Automated dispensing of polyurethane (PU) is often superior to manual silicone dispensing in quality and costs. Most Asian manufacturers of electronic components for solar inverters use manual dispensing techniques for potting or filling (e.g., regulators, transformers or PCBs with silicone). Shifting to polyurethane as the dispensing material, however, results in improved properties and lower production costs for the manufacturer.
When combined with a precisely defined mix ratio and exact metering of the dispensing quantity, these advantages develop their full potential. Therefore, using an automatic dispensing machine significantly increases production reliability, quality and productivity. Return on investment is almost immediate.
The solar inverter is the core element of every photovoltaic (PV) system. The inverter converts the direct current generated by the solar modules into alternating current, which is needed for feeding the generated power to the mains grid. Power inverters also fulfill important monitoring and control functions to ensure that the PV system operates at optimum performance.
The intelligent electronic control registers all operational data, safeguards the system from overloads and stops grid feed-in if necessary. A defective power inverter is the most common cause of failure in faulty PV systems. Statistics show that the power inverter must be changed at least once during the presumed 20-year lifetime of a solar system. Without regular checks, the average time between failures is 5-10 years. Considering how critical this component is for reliable operation of PV systems, high quality should be a matter of course.
Protecting Sensitive Components
Power inverters carry highly sensitive electronic elements, such as chokes and transformers, and these subassemblies need to be protected from outside influences. High-quality treatment with protective resins reliably keeps the parts safe from moisture, heat, dust or other contaminations. At the same time, potting resins or coatings of high quality dissipate heat effectively and prevent the parts from overheating. The resin coating can also be used to muffle noise. Most importantly, however, the layer of resin stabilizes the subassemblies and keeps them in place. This increases the service life of the complete unit.
The coating, potting or filling process is done directly in one of the assembly’s compartments or separately in small casings, which are then mounted on the part and connected to build the final power inverter. Optimum dispensing quality considerably increases a solar inverter’s service life.
The selected casting resins need to meet very specific requirements. One necessary characteristic is the ability to bond reliably with the housing surface, which may be made from plastic, aluminum or steel. Electronic components often generate a large amount of heat. Therefore, the dispensing medium must be thermally conductive and at the same time highly temperature resistant (e.g., thermal insulation class B or F). In other words, the resin must be able to handle permanent temperatures of 130°C or 155°C, respectively. The really important criterion in this case is that the resin must be able to effectively dissipate any generated heat. This requires a thermal conductivity of at least 0.6 W/mK.
Another common requirement is the ability to reduce noise generated by vibrations (e.g., the widely known 50 Hz hum). This requires the resin to be rather elastic or viscoplastic in order to absorb the sound caused by the vibrations. Additional preferred properties include good electrical insulation and flame retardancy, self-extinguishing properties according to UL 94 V0 standards (if possible), and low water absorption.
In Case of Failure
Power inverters are generally manufactured in medium to large batches. While most European manufacturers carry out dispensing as semi-automated or fully automated production steps, Asian manufacturers often use manual dispensing. Since the inverter is a module of major importance, its failure means that the complete PV system is down. Inverters must have a long service life—and consumers must demand flawless quality from their providers.
Nevertheless, many cases have been reported where this component failed before the warranty period had expired. The reason in these cases was often insufficient dispensing quality caused by air trapped in the resin. Not only does this type of failure annoy the owner of the PV system, who suffers losses due to interrupted power feed-in, but it is also inconvenient for the supplier, who must carry the costs for replacing the defective part within the warranty period.
Many power inverter manufacturers lack experience in handling casting resins and in carrying out proper dispensing. As a result, they sometimes choose unsuitable resins or make mistakes in the dispensing process itself. If manufacturers are willing to meet expectations in terms of inverter service life, they have to keep an eye on dispensing quality.
Certainly, manufacturers must keep their costs as low as possible to survive in the highly competitive PV market. Nevertheless, they should make the right choice when deciding where to save money. Since dispensing is carried out on the completely assembled part, rejects have the highest possible costs. At this production stage, other electronic parts and subassemblies are already mounted on the assembly. Even though these elements might still be fully functional, they would have to be discarded together with the faulty part. Low dispensing quality therefore means higher costs.
To guarantee the necessary level of production quality, it is helpful to use the best-suited potting resin and the ideal dispensing method. In the case of solar inverters, automated PU dispensing can turn out to be more economical than manual dispensing of silicone.
Value and Performance
When comparing material characteristics, the less-expensive PU resins show better adhesion on many substrates than silicone. Since its glass-transition temperature can be adjusted in a wide range of -60–100°C, PU is relatively insensitive to temperature changes (e.g., temperature cycling and thermal shock tests). Due to its thermal conductivity, PU provides excellent heat dissipation properties. The medium shows good insulating characteristics and meets the required flammability criteria.
The two-component material is highly elastic and tear resistant, which means it can compensate for vibrations very well. Its elasticity is also the reason why PU is highly resistant to mechanical stress. By adjusting the formulation, it is possible to meet the exact requirements of the manufacturer and to specify characteristics such as rigidity, pot life, color, or flowability.
A very short curing time supports a seamless production workflow and allows fast further processing of the treated assemblies. Compared to silicone-dispensing masses, the PU-based media show low water vapor permeability and low thermal expansion. In addition, when looking at the costs of raw materials, PU is around 70% less expensive than silicone.
Increased Precision and Quality
An automated dispensing system increases the overall precision and quality of dispensing. The exact metering of both components guarantees that the mix ratio always remains constant. With manual dispensing, on the other hand, this precision cannot be guaranteed. If resin and hardener are mixed by a machine, the resulting material blend is completely homogeneous. The contained filler materials are distributed evenly because the material is constantly stirred and circulated. The fillers cannot collect at the bottom of the preparation unit’s tank. Additional controlled heating of the material can influence the flowability of the dispensing medium and ensure that it fills all nooks and crannies of the electronic part.
The one thing that most likely jeopardizes full functionality of a protective coating is air trapped in the resin. Material in the shipping pails always contains some amount of air bubbles. Manual mixing of the components usually stirs in additional air. The bubbles not only distort the mix ratio, but they also skew the dispensing quantity. Even if the coating looks smooth and regular from the outside, any air trapped within will compromise its functionality.
The gaps in the dispensing material reduce thermoconductivity and the protective function for the assembly. In the worst case, moisture gets into contact with the electronic parts, and the resulting corrosion causes the inverter to fail. The solution is to prepare the material in a vacuum. Evacuating the material container while simultaneously stirring the material in a controlled way removes all of the trapped air.
When dispensing manually, it is hard to prevent variations in the filling quantity of the assemblies. Estimating the metering quantity by eye leaves a high margin of error. Automatic dispensing, on the other hand, uses exact and constant metering of the dispensing quantities. All relevant process parameters, such as dispensing quantity, pot life, vacuum value and temperature, are monitored automatically. Any deviations from the defined parameters are detected early and can be adjusted right away, avoiding the resulting rejects. This type of quality control also gives manufacturers the additional benefit that they can prove a flawless production process to their clients.
The complete production process becomes cleaner and more reliable when dispensing machines are used. Potential hazards, such as those caused by spilled material, are eliminated. Automatic equipment for material preparation also helps save material, because the feeding units are designed to empty the barrels and cartridges completely.
Material preparation in a vacuum ensures that the resin cannot be contaminated by air humidity. Any air contained in the resin is removed. Even the best resin can only develop its ideal characteristics if it is properly handled.
Automated or semi-automated production offers many advantages for the manufacturer and for the customer. Thanks to the higher and more reliable quality level of the manufactured electronic parts and the use of a less expensive casting resin, the investment pays for itself in almost no time. This helps manufacturers to remain competitive in the market while also producing a higher quality product. The number of expensive rejects decreases, which in turn lowers the total production costs compared to manual dispensing.
The author would like to thank Andreas Arlt, key account manager of Renewable Energy at WEVO-CHEMIE GmbH, for his contributions to this article.