The Optimal Passive Thermal Management ing and Electrically-Isolating Power Semiconductors to Within 33-micron (1.3 mil) of The Heat Sink Jim Fraivillig Fraivillig Technologies 3315 Toro Canyon Road Austin, Texas 78746 Phone: 512-347-1626 Fax: 512-347-1583 e-mail: jim@fraivillig.com Abstract ing power semiconductor devices to aluminum heat sinks via bonded copper patches PowerSites TM is a very effective passive cooling method. The thermal resistance of electrically-isolated PowerSites TM (I-Series) is about 0.06 o C/W/in 2, while the non-isolated version (N-Series) is only 0.03 o C/W/in 2. The fast-bonding thermoplastic polyimide adhesive on the PowerSites TM, as well as the solder used to mount the power devices, facilitate the automation of assembly. (The PowerDial machine mounts power devices on heat sinks.) This cost-effective technology provides better thermal management than conventional interface materials, with automated assembly and no hardware. Key words: Thermal Management, Cooling Technique, and Thermal Resistance. Power semiconductor devices generate significant waste heat. Power levels and packing densities are high, and getting higher. Better thermal management is becoming more important. Aluminum heat sinks are effective for passive thermal management. Interface materials assist thermal transfer and provide electrical isolation between the power device and the heat sink. The performance of interface materials is dependent on the elimination of air, the distance between the device and the heat sink, and the materials thermal conductivity. Filled rubber, grease, and wax are conventional interface materials. These materials are generally used in conjunction with fabric, film, or foil substrates. Filled rubber, grease, and wax are mature technologies. Improvements in performance are expected to be incremental. Attachment hardware and labor are required to mount devices to heat sinks. 1. Background, Part I 2. Background, Part II Many power devices (such as TO-220 and TO-247) are tinplated, which allows soldering. ing is a proven, reliable component mounting technology. Power semiconductors are now soldered to conventional circuit boards, as well as printed-and-etched Insulated Metal Substrate boards (IMS). 1 ing devices directly onto heat sinks would maximize thermal transfer. PowerSites TM are within-a-hair of this goal. Distance between metal surfaces. 164
The Optimal Passive Thermal Management ing and Electrically-Isolating Power Semiconductors to Within 33-micron (1.3 mil) of The Heat Sink Table 1. PowerSitesTM characteristics. PowerSite Electrical Metal-to-Metal Separation I-Series Isolated 33-micron 0.5 of hair (1.3 mil) N-Series Nonisolated 4-micron (0.15 mil) 0.06 of hair 3. PowerSite TM Description Thermal Resistance ( o C / W / in 2 ) 0.3 0.2 0.1 Non-Isolated olated Isolated PowerSites I-Series N-Series Conventional Rubber Pad Thermal Grease e PowerSites TM are solderable copper patches bonded at high temperature and pressure to aluminum heat sinks with thermoplastic polyimide adhesive. If electrical isolation is not required, only copper foil with a minimal thickness of adhesive coating is used (N-Series). If electrical isolation is required, a layer of 25 micron Kapton MT (1-mil) 2,3 is incorporated in the structure (I-Series). The power device is then soldered to the copper patch. [Figure 1] 32 micron I-Series FT TPI Film PI Adhesive Kapton MT PI Adhesive Heat Sink Figure 1. PowerSite TM cross-section. N-Series 4 micron 0 New Figure 3. Thermal transfer comparison. The I-Series thermal resistance is a fraction of the value of conventional rubber-based interface materials, and is comparable to thermal grease. The non-isolated N-series is even lower. Thermal transfer of PowerSites TM is optimized by the following: 1. The elimination of air during the high-temperature / highpressure bonding process. The PowerDial bond process is optimized to squeeze out the air before the adhesive is set. 2. Minimal non-metallic distance between the device and the heat sink. The 4-micron adhesive thickness is the lowest possible to obtain good bonding and conformance. The dielectric film thickness of the I-Series is also minimized at 25 micron (1-mil). [Figure 1] 3. Maximized thermal conductivity of the bonding and insulating plastics. Kapton MT has 3X the thermal conductivity of conventional Kapton H-type film. Old There is no hardware required, in contrast to conventional interface materials [Figure 2]. The mounting process is easily automated by the PowerDial machine, which bonds PowerSites TM and solders devices to heat sinks. The thermal transfer between the power device and the heat sink is excellent, even with the isolated I-Series, which provides over 4000 volts of dielectric separation [Figure 3]. The fast bonding characteristic of the thermoplastic adhesive allows a full-pressure bond cycle in seconds, rather than the hours required for thermoset adhesive systems. After the adhesive is adhered, the PowerSite TM can immediately withstand soldering. This feature facilitates single-unit robotic manufacturing. The tack bonding characteristic of the thermoplastic adhesive allows the PowerSite TM insulation and copper patches to be precisely placed on a hot heat sink without hard tooling. This feature facilitates fast, flexible automated assembly. (Note: The tack bond is followed by the final high-temp / high-pressure bond step.) Polyimides, both the dielectric and adhesive, retain their properties including adhesion over a wide range of extreme environ- Figure 2. mounting comparison. 4. PowerSite TM Components 4.1. Adhesive thermoplastic modified-polyimide with a thickness of only 4-micron (0.15 mil) per bond line. This thinness maximizes thermal transfer. 165
ments. This includes prolonged exposure to high temperature, cryogenic temperature, and thermocycling in between. Figure 4 shows a representative relative change in capacitance between a PowerSite TM I-Series copper patch and the aluminum heat sink with prolonged exposure at 250 o C. (This measure is an excellent non-destructive test of the integrity of a bond line. A significant drop in capacitance would signify a degradation in the bond between the soldered-on device and the heat sink, which is bad news from a reliability standpoint.) The PowerSite TM capacitance has a moderate drop with the first thermal cycle, but then levels off and can actually increase slightly. The polyimide adhesive, although thin, adds dielectric strength to the structure (I-Series). The 2 x 4-micron coatings adds about 1000 volts. Capacitance (PowerSite-to-Heat Sink, I-Series, N orm alized) 1 1i 10 100 Time (hours) Figure 4. Thermal durability (Heat aged @ 250 C). 5. PowerSite TM Assembly Process The process for mounting of PowerSites TM on heat sinks is characterized by four stages, in which temperature, pressure and time are closely controlled [Figure 5]: Temperature ( C) 300 200 100 25 Preheat Tack Bond Full Bond Add Material TPI PowerSite Placement Figure 5. PowerSite TM process. Epoxy Dot Mounting Time (10 second intervals) Reflow 4.2. Dielectric layer (I-Series only) 25 micron Kapton MT film (1-mil). Polyimide durability is unsurpassed in demanding applications. MT has a thermal transfer rate about 3X that of conventional unfilled polyimide film. Uncoated MT has a dielectric strength with an average of 4200 volts and a minimum of 3000 volts. MT film has a very high punch-through strength, even at the PowerSite bond temperature. (Similar ceramic-filled polyimide films are used in high-stress superconducting magnets.) 4.3. able patch conventional copper foil (electronic-grade). Bondable treatment on one side. Antioxidant treatment on the other side, which allows soldering after high temperature lamination with no cleaning. 35 micron copper (1 oz) is sufficient, although other thicknesses can be used. Other solderable metals can be used, but these add significantly to cost. 5.1. Preheat the aluminum heat sink is brought up to over 200 o C. This would generally be achieved by placing the heat sink on a hot surface. 5.2. Tack bonding the insulation and copper patches are precisely placed on the hot heat sink. Only a few seconds with very little pressure is required for placement bonding. This is done with a robotic arm. Some time is allowed after placement for the added components to heat up and soften, before the next step. 5.3. Full bonding a plunger driven by air pressure at 200-600 psi is applied to the PowerSite TM, which squeezes out all air and permanently bonds the patch to the heat sink. The optimum press temperature is 220-260 o C. Full pressure needs to be held for 5-30 seconds, depending on the size of the heat sink. This is the longest process time and the rate-limiting step in the manufacturing sequence. 4.4. conventional eutectic 63/37 tin/lead, but higher temperature solder can be used, if required by the application (need to exceed 183 o C during operation). 5.4. Placement and soldering of devices the heat sink needs to be cooled down to allow the SMT epoxy 166
The Optimal Passive Thermal Management ing and Electrically-Isolating Power Semiconductors to Within 33-micron (1.3 mil) of The Heat Sink and solder to be placed on the PowerSite TM. The power semiconductor device is then added the epoxy holds the device precisely until the solder reflows and cools. PowerSite TM technology is very conducive to automation. 8.1. Pick-and-place copper patches are fed and cut from a slit roll of foil, and precision-placed on the heat sink. [Figure 6] This is the most economical scenario. 6. PowerSite TM Advantages Low thermal resistance a step-change improvement over conventional interface materials. [Figure 3] This technology allows devices to run cooler and/or smaller heat sinks to be used. High dielectric reliability (I-Series) no chance of cut-through during device mounting with solder. No required hardware screws, clips, etc. No modifications to heat sinks to accept mounting hardware. No pressure dependency to maximize thermal transfer. No property degradation during operation, even in extreme environments. Grease or wax can flow out of the interface joint; screws and clips can loosen up. Fast bonding allows economical single-unit production with automation. Low overall cost PowerSite TM technology is cost-competitive with most conventional methods of device mounting with interface materials, especially when associated hardware and labor are accounted for. 7. PowerSite TM Limitations Large heat sinks require additional heat to reach bonding and soldering temperatures. This slows down the process, or increases the heater requirements. Non-flat heat sink backsides require special custom fixturing. Two-sided soldering on heat sinks is technically challenging. The CTE mismatch between copper and aluminum becomes an issue when the thickness of the metals is comparable (not likely in most applications) the bonded composite structure may bow. The surface of the incoming aluminum heat sink must be sufficiently free of contaminates to ensure a good bond. Removal of the power device for rework requires desoldering. Figure 6. PowerSite TM samples. 8.2. Punch-and-place copper patterns are punched from a roll of foil, and transferred to the heat sink with a vacuum plate. This allows complex shapes, but has limitations on minimum line and space dimensions. 8.3. Flip-Flex reversed-bared printed circuits (no insulation on the bottom layer) are placed and bonded on the heat sink. This allows complex shapes and fine lines and spacing [Figure 7]. 8. PowerSite TM Formats (Patent Pending) The key to PowerSite TM thermal performance is the closeness of the soldered copper site to the aluminum heat sink. The copper can be applied to the heat sink in several formats, all of which would use the all-polyimide bond film between the copper and the heat sink and would have similar thermal performance: Figure 7. Flip-Flex The International Journal of Microcircuits and Electronic Packaging, Volume 22, Number TM exploded view. 2, Second Quarter 1999 (ISSN 1063-1674) 167
9. PowerSite TM Automation The PowerDial TM machine [Figure 8 below] automatically mounts PowerSites TM onto heat sinks at a fast production rate, as many as 6 units per minute. Different heat sink constructions may require different PowerDial configurations. The operating conditions for PowerSite TM bonding would be optimized for each construction; after that is done, the operator s only challenge would be keeping the machine fed with heat sinks, power devices and PowerSite TM materials. Figure 8. PowerDial TM machine. 10. PowerSite TM Applications PowerSite TM technology is valuable wherever thermal management is critical, such as power converters, automotive controllers, and computers. References 1. Thermal Clad Design Guide, Bergquist Company, Rev. 11, April 1996. 2. Kapton polyimid film, Summary of Properties, Dupont Company, H-38492-1, August 1995. 3. Kapton MT polymide film product bulletin, H-38497-1, 1993. 168