Cooling IC Packages in Restricted Enclosures
04/01/2004
THERMAL GAP FILLERS AND THERMALLY ENHANCED SURFACES
BY MIKSA DESORGO
More ICs are being used in tighter spaces and increasing numbers of ICs are approaching or exceeding their operating temperature limits. In turn, designers must contend with thermal overload problems — a major cause of electronic systems failure.
High-powered microprocessors are kept within temperature limits by using thermal management solutions such as heat sinks, fans and thermal interface materials. These cooling techniques generally are unavailable for cooling low- to moderate-power ICs. There is no room for sinks or fans, and they are located where cooling airflow is limited. In the case of tightly sealed enclosures, cooling airflow does not exist. Different approaches are needed to remove excess heat from these ICs.
The Path to Cooler ICs
Heat generated inside a plastic IC package takes a tortuous path from the semiconductor die out to the ambient environment. The heat generated within the IC ultimately must be conducted to the enclosure wall, through the wall and lost to the ambient air by convection. The package type determines whether heat will leave the IC via the leads to the PCB or is lost through the case. Plastic packages transfer much of the heat to the PCB through the leads, while more thermally enhanced packages tend to improve efficiency and lose most excess heat through the case. Thermal gap fillers were developed to improve heat transfer from ICs to the enclosure wall by replacing large air gaps with a more conductive medium.
Thermal Gap Filler Pads
Thermal gap fillers are soft silcone gels originally provided in sheet form. They consist of thermally conductive ceramic particles dispersed in RTV silicone gel. Most of these materials are reinforced with a layer of aluminum foil or woven fiberglass to improve handling and dimensional stability. They can be molded into any shape to fit over multiple components (Figure 1). Gap fillers molded with a ribbed surface provide for greater deflection.
 Figure 1. A thermal gap filler pad molded to fit over a row of ICs. |
The first gap fillers were developed 15 years ago and have been evolving through continuous improvements and modifications to meet new requirements. Today's gap filler pads offer a wide range of thermal performance (from 1 to 10 W/m-K thermal conductivity). Gap filler pads are available in thickness ranging from 0.25 to >5 mm. They typically have a sufficiently tacky surface to provide adhesion for simple drop-in application onto IC packages. They're also available with pressure-sensitive adhesive for permanent mounting.
Compression and Deflection
Gap fillers deflect, rather than compress, when a force is applied to them. The extent of deflection depends on the filler loading and the silicone gel formation. High thermal conductivity requires high filler loading and results in a stiffer, less conformable product. This compression deflection relationship is an important consideration when the dimensional tolerance of the gap is large. Gap filler thickness can be deflected 20 to 30 percent, with as little as 10 to 12 psi compressive force. With higher thermal conductivity, other fillers may require 100 to 200 psi compressive force to achieve the same degree of deflection. This force is sufficient to damage both ICs and PCBs, and must be avoided.
Dispensable Gap Fillers
A recent development in the thermal gap filler arena is a single-component, pre-reacted and fully cured product that can be dispensed from syringes or similar containers, as shown in Figure 1. These form-in-place materials are compliant and can accommodate loose tolerances or varying component heights that would require custom-molded shapes in traditional gap fillers (Figure 2). They require less than 1-psi compressive load to undergo greater than 50 percent deflection, which makes them suitable in gap-filling applications where a pad may overstress component leads or damage the PCB.
 Figure 2. Dispensable gap fillers accommodate loose tolerance and varying component heights. |
Dispensable thermal gap fillers are similar to traditional two-component RTV gels, but they are pre-reacted and then placed into containers ready for dispensing. They require no further cure. They do not require refrigerated storage and do not have filler settling or liquid separation. The material can fill gaps in excess of 7 mm. Thermal conductivity of these materials is approximately 1 W/m-K.
Enhanced Performance Walls
Both types of gap fillers improve heat transfer from the IC to the enclosure wall by eliminating air gaps. Further improvement in the cooling process is gained by spreading the heat over a large area of the wall and reducing its thermal resistance. Heat spreading characteristics of a plastic wall can be greatly improved by coating the plastic with a thin metal layer such as sprayed-on conformal metal plating that provides electromagnetic interferenece shielding. Alternatively, a metal foil laminate such as a copper shielding tape can be applied to the enclosure surface.
The cooling process can be further improved by replacing the plastic wall with a metal cover. This enhances heat spreading within the wall and improves the heat transfer through the wall.
Results
The following example shows how these techniques can be combined to cool an IC. A 100-lead PQFP on a PCB is placed in a 25 × 100 × 150-mm Lexan box. The air gap between the IC case and the box cover is 2.5 mm. If the IC is powered to dissipate 2 W, the IC junction temperature will reach an unhealthy 120°C. Replacing the air gap with a 2.5-mm gap filler pad reduces this temperature to 105°C.
Placing a 38 × 38-mm 1-oz. copper foil between the gap filler and Lexan cover further reduces the junction temperature to 91°C. Replacing the Lexan cover with a 1-mm-thick aluminum cover reduces the junction temperature to 71°C. This represents a 50°C reduction in the junction temperature, because the thermal path to the ambient was improved through the use of gap fillers and enhanced surfaces.
MIKSA DESORGO, senior scientist, may be contacted at Parker Hannifin's Chomerics Division, 77 Dragon Court, Woburn, MA 01888; (781) 939-4643; e-mail: [email protected].