#3 | 2019
SYN] Magazine

A powerful Compound for higher Performance

Temperature management plays a key role for the high-voltage batteries of electric vehicles. Battery cells can only deliver maximum performance within a certain temperature range and must not overheat. In order to effectively transfer the heat caused by cell operation to the surroundings, a thermal compound is applied to the battery tray – this is a critical step in the joining process. Heat transfer compounds provide support for the active thermal management of large battery packs used in electric vehicles. They transfer the heat caused by the charging and discharging of the cells to appropriate cooling structures. This way, the battery can operate in its optimum temperature range and does not overheat. This is important in order to meet the market requirements faced by modern electric vehicles in terms of safety, performance, range and short charging times. During the joining process in battery production, a material containing thermally conductive fillers is applied to the battery tray with high precision, avoiding air inclusions. The cell units are then installed on the liquid material. Atlas Copco tightening systems can take into account the behavior of viscous heat transfer compounds on the joint, press the compounds into place in an even layer and ensure optimum contact between the housing and the battery module. 

 Defining the optimum Application Pattern

In order to ensure the thermal conductivity of the compound, precise application without air inclusions is essential. This is a challenge because the compound is often applied in large quantities at a high flow rate. Depending on the joining process, the material properties and the shape of the parts, there are various application patterns to ensure that the modules are bonded to the compound without any air inclusions. These include parallel lines, meanders or a bone-shaped application pattern. Comprehensive testing is normally required to define the optimum application pattern in each case.
At our Innovation Center in Bretten, we bring battery manufacturers, equipment producers and material suppliers together with our joining experts. “Together, we develop the right process in the test cells and adjust the material, metering equipment and process to the specific requirements of the project,” says Udo Mößner, battery joining expert in the Technical Sales Department of Atlas Copco IAS. In cooperation with a renowned research institute, Atlas Copco is also working on new simulations to determine the best possible application pattern on the basis of material properties and pressing forces. This is a method which may save time and money in the future.

System layout
A typical system layout from Atlas Copco's SCA product line for applying two-component thermal compounds.

 Quality Monitoring in the Cycle

The width, position and continuity of the bead can be continuously monitored by a camera integrated in the metering head. Application errors such as gaps in the adhesive bead are immediately detected and can then be corrected. Modern systems from Atlas Copco offer a bead correction function for automatically remedying gaps in the compound. That keeps the cycle time short and reduces reworking and quality assurance expenses.

 Compensating for Tolerances: as much as necessary, as little as possible

The economical use of heat transfer compounds is not only thermally efficient but also saves costs. However, when dosing the material, it is essential to take into account tolerances in the fit between the battery tray and the cell module. The tolerances on the various parts result in gaps between 0.5 and as much as 3 mm. In the production process, manufacturers often apply too much material to ensure that the gap is adequately filled even if the maximum tolerances are reached. Many manufacturers, plant construction contractors and metering specialists are therefore working intensively to ensure that precisely the quantity of material required is applied. The experts at Atlas Copco are developing a solution for the measurement of the housing and cells and the precise determination of the tolerances on the gap between each component combination using a 3-D scanner. This way, the quantity of material required to precisely fill the gap may be calculated. The volume is then precisely controlled by the metering system and not using the robot speed, as was previously the case. “Volume adjustment using the controller is much more precise. As regards the process, it is a great benefit if you no longer need to work on the robot program. This allows material savings of up to 50 percent compared with conventional solutions,” says Udo Mößner.

Battery tray scan
The gap to be filled can be calculated on the basis of measurements of the battery compartment and the battery modules (left). This allows precise metering of the heat transfer compound. Application is monitored by an integrated vision system – any errors are immediately detected and corrected.

 Injection: first fix the Modules, then fill the Gap

Some manufacturers have decided not to press the battery modules into the heat transfer compound but to inject the compound into the gap. The gap is filled from back to front. This approach could also save material. However, the main advantage is that no force is applied to the sensitive battery cells and the risk of air inclusions or uneven tightening on the soft material is minimized. The disadvantage is that visual inspection of the joint is not possible. “We have already carried out some tests with the injection of heat transfer compound at our Innovation Center. To a very large extent, the feasibility of using this approach depends on the customer’s process and the individual material. A low-viscosity compound must be used. If the gap is too small, it may be necessary to use a higher pressure for injection, which can also cause damage to the cells,” says Mößner.

Module tightening
The module is pressed evenly onto the heat transfer compound and screwed into place using special Atlas Copco nutrunners — the result is a clean contact surface without air inclusions.

 Special Equipment for Protection against Abrasion

All heat transfer compounds have a high filler concentration to ensure heat transfer. These fillers typically consist of aluminum oxide or aluminum hydroxide, abrasive substances that can cause rapid wear on the inner surfaces of plant components. Where especially high flow rates are expected, for example at valve seats, carbide components may be used. In addition, the diameter of the parts should be as large as possible in order to reduce the flow speed. This approach allows wear to be minimized. For reliable, productive handling of heat transfer compounds, robust, specially designed pump and metering components are needed. The SCA product line offers special components with maximum durability.

Learn more about our joining solutions for battery assembly and electromobility