventional insulators coated with thermal grease, or with one-component thermal interface materials.
A thermally conductive insulating material provides electrical isolation because the two metal surfaces are separated by the dielectric material. Thermal contact resistance has been minimized because the air gaps have been eliminated and replaced with a material whose thermal conductivity is much greater than that of air.
To perform successfully, thermal interface materials must have high dielectric strength, high thermal conductivity and sufficient pliancy to conform to both microscopic and macroscopic surface irregularities. They must also be sufficiently durable to survive a variety of assembly, use and environmental conditions.
Composition - Thermal interface materials generally consist of a thermo-set elastomeric binder containing a dispersed, highly thermally conductive ceramic filler. This mixture is generally reinforced with glass fiber, metal foil, or dielectric film. The elastomer binder is typically a silicone molding resin cured at high temperatures and high pressure. Urethane elastomers have been introduced for use where silicone cannot be tolerated due to possible contamination. Ceramic fillers are added to the elastomer to increase its thermal conductivity. Typical fillers are boron nitride, aluminum oxide and magnesium oxide.
Thermal Conductivity - Thermal conductivity is a measure of the ability of a material to conduct heat only after heat has entered the material. This ability is determined by the material's composition, i.e., the type and ratio of thermal filler to elastomeric binder, and by the relative amount of reinforcing metal foil, glass fiber or dielectric film. Thermal conductivity is usually expressed in units of Watt/m-K, where, W/m-K = (Cal/sec-cm-°C x 420) = (BTU-in/hr-ft2-°F x 0.14)
Values reported in different literature must be used with caution unless the test method is clearly stated. Thermal conductivity is difficult to measure for thin, resilient interface materials. Many test methods cannot distinguish between contact resistance and sample resistance, leading to unrealistically low values. Other methods based on calculations from "TO-3 Thermal Impedance Data" may seriously overestimate thermal conductivity.
In homogeneous materials, thermal conductivity is independent of physical dimensions. Unreinforced materials can be considered homogeneous and their thermal conductivity is independent of thickness. All reinforced materials are non-homogeneous in that the reinforcing layer is a poor thermal conductor compared to the outer elastic layers which are good thermal conductors. As sample thickness is increased, the reinforcing layer remains constant while the elastomeric layer expands. The ratio of good conductor to poor conductor increases, and the apparent, or total, thermal conductivity increases with increasing material thickness.
When making thermal calculations based on thermal conductivity, care must be exercised to take into account all contact resistances that may be present in the thermal path. Other complications resulting from non-uniform heat flow, e.g. hot spots and thin heat spreader plates, can cause an underestimate of the temperature differential between the junction and ambient, and lead to unsafe operating temperatures.
Thermal Impedance - Like thermal conductivity, thermal impedance is a measure of a material's ability to conduct heat. In addition, thermal impedance describes a material's ability to conform to irregular surfaces and minimize contact resistance.
Thermal interface materials can reduce contact resistance by conforming to two rough mating surfaces and eliminating air gaps. Most of these elastomeric materials are highly loaded with hard ceramic fillers. They require pressure to make them "flow" into the surface irregularities and reduce contact resistance.
The contact resistance is high at low pressures due to poor mating. As pressure is increased, the material begins to flow into the surface irregularities and the contact resistance decreases. At 300 to 500 psi the contact resistance is essentially eliminated because thermal impedance is not influenced by further pressure increase. To illustrate, at 10 psi the thermal impedance due to contact resistance is three times as great as the resistance due to the material itself.
The pressure needed for achieving minimum thermal impedance is easily accommodated in most packages. In fact, secure attachment of components to heat sinks usually requires the same magnitude of force holding the component to the sink. However, recent developments, such as surface mount applications and heat sinking of microprocessors, require that good thermal contacts be made with minimum applied pressure. A new approach is required for applications in which an interface material must conform at very low pressure.
Suitable materials for such applications have been developed based on a precise balance of pliancy and thermal conductivity. By careful adjustment of filler level and binder elasticity, they are made with essentially no contact resistance below 10 psi.
Dielectric Strength - Measured according to ASTM D149, dielectric strength is defined as the AC voltage required to cause a breakdown of the insulating material being tested. The results are reported as the Dielectric Breakdown Voltage for a given thickness or as a Dielectric Strength in volts/mil.
Measurements using ASTM D149 yield values are obtained under controlled test conditions and may not accurately reflect actual field performance. Factors such as corona discharge, frequency, temperature and humidity can significantly affect the long term insulating characteristics of a material. Allowances for creep and strike distance must often be made to meet electrical requirements. One effective technique is to use an interface insulator slightly larger than the base of the device case.
Volume Resistivity - Volume resistivity as determined by ASTM D257 is a measure of bulk electrical resistance. This property shows a strong inverse dependence on humidity and temperature. It is not unusual for volume resistivity to change by a factor of 105-106 when a material is exposed to more than 90% humidity. Increasing temperatures yield similar, though not such drastic changes. These changes are completely reversible. When conditions are returned to normal, the volume resistivity also returns to the original value. Chomerics CHO-THERM products are comparatively much less sensitive than ordinary silicones to both moisture and temperature, typically undergoing a 100 to 1000-fold decrease in volume resistivity.
Elastomeric Properties - Thermal interface materials exhibit properties consistent with highly filled elastomers, such as compression deflection, stress relaxation, and compression set. Each property has a major impact on the long term effectiveness of an interface material.
Compression Deflection - A solid elastomer, as opposed to a foam, is not compressible, but will yield when a load is applied. Under a compressive load, the material will undergo a deflection. The magnitude of the deformation is proportional to the load up to the elastic limit - the point at which the material ruptures and can no longer recover.
Stress Relaxation - If an elastomer is subject to a compressive load it first undergoes deflection while the load is applied. This is followed by a slow relaxation process whereby the initial stress begins to decay. A natural rubber process, this stress decay is brought about by macromolecular rearrangement within the elastomer. The initial load causes the rubber to fill the gaps. There is then no further need for such high pressure. In time the stress decays to a point where it is insufficient to cause further rearrangement. The point, or percent stress loss, is dependent on several factors, including the nature of the elastomer and the level of loading.
Compression Set - If an elastomer is subject to a compressive load for an extended time, a part of the deflection becomes permanent and will not be recoverable when the load is removed. This behavior is important only in designs where the interface material has to be unloaded and reloaded occasionally during its service life. The compression set of Chomerics CHO-THERM materials, per ASTM D395, Method B, is typically 25 to 35% - which means that 6 to 9% of its initial thickness will not be recovered.
Chemical Resistance - Interface materials may come into short term contact with solvents either in solder-flux and cleaning operations or through unintentional exposure to coolants, fluids or lubricants. Contact with any number of solvents will cause swelling of the exposed areas of elastomer interface materials. The severity of the swelling will depend on the type of solvent, duration of exposure and the type of elastomer. Generally, solvents such as ketones, halogenated hydrocarbons and esters cause more swelling than alcohols or aromatics.
While the elastomer is swollen its resistance to abrasion is reduced and care should be taken not to damage the material. The swelling phenomenon is reversible and the interface material returns to its normal state as the solvent evaporates. All physical, electrical, and thermal properties remain the same as before the exposure.
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