New-Tech Europe Magazine | May 2018

the system can respond to short-term increases in the cold plate temperature without encountering thermal runaway. Calculating Current and Thermal Absorption If the desired temperature difference and operating voltage of the power supply are known, the thermal dissipation and operating current can be calculated from the module using function diagrams as presented in the datasheet. As an example, the function diagrams shown in figure 3 can be used to find the heat pumped and supplied current, for hot plate temperature (Th) 50°C, cold plate temperature 10°C, and supplied voltage 12 V. To determine the operating current and thermal absorption: 1. Find ΔT: ΔT = Th – Tc – 50°C – 10°C = 40°C 2. Use the function diagram for Th = 50°C to find the current to maintain ΔT = 40°C, at the supplied voltage: From the diagram, I = 3.77A 3. Find the heat pumped from the function diagram, at I = 3.77A and ΔT = 40°C: From the diagram, Qc = 20.75W Thermal Fatigue in Peltier Modules Thermoelectric coolers can be susceptible to thermal fatigue. Conventionally manufactured units contain ordinary solder bonds between the electrical interconnect (copper) and the P/N semiconductor elements, as well as solder or sinter bonds between the interconnect and ceramic substrate (figure 4). While these bonding techniques normally create strong mechanical, thermal and electrical bonds, they are inflexible, and can degrade and eventually fail when subjected to the repeated heating and cooling cycles that are typical of normal Peltier module operation.

Figure 4. Solder and sinter bonds of a conventional Peltier module

CUI conceived the arcTEC™ structure for Peltier modules to combat the effects of thermal fatigue. The arcTEC structure replaces the conventional solder bond between the copper electrical interconnect and the ceramic substrate on the cold side of the module with a thermally conductive resin. This resin provides an elastic bond within the module that allows for the expansion and contraction that occurs during repeated thermal cycling. The elasticity of this resin reduces stresses within the module while achieving a better thermal connection and a superior mechanical bond, and shows no marked drop-off in performance over time. In addition, a special SbSn (antimony- tin) solder replaces the BiSn (bismuth- tin) solder typically used between the P/N semiconductor elements and the

copper interconnect (figure 5). The SbSn solder has a higher melting point of 235°C, compared to 138°C for BiSn, and so offers superior thermal-fatigue performance and better shear strength. Improving Reliability and Thermal Performance To deliver an additional boost to reliability, the P/N elements of arcTEC structure modules are made from a premium silicon and are up to 2.7 times larger than those employed by other modules. This ensures a more uniform cooling performance, avoiding the uneven temperatures that contribute to the risk of a shorter working life. Figure 6 illustrates the effect on temperature distribution by comparing infrared images of a conventional Peltier module (top) and an arcTEC structure module

Figure 5. arcTEC structure enhancements boost reliability and thermal performance

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