What a heat spreader does
Electronic chips, laser diodes and power devices produce heat in a very small area. If that heat stays concentrated, the device runs hot, may degrade faster and may fail. A heat spreader is a plate or block placed under or next to the device. It carries the heat sideways and downward over a larger area, so that a heat sink, cold plate or other cooling system can remove it more effectively.
A good spreader material needs two things at once. It must conduct heat well, and it must not expand and contract so differently from the device that thermal cycling cracks the joint between them. Meeting both is the reason composites are used.
The problem with single materials
Copper is a common spreader because it conducts heat very well and is easy to machine. But copper expands noticeably more than silicon and many ceramics when heated. A chip bonded to a copper plate experiences stress at every heating and cooling cycle, which can lead to cracked solder or a failed joint.
Materials that expand little, such as some ceramics, are matched better to chips but may conduct heat less well than metals. This is where aluminum nitride substrates, which combine reasonable heat conduction with electrical insulation, find a role. Pure diamond conducts heat extremely well and expands very little, but a plate of diamond is costly and hard to shape. The issue of expansion mismatch is a broad one in assemblies, covered in the article on thermal expansion.
Why add diamond to a metal
A metal-diamond composite tries to take the best of both. Diamond particles, which have exceptional thermal conductivity, are packed into a metal matrix such as copper or aluminum. The metal fills the spaces between the particles and gives a continuous path for heat, as well as machinability and a way to join the part. The diamond raises the effective conductivity, and it lowers the thermal expansion of the composite toward values closer to semiconductor materials.
The idea is that by changing the proportion of diamond, designers can tune the composite. More diamond tends to lower expansion and, up to a point, increase conductivity, but also makes the material harder to produce and to machine. The final properties depend on the diamond content, particle size, particle quality and the quality of bonding to the metal, so published values for specific composites vary widely and should be taken from supplier data for the exact grade.
Copper versus aluminum matrices
- Copper-diamond. Copper has higher conductivity than aluminum, so copper-based composites aim for the highest thermal performance. They are denser and generally more expensive.
- Aluminum-diamond. Aluminum is lighter and cheaper to process, which suits weight-sensitive applications such as aerospace electronics. Its base conductivity is lower than copper’s, but with enough diamond the composite can still perform well.
- Silver and other matrices. Other metals are sometimes used, and some designs include thin carbide-forming layers to improve bonding. Silicon-based and ceramic-bonded variants also exist.
These choices are part of the larger family of metal matrix composites, in which ceramic or other particles modify a metal’s behavior.
The interface problem
The most difficult part of making these composites is the interface between diamond and metal. Diamond does not naturally wet or bond with copper, and a poor interface acts as a barrier to heat flow. Heat has to cross the boundary of every particle, so a weak or gappy interface can cancel much of the advantage of the diamond.
Producers address this in several ways. One approach is to coat the diamond particles with a thin layer of a carbide-forming metal that bonds to both diamond and the surrounding metal. Another is to add small amounts of an alloying element to the matrix. The general lesson mirrors that of any composite: bonding at the boundary governs how well the reinforcement’s properties carry through, which is the central theme of the fiber-matrix interface, even though here the reinforcement is particles rather than fibers.
How the parts are made
Common routes include:
- Infiltration. A packed bed of diamond particles is infiltrated with molten metal, often under pressure, so that the metal fills the gaps.
- Powder metallurgy. Metal and diamond powders are mixed, pressed and sintered or hot-pressed together.
- Spark plasma and hot pressing. Heat and pressure are applied together to consolidate the powder mix quickly.
Processing must control temperature and time, because diamond can convert to graphite at high temperature, especially in the presence of certain metals. The nature of this limit is discussed in the article on oxidation limits. Producers therefore work to keep conditions gentle enough to preserve the diamond.
Machining and finishing
A composite loaded with diamond is extremely abrasive to conventional tools, so cutting is difficult. Parts are often made close to final shape and finished by grinding, wire cutting or electrical discharge machining, depending on the metal and electrical conductivity of the composite. Diamond tooling is the usual choice for grinding, which is a case of one form of diamond working on another. Surface finish also matters, since a spreader must make flat, tight contact with the device to avoid thermal resistance at the joint. Thermal interface materials are often used to fill microscopic gaps.
Where they are used
Typical applications are high-power laser diode mounts, radio-frequency and microwave power amplifiers, power electronics modules and other places where heat flux is high and space is tight. In laser systems, the broader subject of laser heat spreaders includes solid diamond plates as well as composites. Wider thermal management of equipment also draws on heat sinks, fans, liquid cooling and clever layout, not just spreader materials.
Trade-offs
- Cost. Diamond content, difficult processing and difficult machining raise price compared with plain copper or aluminum.
- Performance gain. The benefit shows up mainly where heat flux is high; for ordinary electronics a simple metal spreader is enough.
- Reliability. Thermal cycling and interface quality should be verified by testing, because a poor interface can degrade over time.
- Alternatives. Other options include copper-molybdenum, copper-tungsten, aluminum-silicon carbide and graphite-based spreaders, each with its own balance of conductivity, expansion and cost.
Metal-diamond composites are best seen as a specialist option for demanding thermal problems, chosen when simpler materials cannot meet both the conductivity and expansion requirements.