THERMAL MANAGEMENT

As the electronics industry makes progress through the introduction of ever faster components, the thermal and structural properties of printed circuit boards (PCBs) play an increasingly key role in device functionality. The PCBs on which electronic components are based must be modified in order to support the high-power integrated circuits (ICs), such as graphic chip sets and memory dies, that make possible the extension of electronic capabilities. PCBs now impose limitations relating to their thermal conductivity, coefficient of thermal expansion (CTE), weight, and rigidity.

Several potential solutions currently address some of the difficulties facing electronics designers. In many cases, however, the materials available for PCB construction, while helping to manage one area of concern, may add complexity in other areas. For example, heavy copper can be useful for thermal management, but it is not able to manage the board's CTE, and it substantially increases the weight premium of the PCB.

A new ground base for PCB construction is carbon composite laminate. Carbon composite laminate is a material made from carbon fibers, which possess properties distinctively different from those of the E-glass insulating fibers that are an electronics industry standard. These attractive properties include high thermal conductivity, a negative CTE, very low density, and a high tensile modulus. Composite laminate made from carbon fibers accordingly feature high in-plane thermal conductivity, a very low in-plane CTE, a high tensile modulus, and a weight as light as any equivalent glass-fiber composite. This material can be used selectively in the integral part of the PCB or its substrate.

Thermal Management

In order to manage thermal issues, many electronics engineers and designers are turning to metals such as copper, copper-Invar-copper (CIC), copper-molybdenum-copper (CMC), and aluminum. The thermal conductivity of copper is in the range of 385–400 W/m•k. CIC conductivity performance is 20–30 W/m•k, for CMC 180–220 W/m•k, and for aluminum around 150 W/m•k.

Figure 1. The thermal path of an isotropic metal in a PCB. (click to enlarge)

Normally, metals are isotropic thermal conductors (see Figure 1). Metal layers are used as an integral core of a PCB. Metal core materials spread heat in a circular, multidirectional pattern (that is, isotropically). Thus, for generated heat to be dissipated, it must first travel to the center of the board through several dielectric (non– thermally conductive) layers, and then be drawn out.

Although the thermal conductivity of the metal core may immediately seem to be advantageous, the effective thermal conductivity is actually low owing to the high thermal resistance of the dielectric layers located between the thick metal core and the heat source. Placement of a thick central core in the PCB introduces the additional disadvantage of limiting device functionality by reducing the number of fine signals that can pass through the core.

Fibrous carbon composite laminates, on the other hand, are used as plane layers, most suitably in a ground plane in the stack of a PCB or substrate. The composite can be processed easily and placed in the second layer from the surface of the PCB. Carbon composite laminates dissipate heat in an anisotropic manner. Rather then spreading heat in all directions equally, they conduct it in the lateral x-y plane. The thermal conductivity of the raw carbon fiber ranges from 10 to 600 W/m•k.

Figure 2. In a carbon-embedded PCB, heat travels quickly to the composite layer through a ground via (a) or thermal vias (b) and then is conducted toward a dissipative element such as a wedge lock or heat sink. (click to enlarge)

In carbon-embedded PCBs, heat will travel through the ground vias and thermal vias and then through a very thin dielectric layer, reaching the carbon composite layer after following a much shorter thermal path with low thermal resistance (see Figure 2). The heat will then be rapidly dissipated in the lateral plane, drawn to a mounting wedge lock, chassis, frame, or heat sink. The resulting high in-plane thermal conductivity contrasts with the through-plane conductivity of metal core composites.

Multiple layers can be placed strategically throughout the PCB, in a symmetrical pattern, to maximize heat capacity. The anisotropic conductivity allows for the heat to flow a great distance in-plane quickly. Heat can also be conducted through the thin dielectric directly to the composite situated just below the surface layer. Spreading heat across the entire surface area of the PCB this way eliminates the development of hot spots on the board. Designers, thus no longer constrained, can design in fine signal features without frustration.

CTE Management

As temperature rises, a standard PCB with organic packaging expands at a rate of 17–19 ppm/°C. But designers attempting to create higher-speed electronics are mounting dies on ceramic substrates that typically expand at 6–8 ppm/°C. There can thus be a large expansion mismatch between the board and the ceramic IC substrate, which causes solder joints to crack and introduces reliability issues.

Two different approaches can be taken to handle this CTE mismatch. One is to use a nonwoven aramid material to control the CTE of the PCB. These polymeric materials have the ability to keep the CTE between 9 and 12 ppm/°C. The difficulty with them is that they expand radically in the z-axis—exhibiting more than twice the expansion of FR4 fiberglass epoxy laminate—causing through-hole continuity failures. Also, they are highly moisture absorbent. In addition, nonwoven aramid is difficult to obtain because manufacturers have declared a shortage.

Figure 3. Solder columns can mitigate the mismatch between an organic PCB's CTE and the much lower expansion rate of a ceramic IC substrate. (click to enlarge)

The second method developed to cope with the expansion mismatch was the use of solder columns (see Figure 3). In order to place solder columns on the IC substrate, the solder balls must first be removed. The solder columns replace them. This of course adds cost and additional labor time.

Carbon-embedded PCBs, on the other hand, with their expansion rate of 2–12 ppm/°C, have a CTE much lower than the traditional PCB and are able to match that of the ceramic IC substrate. This enables the designer to maximize the speed of components without requiring additional labor, as is necessary with solder columns. The efficiency of the PCB is very high because, under the circumstances, engineers can attach the die directly to the board.

When typical PCBs are subjected to repeated thermal cycling, especially in harsh operating environments, this leads to low-cycle solder joint failures due to thermally induced strains. These strains again are caused by uneven expansion and contraction rates of various adjacent materials, particularly between IC packages and PCBs. The ability of carbon composite material to lower a PCB's CTE allows engineers to employ ceramic ball-grid-array technology instead of a column grid array.

Figure 4. A carbon-embedded PCB has a CTE low enough to allow direct attachment of the die to the board. (click to enlarge)

Carrying the advantage one step further, the very low CTE of the carbon hybrid substrate makes possible the development of reliable flip-chip or direct-die-attach (DDA) packages (see Figure 4). Reducing a board's CTE by more than 80% and matching it to that of the die itself eliminates the need to use underfill in a DDA application owing to stress of just 0–1% on the redistribution layer bumps. Today's high-performance PCBs allow for higher speeds and reduced form factors while maintaining thermal management. To facilitate satisfaction of the growing demand for higher speeds, the electronics industry increasingly is moving toward DDA and flip-chip packages.

PCB Rigidity

Increasing the speed of components and having the ability to mount the components directly on the PCB allow designers to create new electronic technology without being hampered by thermomechanical limitations. A concomitant benefit of carbon enhancement of the PCB is the greater rigidity it gives the board.

The tensile modulus of a carbon composite laminate is magnitudes greater than that of glass laminate. When carbon composites are embedded in an FR4 or polyimide board, the stiffness increases as much as fivefold over that of the typical PCB, depending on the ratio of the volume of the composite to the volume of the rest of the material in the board. The increased stiffness of the PCB provides much higher reliability under shock and vibration. The stiffening composite layer decreases the need to use metal frames and stiffeners.

In addition, the weight of carbon composite is close to that of FR4 and polyimide (see Table I). To reduce project cost, it often is essential to replace CIC or heavy copper in electronic designs. The higher stiffness and lower weight of carbon composite means that incorporating laminates of this material in PCBs results in a higher stiffness-to-weight ratio than metal-based solutions can provide.

Conclusion

As the electronics industry moves toward compliance with the RoHS Directive, issues with heat arise. To attach components with lead-free solder generally requires substantially increasing oven temperatures. But because carbon-embedded PCBs can distribute heat evenly, using lead-free solder to attach components to the board would not necessitate higher oven temperatures. In fact, the temperature required would be lower than usual.

With their substantial thermal-management and rigidity benefits and no weight premium, carbon composite laminates enable engineers to create faster, stronger, and more-reliable PCBs for electronic medical devices and other appliances. Leading-edge electronic technologies are now incorporating the materials as a forward-looking foundation for the solution of current and likely future PCB design issues.