* When assembling a thermoelectric system, it is important to choose an assembly method (installation) that is suitable for the installation environment and product characteristics. The main assembly methods include clamping, epoxy bonding, soldering, mounting pads, and thermal conductivity tape.
1) Key Considerations
○ When assembling a thermoelectric system, it is important to choose an assembly method (installation) that is suitable for the installation environment and product characteristics. The main assembly methods include clamping, epoxy bonding, soldering, mounting pads, and thermal conductivity tape.Assembly of thermoelectric systems is an important technology that determines the performance and reliability of the system, and the basic principles for assembly must be strictly followed.
• While thermoelectric elements have high mechanical strength under compression, their shear strength is relatively low. Therefore, thermoelectric systems (thermoelectric elements) should not be used as important supporting components that receive mechanical loads.
• All interfaces within the thermoelectric system components must have flat, parallel, and clean surfaces to minimize thermal resistance. High-conductivity thermal interface materials (TIMs) should be used to minimize thermal resistance at contact interfaces.
• The leads of a standard thermoelectric element should be attached to the heat-generating surface (hot side) of the element in contact with the heat sink, and the positive (+, red) terminal of the DC power supply should be connected to the right side of the element. This helps distinguish between the heat-absorbing and heat-generating sides of the element and reduces the risk of polarity connection errors.
• When cooling a target object below ambient temperature, the object should be insulated as much as possible to minimize heat loss to the surroundings. To reduce convective losses, the inflow of external air should be blocked, and direct contact with external structures should be minimized to reduce conductive losses.
• When cooling a target object below the dew point, moisture (condensation) may form on the cooling surface of the thermoelectric element and penetrate into the element, causing significant heat loss or product failure. Therefore, the thermoelectric system should be sealed using RTV silicone or other methods between the perimeter of the element or heat sink and the target object.
• The thickness of the thermoelectric element must be strictly controlled to increase the thermal conductivity (reduce thermal resistance) between the heat sink and target object. In particular, when assembling one or more thermoelectric elements, the thickness tolerance should be managed within ±0.03mm. However, when using one element in a system, a tolerance of ±0.3mm is generally acceptable.
○ Clamping is the most common method of assembly, which involves securing the thermoelectric element between a heat sink and a cold object using screws. To reduce thermal resistance at the junction interface, both the heat sink and the cold object are machined to be flat (within 1 mm/m), and the thickness deviation between thermoelectric elements is kept within 0.06 mm.
○ Screws are symmetrically fastened in a pattern around the thermoelectric element to ensure uniform pressure is applied across the front surface of the thermoelectric element. To minimize conductive loss through the screws, stainless steel screws with a size of M3 or smaller are used, and the use of Belleville spring washers or split lock washers helps to maintain uniform pressure even if the system undergoes thermal expansion or contraction.
Fig 1. General washer configuration
○ The assembly sequence involves removing burrs, dust, and other contaminants from the mating surfaces of the thermoelectric system, and applying a thin coating of thermal interface material (TIM), such as thermal conductivity grease, to all mating surfaces of the thermoelectric system. The heating surface of the thermoelectric module is attached first, and the thickness of the grease should be less than 0.02 mm. Any excess grease should be removed from the mating surfaces after assembly is complete.
○ It is important to tighten the screws evenly across the front of the thermoelectric module to prevent performance degradation or damage to the module. Using a torque screwdriver, tighten the screws gradually, increasing the torque from the center to the outside.
○ The grease applied to the mating surfaces may partially escape during clamping, causing the screws to become loose. Therefore, after at least 1 hour, the screws should be tightened again to ensure that the proper torque is maintained.
Fig 2. Clamping assembly
○ The torque value for clamping the thermoelectric element is calculated as follows, and the appropriate torque value is found by adjusting the torque value after confirming the actual fastening state.
☞ T = ((Sax A)/N) x K x d
▪ T = torque on each bolt
▪ Sa = cycling 25-50 psi, static 50-75 psi
▪ A = total surface area of module
▪ N = number of bolts used in assembly
▪ K = torque coefficient (use K=0.2 for steel, K=0.15 for nylon)
▪ d = nominal bolt diameter
3) Epoxy Bonding
○ Epoxy bonding is a method that applies high thermal conductive epoxy adhesive to directly attach a thermoelectric device to the target surface, and it is applied to limited specific application fields.
○ For commercial devices, since the coefficients of thermal expansion of ceramic substrates, heat sink plates, and cooling targets are different, it is not recommended for large devices, especially in vacuum systems.
○ The surface on which the thermoelectric device is installed must have a flat surface through mechanical processing, but the importance of surface flatness is relatively low. And, as with other bonding methods, the bonding surface must be kept physically clean.
○ The bonding method involves coating epoxy thinly on the hot side of the thermoelectric device and attaching it to the heat sink plate first. Excess epoxy is then moved back and forth under load on the thermoelectric device to flow out of the bonding surface and be removed.
○ During the curing time of the epoxy, it should be fixed while under load, and in the case of heat-cured type, it should be cured at a temperature below the maximum operating temperature of the thermoelectric device. (For general modules, it is typically cured at 200℃ or below, following the specifications of the thermoelectric device manufacturer. Note that TEGway flexible thermoelectric devices must be worked at a curing temperature below 100℃ (below the decomposition temperature of the internal filling foam).
Fig 3. Epoxy bonding Fig 4. Soldering joint
4) Soldering Joint
○ Soldering joint is a method of metallizing the outer surface of a thermoelectric element and joining it to a target object (or heat sink) by soldering. Careful control of the soldering temperature is required to prevent overheating and damage to the thermoelectric element.
○ Soldering joint should be applied only to the parts where excessive stress is not concentrated on the thermoelectric element, and if it is not used as a structural component of the product, both sides of the thermoelectric element can be soldered.
○ The maximum operating temperature of the thermoelectric element must never be exceeded during the soldering process, and it is desirable to use it for thermoelectric elements of 15X15㎜ or smaller.
○ The joint surface to be soldered should have a flat surface through mechanical processing, and it is necessary to remove foreign substances, as well as degrease and remove oxide films. The joint surface of the heat sink or other object may require tin (Sn) treatment and application of flux, and it should be fixed with appropriate weight during the soldering process. After completing the soldering work, the remaining flux should be removed.
5) Mounting Pads, etc.
○ Silicon-based mounting pads are sometimes used as a type of Thermal Interface Material (TIM) instead of thermal conductive grease.
○ Mounting pads used for assembling semiconductor components may have high thermal resistance at the interface, but their use is expanding due to their high productivity and the ability to reduce cleaning time after assembly.