® Application Note ® Heat Sinks for Data Converters ADC-AN-08 by Joe Coupal Design engineers constantly look for ways to reduce the effects of high operating temperatures in hybrid data converters. Heat sinks provide a proven solution. Electrical performance of high precision electronic components, such as hybrid type Data Converters, is very temperature sensitive. These devices generally operate much more effectively at room temperature (+25°C) than at higher temperatures. For the most part, the lower the temperature, the better the electrical performance, the reliability, and the mean time before failures (MTBF). Users of hybrid products frequently ask questions regarding thermal dissipation. They usually want to know if it is normal for the hybrid to feel hot when touched. Power densities of some hybrid components are quite high and it is normal for a hybrid to feel hot. What users are actually referring to is a parameter known as the case temperature. The case temperature is largely a function of the internal circuitry and circuit connections. The power dissipated in these circuits causes the temperature to rise; this temperature is generally specified as the junction temperature. Following is a brief overview of the differences between the case and junction temperatures. With no power applied, the junction (chip) temperature, Tj, will be the same as the ambient or room temperature, Ta. The dissipation of power in the device raises the junction temperature. The heat produced flows outward to the case. This temperature rise must be kept within acceptable limits or the data converter will suffer physical damage, resulting in decreased electrical performance or electrical failure. The thermal problem is easily understood using a simple thermal analog of Ohm’s law, in which current is replaced by power (Pj), voltage by temperature, and electrical resistance by thermal resistance, T. Formula Measurement Tj–c Thermal resistance between junction and case, °C/W Tc–a Thermal resistance between case and ambient, °C/W Tj–a Thermal resistance between junction and ambient, °C/W Tj–c=Tj–c×Pj Rise in junction temperature above case temperature Tc–a=Tc–a×Pj Rise in case temperature above ambient temperature Tj=Pj×(Tc–a+Tj–c)+Ta Junction temperature Tc=Pj×Tc–a+Ta Case temperature Note: Pj = power dissipation at the junction Table 1. Calculating Temperature Rise Junction-to-Case Thermal Resistance The thermal resistance between a semiconductor devicejunction and the hybrid package (case) is a function of: 1. the semiconductor device size and material 2. the die and substrate mounting material and its thickness 3. the substrate material and size 4. the hybrid package material and size Therefore, the junction-to-case thermal resistance iscontrolled by the hybrid manufacturer and cannot be modifiedexternally by the user. Users looking to lower the temperatureof the converter need to reduce the case-to-ambient thermalresistance. Use of heat sinks, air flow, thermionic cooling, andcombinations thereof are effective design considerations. Heat Sink Physical Description Ohm’s Law v I×R Thermal Version T P×Q Thermal resistance is a constant; the rise in temperature is proportional to the power dissipated at the junction. Table 1 shows the thermal resistance constants of interest and the equations to calculate temperature rise. DATEL • DATEL has developed aluminum heat sinks for 24-pin DDIPand 32- and 40-pin TDIP packages. The DDIP has 0.600"spacing between the rows of pins and the TDIP has 0.900"spacing. The heat sink itself consists of a base and a cover, asFigure 1 shows. The DIP leads slip through holes in the base while the coverfits over the top surface of the component. To ensure themaximum heat flow from the component to the heat sink, acompressible, thermally-conductive silicone preform is appliedto the top and bottom of the component. The two pieces arethen fastened together. This process allows the data converter’s entire top andbottom surfaces to be in thermal contact with the heat sink.The base of the heat sink fastens to the printed circuit boardand does not interfere with mounting dual in-line packages.For optimal heat transfer, a large copper plane should belocated beneath the heat sink. 11 Cabot Boulevard, Mansfield, MA 02048-1151 USA • Tel: (508) 339-3000 • www.datel.com • e-mail: [email protected] ADC_AN_08_AppNote Page 1 of 4 ® Application Note ® Cover Base PC Board Assembled Unit (DIP not visible) Figure 1. Datel’s heat sinks for converters. The recommended lead length for hybrid components using these heat sinks is 0.200" or greater. All the critical dimensions of the DATEL heat sinks appear at the end of this application note. The model numbers are as follows: DATEL • 24-pin DDIP heat sink 32-pin TDIP heat sink 40-pin TDIP heat sink 11 Cabot Boulevard, Mansfield, MA 02048-1151 USA • Tel: (508) 339-3000 Model #: HS-24 Model #: HS-32 Model #: HS-40 • www.datel.com • e-mail: [email protected] ADC_AN_08_AppNote Page 2 of 4 ® Application Note ® Figure 2. MTBF Increase Using Heat Sinks Case Temp. (Tc) Junction Temp. (Tj) Without heat sinks 62.7°C 67.6°C With heat sinks 39.8°C 44.7°C Benefits of Using Heat Sinks Increased MTBF Using a 24-pin heat sink on a DATEL ADS-117 samplingA/D converter reduces the case temperature by 37% andincreases the MTBF by 250%. Refer to Figure 2. on thispage. Thermal Resistance Reduced Heat sinks developed by DATEL are effective in reducingcase-to-ambient thermal resistance, as the following analysisdemonstrates. But first, an example to determine theexpected temperature rise of a converter above roomtemperature (Ta = +25°C) during operation. Device tested: ADS-117 Sampling A/D Resolution: 12-bits Conversion rate: 2.0 MHz Power dissipation: 1.64 Watts (measured) Thermal resistance: (junction temperatures determinedusing infrared measurement techniques) Tj–c Tc–a Tj–a 3°C/Watt 23°C/Watt 26°C/Watt Using the equations in Table 1, the junction and casetemperatures are found to be: Tc-a = 37.7 °C Tc-a Tc = 62.7 °C Tj-c = 4.9 °C Using DATEL’s ADS-117 as an example, let’s compare thecase and junction temperatures both with and without heatsinks. Using heat sinks reduces the thermal resistances bygreater than 50 percent. For the ADS-117 example, thistranslates into a reduction of the case and junction temperaturesby 37 percent and 34 percent, respectively. Summary Using DATEL’s heat sinks drastically reduces the case-to-ambientthermal resistance. This in turn lowers the junction, orchip, temperature further ensuring the optimum electricalperformance of the device. Based on the data presented, Tc-ashould be less than 10 ºC/W for any of the three package types when using heat sinks. Another advantage directly related to the reduction ofjunction temperatures is the increase in MTBF (by approximately250%). This leads to higher customer satisfaction andreduced long term costs. 24-PIN DDIP PACKAGE 32-PIN TDIP PACKAGE 40-PIN TDIP PACKAGE 23°C/Watt 18°C/Watt 17°C/Watt 9°/Watt 7°C/Watt 6°C/Watt Without heat sink With heat sink DATEL Tj = 67.6 °C Typical case-to-ambient thermal resistance measurementsfor hybrids with and without the heat sinks appear below.These measurements were made in free air using no additionalair flow, which would have further reduced the case-to-ambient thermal resistances. • 11 Cabot Boulevard, Mansfield, MA 02048-1151 USA • Tel: (508) 339-3000 • www.datel.com • e-mail: [email protected] ADC_AN_08_AppNote Page 3 of 4 ® Application Note ® Mechanical Dimensions Inches (mm) 2.50 (63.50) 0.23 (5.84) 0.11 (2.79) 1.23 (31.24) 0.63 (16.00) 0.30 (7.62) Heat Sink for 24-Pin TDIP Packages Cover, edge view Cover, top view 2.50 (63.50) 0.60 (15.24) 2.25 (57.15) 0.075 Dia. Typ. Tapped for #4-40 UNC Thread 4-Places 1.56 (39.62) 0.60 (15.24) 0.30 (7.62) 0.36 (9.14) 11 spaces at 0.100=1.100 Base, edge view Base, top view 2.50 (63.50) 0.23 (5.84) 0.11 (2.79) 1.23 (31.24) 0.63 (16.00) 0.30 (7.62) Heat Sink for 32-Pin TDIP Packages Cover, edge view Cover, top view 2.50 (63.50) 0.60 (15.24) 2.25 (57.15) 0.075 Dia. Typ. Tapped for #4-40 UNC Thread 4-Places 1.56 (39.62) 0.45 0.90 (22.86) (11.43) 0.33 (3.38) 15 spaces at 0.100=1.500 Base, edge view Base, top view 2.90 (73.66) 0.23 (5.84) 0.11 (2.79) 1.23 (31.24) 0.63 (16.00) 0.30 (7.62) Cover, edge view Heat Sink for 40-Pin TDIP Packages 0.60 (15.24) Cover, top view 2.90 (73.66) 2.65 (67.31) 0.075 Dia. Typ. 1.56 (39.62) Tapped for #4-40 UNC Thread 4-Places 0.45 (11.43) 0.90 (22.86) 0.33 (3.38) 19 spaces at 0.100=1.900 Base, edge view DATEL 11 Cabot Boulevard, Mansfield, MA 02048-1151 USA Base, top view . makes no representation that the use of its products in the circuits described herein, or the use of other technical information contained herein, will not infringe upon existing or future patent rights. The descriptions contained herein do not imply the granting of licenses to make, use, or sell equipment constructed in accordance therewith. Specifications are subject to change without notice. ITAR and ISO 9001/14001 REGISTERED www.datel.com • e-mail: [email protected] ADC_AN_08_AppNote Page 4 of 4