McDonnell Douglas Electronic Systems Company. P.O. Box 516. St. Louis, Missouri 63166. ABSTRACT. The demand for increasing optical power levels from ...
Stackable wafer thin coolers for high power laser diode arrays
R. E. Hendron, C. C. Becker, J. L. Levy, J. E. Jackson McDonnell Douglas Electronic Systems Company P.O. Box 516 St. Louis, Missouri 63166
ABSTRACT The demand for increasing optical power levels from GaAs laser diode bars has led to the development of more advanced cooling techniques. The waste heat from individual diode bars has grown so large that they must be mounted directly to coolers with very high heat removal capabilities. In addition, the desire for a compact two dimensional array of these bars requires that the coolers be extremely thin and only slightly wider than the bars themselves. Three different versions of this type of cooler have been developed and one will be assembled into a stack of ten. Water is used as the coolant for each version. The stack is fed from above through one main inlet header that is designed to distribute equal flow rates in parallel to the ten coolers. Sealing techniques have also been developed that prevent leaks from occurring in the header region while allowing the stack to be easily disassembled and reassembled. These stacks have been designed to remove 500 Watts of heat from a 2.25 cm x 1.6 cm x 4.7 cm envelope while maintaining diode bar junction temperatures below 40°C with variations less than 10°C across the array. Since the coolers are designed to operate in the laminar flow regime, the flow rates and pressure drops across the stacks will be very small (less than 50 psi for 100-200 kg/hr). Detailed thermal performance testing will be done on the individual coolers to determine their heat transfer capability and temperature uniformity for heat fluxes from 10 to 250 W/cm2. A stack of ten cooler mock-ups was assembled and tested to validate the sealing techniques. Previous results have indicated that these stackable wafer thin coolers will be capable of removing high heat loads effectively from a very small volume while maintaining good junction temperature uniformity.
1. INTRODUCTION Stackable wafer thin coolers are very important for removing heat fluxes over 100 W/cm2 from high power CW laser diode arrays. These 1 mm thick coolers must be stacked in a compact arrangement in order to minimize the spacing of the diode bars, which are indium soldered directly to the coolers. A typical two-dimensional array is shown in Figure 1. Ten diode bar/cooler assemblies are stacked up with very small integral seals between them to prevent leaks. The stack is held together by a special clamping/manifold fixture. This paper will discuss the design and fabrication of stackable wafer thin coolers that meet the thermal requirements of the PILOT Mod Fab contract. It will also describe the equipment and procedures required to do detailed performance testing on these coolers.
An individual stackable cooler is shown in Figure 2. This cooler must meet several important requirements. First it must have a very high thermal conductance in order to keep the diode bar temperature near 40°C, which will improve the lifetime and efficiency of the bar. The coolers must also have very
330 / SPIE Vol. 1219 Laser-Diode
Technology andApplications 11(1990)
CLAMPING SPACER CHIC
ALIGNMENT PLATE
CHIC COOLER
CLAMPING
COOLANT
CLAMPING! FIXTURE
SEAL
COOLANT INLET COOLANT OUTLET
FIGURE 1. TWO DIMENSIONAL LASER DIODE ARRAY WITH STACKABLE WAFER THIN COOLERS good surface temperature uniformity to minimize wavelength variations across the array. The amount of gain in the waveguide is dependent on the difference between the wavelength of the injected signal and the peak wavelength of the gain curve. Another requirement of the cooler is that it operate in the laminar flow regime to minimize pressure drops. It must also be very strong and stiff, so that it will not break or deform during stacking, and will not move with time once optics are aligned to the stack.
Three different coolers have been developed to meet these requirements. The first is a Compact High Intensity Cooler (CHIC) produced by Sundstrand. The other two are double pass microchannel coolers, which were designed by McDonnell Douglas. One of the microchannel coolers is made of copper, and the other is made of beryllium. These stackable coolers will be tested in three separate phases. In the first phase, detailed thermal testing will be performed on individual coolers using a heat flux amplifier to produce fluxes up to 250 W/cm2. This phase will provide information on each cooler's overall thermal conductance, surface temperature variation, and pressure drop. The second phase will consist of testing a stack of ten coolers using three different sealing techniques. This phase will determine the best method of sealing the coolant manifold to assure a leak-free stack and will also examine the uniformity of the flow distribution in the stack. In the third phase, optical techniques will be used to measure how much the coolers deflect, expand, and jitter as the coolant temperature and flow rate to the stack are varied.
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2. COOLER DESIGN AND FABRICATION
The CHIC was selected as the baseline for this program because it has proven performance and Sundstrand has extensive experience in fabricating high heat flux coolers(l). The CHIC obtains high heat transfer coefficients using a fluid jet impingement technique, as shown in Figure 3. The cooler is constructed of chemically etched copper platelets which are stacked up and diffusion bonded in a vacuum furnace. Two versions of a CHIC have already been fabricated with a 1 mm thickness.(2) The first version had a very large pressure drop and poor performance, but the secondS design showed excellent thermal performance and temperature uniformity.
LENSHOLDERS
COOLANT INLET/OUTLET
FIGURE 2. DIODE BAR IS MOUNTED DIRECTLY TO STACKABLE COOLER
FIGURE 3. COMPACT HIGH INTENSITY COOLER (CHIC) USES FLUID JET IMPINGEMENT COOLING TECHNIQUE
The other type of cooler selected for this study is the double pass microchannel cooler which is shown in Figure 4. The key feature of this cooler is a lower regenerative flow path that absorbs much of the heat input, which results in a more uniform surface temperature. Recent thermal and optical testing has indicated that this cooler design may have the best overall performance of any wafer thin cooler that has been produced (2,3). However, fabrication problems resulted in very large pressure drops in early versions of this cooler. Since that time, we have modified the double pass cooler design and evaluated alternate fabrication techniques. A thermal model was developed to characterize the thermal and hydraulic behavior of double pass microchannel coolers made of several different materials using various channel geometries. We used this model to optimize the design of the cooler within the packaging constraints. Several important differences exist between this new design and the original design. The original design had microchannels in the upper pass which were coupled with much wider channels in the lower pass. This was done to minimize pressure losses. However, the use of microchannels in both passes results in a significant decrease in surface temperature as well as greatly improved surface temperature uniformity, as shown in Figure 5. These improvements more than compensate for the added pressure drop. Optimization of the microchannel dimensions was done by analyzing the change in heat transfer ability as channel width and height are varied. As shown in Figure 6, the surface temperature of the cooler can be reduced substantially by
332 / SPIE Vol. 1219 Laser-Diode Technology andApplications 11(1990)
OLD DESIGN
HEAT INPUT
I .8 mm
1
k:
>1
10.0mm NEW DESIGN
OUTLET
HEAT INPUT
FLOW PASSAGE
uuuuu OURET HEADER
INLET
FLOW PASSAGE
FIGURE 4. REVISED DOUBLE PASS MICROCHANNEL COOLER IS DESIGNED TO PERFORM BETTER THAN ORIGINAL VERSION.
MICROCHANNELS ON TOP ONLY
42
0 w
MICROCHANNELS ON BOTtOM ONLY
35—..-.-.--..------
5 2.35
SURFACE
w
SURFACE
28 -
i4IIII
w w
I0
50
100
250 300
200
150
0
350
POSITION (1 o in)
50
100 150 200
300
250
350
POSITION (10 in)
MICROCHANNELS ON BOTH SIDES
0 w
35
0
0 = 100
SURFACE
28-
W/c
n=5kgThr
50
100
150
200 250
300
350
POSITION (1 o in)
FIGURE 5. ThERMAL ANALYSIS INDICATES THAT MICROCHANNELS IN BOTH FLOW PASSES GREATLY IMPROVES ThERMAL CONDUCTANCE AND SURFACE TEMPERATURE UNIFORMITY.
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w
100
w a-
80
0 — 1002 M - 15 Kg/hr INLET 15°C DEPTH
w 60
CI)
w
0
40
/7N.DEPTH
w
>
20 10
0
20
30
40
50
MICROCHANNEL WIDTH (1 0 IN.)
FIGURE 6. PEEP, NARROW CHANNELS PRODUCE THE BEST THERMAL PERFORMANCE
minimizing
the width of the channels, and maximizing the height. However, limitations of current fabrication technology restrict the narrowness of the channels, and packaging constraints limit their height. The most significant change to the design is the reduction in overall thickness from 1.8 mm to 1.0 mm. This allows a much higher packing density of diode bars in the array. The upper, lower, and middle walls are as thin as possible while maintaining enough mechanical strength to hold their shape under high internal coolant pressure. Another critical issue in the design of microchannel coolers is the selection of a material that has good thermal and mechanical properties, and can be easily machined or chemically etched. There are several materials which will work fairly well, but none of these have an ideal combination of all required properties. Figure 7 presents a summary of the most important properties of the materials which have been considered for microchannel coolers.
THERMAL CONDUCTIVITY
- (W/mK)
YOUNGS MODULUS (PSI)
TENSILE STRENGTh (PSI)
32-35,000
COEFFICIENT OF THERMAL EXPANSION
MACHINABILITY
(1/SC)
RAW MATERIAL COST
GaAs=6.9x106
ELECTRICAL RESISTIVITY p.-
1 7.6 x 10 -6
POOR
LOW
NO
LOW
17 x 10 6
17.6 x i06
POOR
LOW
NO
LOW
250
50 x 10 6
9.0 x 10 6
GOOD
HIGH
YES
HIGH
75% W Cu (ELKONITE)
190
34 x 10 6
g.i x io 6
pQ
HIGH
NO
LOW
BERYLLIUM
159
41 x 10 6
1 1 .4 x 10
GOOD
HIGH
YES
LOW
SILICON
140
16 x 10 6
GOOD
LOW
NO
SEMICONDUCTOR
OFHC COPPER
391
1 7 x 10 6
ZIRCONIUM COPPER
367
BERYLLIUM OXIDE
60-90,000
2.6 x 10 -6
FIGURE 7. MATERIAL SELECTION REQUIRES TRADEOFFS OF SEVERAL IMPORTANT PROPERTIES
334 I SPIE Vol. 1219 Laser-Diode Technology andApplications II (1990)
Oxygen-Free High Conductivity (OFHC) copper offers the advantage of high thermal conductivity, good ductility, and low cost. However, it has poor strength and stiffness and is difficult to machine to tight tolerances. When copper is alloyed with zirconium, the tensile strength can be doubled while preserving its other desirable properties. Tungsten-copper alloys have the advantage of higher strength and stiffness along with a very good match of thermal expansion coefficients with gallium arsenide. The disadvantages of tungsten-copper are lower thermal conductivity and reduced machinability. Beryllium has both good strength and machinability, but is easily corroded by water and requires a protective coating (alodyning or anodizing). Beryllium oxide is a commonly used ceramic heat sink material which is quite machinable and stiff, but is very brittle. Single crystal silicon has the unique ability to be micro-machined by anisotropic etching (3). This allows one to make extremely narrow microchannels. The major weaknesses of silicon are relatively poor thermal conductance and brittleness. The thermal properties of these materials are all very good, and there would be little difference in the thermal performance of a microchannel cooler made from any of them. The critical factors in selecting a material are, therefore, mechanical properties and machinability. We decided to have prototypes made of both copper and beryllium fabricated and tested. The detailed parts of the cooler will be machined using an electron discharge machining technique, and will be diffusion brazed to make the final assembly. The fabrication will be done by Electrofusion Corporation. A preliminary assessment of using coolants other than water with microchannel coolers was also performed. Water has the best overall thermal properties, but its relatively high freezing point of 0°C prevents it from being used in aircraft or space applications. If ethylene glycol is mixed with water, its freezing point drops significantly. However, its viscosity becomes very large at colder temperatures, resulting in extremely large pressure drops as it is pumped through the tiny microchannels. Ammonia has good thermal properties, but it must be used in a pressurized system so that it stays in the liquid state as it flows through the stack. The toxicity of ammonia also makes it extremely dangerous. Freons such as R-ll and R-22 are nontoxic and have very low freezing points. However, they have relatively poor thermal properties and must be supplied to the stack at temperatures between -60°C and -40°C to perform as well as water supplied at 15°C. I
COOLANT MANIFOLD AND SEALING
Two of the biggest challenges in this program were developing a sealing technique and a manifold for the cooler stack that meets the thermal requirements. In order to obtain uniform diode bar temperatures across the array, the coolant manifold must distribute equal flow rates to the ten coolers. It is also extremely important that the manifold be leak-free, since water would damage both the diode bars and the electronics. The coolers will receive equal flow rates only if the pressure drop across each parallel branch is the same. To achieve this, the coolers must have roughly the same pressure drop and the inlet and outlet headers must have pressure drops negligible compared to the coolers. However, the manifold must also be as small as possible to minimize the overall width of the stack The inlet and outlet ports are on the same side of the cooler in the manifold design we selected.
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This is done to allow more space for the seal and more width for the header. The overall cooler width is critical because it directly affects the horizontal spacing of the unit cells, which in turn affects the fill factor of the array. We expect that this manifold design will succeed in supplying equal flow rates to the stack. The seals that are used in the stack are another important consideration. They must be very small to minimize the unit cell spacing in both the horizontal and vertical directions. The seals must also withstand at least 50 psi of water pressure without allowing leaks. It is also desirable for the stack to be easily assembled and disassembled if a diode bar must be replaced. In addition, the seal should be electrically insulating so that current can be supplied to each diode bar individually.
Three different types of seals were fabricated, as shown in Figure 8. The most promising technique is a Parker integral seal. It consists of an anodized aluminum retainer, with ethylene propylene seals vulcanized to the inner walls. The retainer prevents the seal from being overcompressed by absorbing any force that is greater than the amount required to compress the rubber down to the thickness of the metal. Two alternate techniques are also being considered. One is an 0-ring assembly based on the same concept as the integral seal, and consists of two small 0-rings constrained within an aluminum retainer. 0-rings made of both silicone and ethylene propylene will be tested. Although the 0-ring and retainer are separate units, the seal compression is at its optimum value when metal to metal contact is obtained across the frame. The other sealing option is a simple rubber gasket. A gasket has the advantage of more sealing area to prevent leaks, but it can also be overcompressed, resulting in extrusion of the rubber or at least inconsistent spacing of the diode bars.
4. TEST PROCEDURE
4.1 Thermal Testing In the thermal testing phase, an individual cooler will be tested at heat fluxes from 25 W/cm2 to 250 W/cm2 using flow rates from 5 to 20 kg/hr. This phase will be very similar to the testing done on five types of wafer thin coolers in late 1987 by McDonnell Douglas (2). The purpose of this phase of the testing is to determine the overall performance of each cooler, including its thermal conductance, surface temperature uniformity, and pressure drop.
The test system which will be used is illustrated by the schematic in Figure 9. A chiller removes the waste heat from the primary flow circuit through a tube shell heat exchanger. A gear pump forces distilled water through the primary circuit containing the cooler. Upstream of the cooler is a 10-micron stainless steel filter and a mass flow meter. The coolant is supplied from a distilled water reservoir open to atmospheric pressure. The flow rate is controlled by adjusting the input voltage to the pump. Visual pressure gauges will be placed upstream of both the filter and the cooler, and a differential pressure transducer will monitor the pressure drop across the cooler. In-line thermocouples will be placed at the inlet and outlet of the cooler to verify the amount of heat being input to the coolant. The water lines will be made of
336 / SPIE Vol. 1219 Laser-Diode
Technology andAppilcations 11(1990)
WATER
VULCANIZED EThYLENE PROPYLENE SEAL
INTEGRAL SEAL
ALUMINUM FRAME
0-RING ASSEMBL V
ALUMINUM RETAINER -
SILICONE GASKET
FIGURE 8. THREE DIFFERENT SEALING TECHNIQUES WERE DESIGNED AND TESTED
FiGURE 9. COOLING SYSTEM FOR WAFER ThIN COOLER TESTING.
teflon tubing, and an anticorrosion/biocide mixture will be added to the water supply. The high heat fluxes will be input to the cooler using a heat flux amplifier similar to the one used in the 1987 testing. The amplifier/cooler assembly is shown in Figure 10. The amplifier reduces the heat conduction area from 25 cm2 where the heaters are, down to a 1 cm x 0.5 cm rectangular area where the heat is input to the cooler. This allows 125 watts of heater power to supply 250 W/cm2 of heat flux. Four ceramic heaters will be placed at the large end of the amplifier, and will be clamped against thermal pads to prevent the heater from burning out. The amplifier will be made out of high conductivity copper so that uniform heat fluxes are sustained through each cross section. A sheet of .002 inch indium foil will be clamped between the amplifier and cooler to provide a good interface conductance. The amplifier and cooler will both be gold coated to further enhance the interface conductance. A 1.5 mm thick teflon insulator is placed between the cooler and the clamp to minimize heat leaks.
FIGURE 10. THERMAL TEST FIXTURE WITH HEAT FLUX AMPLIFiER.
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An important feature of the heat flux amplifier is its ability to extrapolate the temperature distribution at the surface of the cooler without interfering with the heat flow. The thermocouple locations on the heat flux amplifier are shown in Figure 11. There are eight thermocouples in each of three planes on the amplifier. The ninth temperature in each plane will be extrapolated during the data reduction process. Also, the thermocouple readings in planes B, C, and D will be used to extrapolate the temperatures at plane A, which is the interface with the cooler. Because of the excellent interface conductance produced by indium foil, these temperatures will be very close to the actual surface temperatures of the cooler. HEATER
CLAMP
I1TERFACE SURFACE
WAFER ThI1 COOLER
FIGURE 11. COOLER SURFACE TEMPERATURES ARE EXTRAPOLATED FROM THREE PLANES OF THERMOCOUPLES IN HEAT FLUX AMPLIFIER.
All
thermocouple, pressure transducer, flow meter, and heater inputs will be monitored by a data acquisition system. The temperatures will be monitored continuously on the computer screen, and once steady state is reached for a test run, a complete data scan will be stored in the computer. the acquisition system will perform all of the data manipulation, including determination of heat transfer coefficients and surface temperature variations.
4.2 Stack Testing In this phase of the testing, ten CHICs will be stacked up using the three different sealing techniques. The purpose of this phase is to validate the sealing technique as well as to determine if the flow is being distributed equally to the ten coolers. Some preliminary leak tests have already been performed using the seals and ten mock-up versions of the stackable thin coolers. A visible leak test using water was conducted, as well as a bubble leak test using pressurized nitrogen gas. The results are presented in Section 5. The flow distribution in the stack will be determined by inputting a known amount of heat into each CHIC, and measuring a temperature difference between two points near the cooler inlet and outlet. This temperature difference can be used to determine the flow rate through the cooler because characteristic curves of temperature difference versus flow rate will be developed for each cooler prior to stacking. Pressure drop versus flow rate will also be measured for individual coolers. Each cooler in the stack will have two thin Kapton foil heaters mounted to it along with two thermocouples, as shown in Figure 12. One heater will be on top, one will be on the bottom, and they will be insulated from the air and the coolers around them using thin silicone pads. The twenty heaters will be connected in parallel with a known voltage. The flow distribution tests till only be run with the integral seals and CHICs. Leak tests will be performed for all of the cooler and seal combinations.
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4.3 Stability Testing The purpose of this phase is to measure small deflections or vibrations of the front edge of the coolers that could cause optical misalignment. If a diode bar changes its position by as little as 1 jm once the array optics are aligned to the stack, the optical alignment can be significantly degraded. Therefore, a theodolite will be used to detect these small displacements as a function of time, flow rate, coolant temperature, and waste heat input to the coolers.
The theodolite projects a beam of light, which is reflected back to the instrument from a polished surface on the front of a cooler. When the stack is monitored from the top, angular displacements of 15 microradians can be 0.38 pm vertical motion of the measured. This accuracy corresponds to front of the cooler, which is within the 1.0 pm required.
5. TEST RESULTS The only tests that are complete at this time are preliminary evaluations of the three sealing concepts using a stack of ten mock-up coolers. The actual stackable thin coolers are scheduled for testing from January - March 1990. All four of the seals, including both versions of the 0-ring assembly, completely sealed the stack from visible water leaks at 50 psig internal pressure, even at clamping forces below 20 lb. Therefore, we performed bubble leak tests on the seals at various clamping forces to obtain quantitative data for comparison. Nitrogen gas pressurized to 50 psig was applied to the stack, and the clamping force was increased until there was no visible bubble formation when the stack was submerged in water. The results of these tests are summarized in Figure 13. The integral seals eliminated nitrogen leaks at the lowest force, while the gaskets and ethylene propylene 0-ring assemblies worked at higher forces. The 0-rings made of silicone, which is a softer rubber than ethylene propylene, never completely eliminated bubble formation. Another important property of the seals is the force required to achieve metal to metal contact, which is necessary for uniform spacing of diode bars in the stack. It is desireable to minimize this force, since the coolers must eventually support the same load as the seals. Higher forces increase the chances of deforming or breaking the coolers. The silicone 0-rings 1eflected completely at the lowest force, followed by the integral seals, and the ethylene propylene o-rings. The gaskets continue to deflect even at very high clamping forces TWO 25 WATT KAPTON HEATERS (TOP AND BOTTOM)
THERMOCOUPLES
SEAL TYPE INTEGRAL SEAL
FORCE REQUIRED TO FORCE REQUIRED FOR METAL-METAL CONTACT ELIMINATE LEAKS
22 lb
68 b
56 lb —
79 b 45 b
45 lb
—
0-RING ASSEMBLY — ETHYLENE PROPYLENE — SILICONE
GASKET
COOLANT INLET/OUTLET
FIGURE 12. HEATERS ARE MOUNTED TO BOTH SIDES OF THE COOLER DURING FLOW DISTRIBUTiON TESTS
FIGURE 13. ThREE OF ThE SEALING CONCEPTS PREVENT NITROGEN LEAKS WITh SMALL CLAMPING FORCES.
SPIE Vol. 1219 Laser-Diode Technology andApplications 11(1990)!
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6. CONCLUSIONS An advanced cooling technique has been developed to remove very high heat flux levels from two dimensional arrays of GaAs laser diode bars using stackable wafer thin coolers. The thermal performance of these coolers is expected to surpass previous versions, and a unique sealing technique has been designed to allow these coolers to be packaged in a very compact arrangement. The seals and clamping fixture have been tested with mock-up versions of the coolers, and they have worked very well. Detailed thermal, hydraulic, and stability testing will be completed at McDonnell Douglas by March, 1990. 7 .
ACKNOWLEDGMENTS
The authors would like to express their appreciation to J. R. Lapinski for performing the early design work for this technology, R. L. Hood for his valuable mechanical design assistance, and H. W. Stimmell for his work in planning and setting up the tests. This work was supported in part by the Department of the Air Force under contract #F296O1-88-C-O039. We also want to thank Chip Chandler of Sundstrand, Wayne Marcellin and Rob Hardestry of Electrofusion, and Lance Cannon of Parker Seal for their extremely important work in design and fabrication of the coolers and seals.
8. REFERENCES 1. T. S. Bland, R. E. Niggemann, and M. B. Parekh, "A Compact High Intensity Cooler (CHIC)", SAE paper #831127, 13th Intersociety Conference on Environmental Systems, July 11-13, 1983. 2. M. G. Grote, R. E. Hendron, H. W. Kipp, and J. R. Lapinski, "Test Results of Wafer Thin Coolers at Heat Fluxes from 5 to 125 W/cm2", SAE paper #880997, 18th Intersociety Conference on Environmental Systems, July 11-13, 1988. 3. 5. M. Kastigar, R. E. Hendron, J. R. Lapinski, G. R. Hertzler, "Wafer Thin Coolers for Continuous Wave (CW) Aluminum Gallium Arsenide/Gallium Arsenide (AlGaAs/GaAs) Monolithic Linear Diode Laser Arrays", Laser Diode Technology and Applications, Proc. SPIE 1043, pp. 359-367, 1989. 4. D. Mundinger, R. Beach, W. Benett, R. Solarz, V. Sperry, "Laser Diode Cooling for High Average Power Applications", Laser Diode Technology and Applications, Proc. SPIE 1043, pp. 351-358, 1989.
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