Precision dispensing of thermally conductive resins is one of the toughest challenges for all dosing systems. The physical properties of the material, in fact, are hardly compatible with the mechanical sophistication of the machines called to manage it: the dissipation fillers stress and wear out all the components with which to come into contact, from the degassing and feeding systems to the dosing heads. Let's review some of the main technical issues in the management of thermally conductive resins, as well as the most modern production solutions.
The electric automotive
The global boom in electric mobility is revolutionizing the automotive industry and its vast supply chain, also in Italy. Despite the collapse in overall registrations, in our country in the first 9 months of 2020 the market for "full-electric" or "plug-in hybrid" cars exceeded 3% of the total (+ 2% compared to 2019), reaching 30,000 units. That's a 155% growth. What role do dosing systems play in this scenario?
The heart of any electric vehicle is the lithium-ion high-voltage (HV) battery that stores, stores and delivers the energy needed to run the engine. The key design criteria for these batteries are high energy density, low production costs, weight reduction, scalability and mechanical stability. A very important component is the battery housing which protects against harmful environmental influences such as dust or humidity. The heat dissipation properties are critical.
The thermal factor plays a central role on the duration, capacity and above all on the operational safety of the HV battery. According to the Arrhenius equation, reducing the average temperature of a battery by 10 °C means doubling its operating life. Optimal operating temperatures are between about 20 and 40 °C. To achieve this, every electric vehicle is equipped with an active battery management system (BMS) that works by air or by a closed-circuit coolant. Furthermore, new passive materials are applied to dissipate heat by contact thanks to their physical and chemical characteristics.
Advantages of thermally conductive pastes
The pre-formed adhesive pads that are still often seen in small processes have some contraindications. First of all, it is an expensive material and not always consistent in thickness. Furthermore, it must be applied mechanically, by pressure, which can be detrimental to electronic components. It cannot be thinned beyond a certain limit, on pain of the impossibility of handling it. Again, it does not flexibly adapt to uneven substrates. More importantly, it is not suitable for certain crucial production sectors: for example, it is not compatible with the severe 5G or 6G tests of the automotive sector, because it does not guarantee stability over time with respect to the vibration of the vehicle in motion.
In light of all this, more and more often we have switched to liquid dissipation substances or thermally conductive pastes. For example, by dosing appropriate quantities of these resins between the battery modules and their housing, the thermal resistance of the housing can be reduced by up to 40-50%, significantly lowering the overall temperature of the battery pack.
The mechanical and thermal dissipation properties guaranteed by these resins are crucial for the entire automotive sector: in fact, they also find application in on-board networks, in the electronic components management, in LED areas, in sensor technology, in the charging infrastructure and in inverters. This is the type of highly specialized process that has favoured manufacturers of precision dosing systems in entering the field.
The technological challenge
The ability to conduct heat of a thermally conductive resin (TCA) is given by the high concentration of fillers, generally aluminium oxide, silver or boron nitride. Fillers are irregular fragments, spheres or cubes, often featuring very high levels of hardness and sharp-edged profiles. For this reason, as the dissipative properties of the resins increase, their specific weight also increases.
Today the market is moving towards silicone, epoxy and polyurethane resins, with fillers that bring their specific weight to over 3.5 g/ml, well beyond aluminium specific weight. Polyurethanes reach a dissipation coefficient of 1.6 K (W/°K m), and temperature ranges from -50 °C to +160 °C. Epoxy and silicone resins reach a thermal conductivity of over 4.5 K. Then there are special resins with silver or gallium fillers that respectively reach up to 8 K or even 13 K, but they have prohibitive costs for the industrial scale and their use is limited to highly specialized productions.
The presence of abrasive fillers poses enormous challenges for the delivery systems of these resins. Almost every type of pump undergoes severe mechanical wear which leads to a progressive reduction in delivery precision, increasing maintenance costs and significant downtime. To minimize these problems, it is essential to use the right preparation and dispensing systems. Gear pump systems, for example, are not suitable for handling abrasive materials, because mechanical interference would quickly affect the operating life of the pump. The state of the art for dispensing two-component resins for heat dissipation are piston volumetric dosing systems with disposable static mixer. These are the systems in which Atra has specialized, in exclusive collaboration for Italy with the German partner Scheugenpflug.
The advantages of the volumetric piston pump
The market offers many different technologies, but years of research and field work with numerous applications have convinced Atra that the piston is the best performing. In the progressive cavity pump (PCP), for example, the sliding between the rotor and the stator involves the wear of all the rotating parts. The most modern ceramic screws slightly reduce the problem, which however remains for the inner wall of the polymer stator. In the piston pump too, of course, the abrasive pastes involve inevitable mechanical rubbing, but some exclusive technical measures developed by Atra and Scheugenpflug have improved the resistance of each part involved.
First, while the piston is working, its rear part is properly lubricated. Furthermore, the resin inlet valves (another critical element) are made with special membranes that allow to control the passage of the abrasive fluid without direct contact. During the closing action, the elasticity of the membrane reduces the load of the abrasive particles present on the metal seats of the valve. The flexibility of the membrane thus avoids wearing down its seat.
These special technical solutions are also found in the Scheugenpflug material feeding and degassing unit. To avoid abrasion of the material inlet valve, a special seal prevents the material from being crushed between the metal parts. The fillers are forced upon contact with the elastomer that actually absorbs the deformation.
These unique technical solutions are finding increasing recognition in companies of all sizes. An important international integrator in the automotive sector, for example, had previously chosen cavity pumps to dispense highly abrasive material during the processing of power modules. The company produces many thousands of pieces a day, 24/7, on multiple lines. However, there was a rear seal problem, a systematic wear of the crankshaft seal, and after a certain number of cycles the dripping of material was inevitable. Once the overall costs of maintenance, spare parts and line stoppages were assessed, the company turned to Atra to install DOS P volumetric piston systems.
Another considerable advantage of piston heads is that a single dosing head can work simultaneously in parallel on multiple pieces, up to a maximum of twelve. In order to dose several pieces at the same time with a two-component resin, the Atra system can use a single machine, with a single motor, while other manufacturers should field a motor for each pump, with evident greater investments, maintenance and cost of materials.
Being a single system with multiple outlets, Atra’s multiple head guarantees perfect uniformity of dosing from each of the nozzles. On the contrary, any independent systems involve possible delivery differences caused by electrical variations between the different motors, or mechanical or asymmetrical wear between the different heads. These discrepancies can increase exponentially over the life of the system.
The piston pump features important advantages also from a maintenance point of view. In the Atra DOS P TCA, for example, the internal diaphragm wears much slower than the stator of a cavity pump, is smaller, cheaper, and can be replaced in minutes. It is also interesting to note that, although the price of the dosing head increases as its size and number of nozzles increase, a multiple single motor system becomes proportionately more and more competitive as its complexity and number of points increase.
The cost/benefit ratio between pad and 2K dosage
Also from an economic point of view, for projects involving large production volumes, automation and precision, the use of a liquid dissipation interface tends to be more advantageous than a solution based on pads. Despite the initial investment for the system, in fact, the finished work present unparalleled advantages.
Thanks to the very high precision and reliability of the DOS P TCA heads, for example, it is possible to customize and modify the application on the piece, even if the project changes during production. The application as a whole is up to three times faster. The thermal conductivity (with the same material) is higher, it adapts better to the substrate and does not mechanically stress the sensitive components during application. The costs of storage, handling and assembly are reduced. More importantly, it is possible to reach very thin thicknesses (reducing the cost) and at the same time to improve the heat exchange.
The accuracy of mixing dosage, quantity dispensed and position on the piece is also essential in guaranteeing the dual function of a TCA resin: thermal conductivity and adhesive properties between the substrate and the dissipation medium. Even in materials with a high dissipation coefficient K3 or K4, in fact, a too thin layer will make complete contact more difficult, while the application of a too thick layer will have an insulating effect (in addition to increasing costs). Furthermore, a too high concentration of filler would improve the thermal dissipation properties but would lead to the immediate degradation of the adhesive properties. On the contrary, a too low concentration would risk not dissipating heat correctly.
In conclusion, piston volumetric dosing systems with disposable static mixer are the most reliable choice in the field of dispensing two-component resins for heat dissipation. They guarantee very high volumetric precision, reduced maintenance costs, fast and repeatable dispensing, possibility of working in parallel multiple dosing on several pieces at the same time with a single motor. It is important to remember these aspects when choosing a system, because apparently advantageous systems from a price point of view often turn out to be problematic in terms of maintenance and spare parts.