We take a look at key aspects regarding the chemicals involved in the assurance of high reliability inverters including the solder paste, solder flux, defluxing cleaning agents and conformal coatings. Key parameters covered include toxicity of the chemicals, energy usage, water usage in the cleaning process, and shorter time cycles
by Rodrigo Aguilar, Inventec Performance Chemicals
Inverters represent critical system components which enable power quality, efficiency, cost, safety and protection in green energy systems (solar and wind technologies primarily); once performing in the field, inverters get exposed to harsh environmental conditions such as dirt, snow, humidity, thermal stress, etc., thus the demand of long term reliability becomes critical. Furthermore, increase in the reliability and manufacturing quality of the inverter will drive to greater confidence in avoiding unexpected failures, reducing warranties and aftermarket services. By doing this and implementing a sustainability approach in manufacturing, inverter producers can create a real competitive advantage.
Solar inverters are the core of a photovoltaic system. Several issues can affect their performance thus jeopardizing the expected lifetime of 10 to 15 years. Among those issues we can include critical component failures (electrolytic capacitors, switching devices, etc), thermal shock, thermal overload and over current and voltage.
Environmentally compliant PCBAs and reliability
The printed circuit boards assemblies used inside the inverters are commonly processed using materials compliant to environmental legislations such as RoHS and WEEE, making it even more difficult to assure the expected lifetime of over 10 years. Solder paste, solder flux, defluxing cleaning agents and conformal coatings must all be considered both in terms of performance as well as environmental sustainability. Key parameters include toxicity of the chemicals, energy usage, water usage in the cleaning process, and cycle time.
Balance of system components contribute to 10% of the total photovoltaic system. At the same time inverters represent 60 to 80% of the unscheduled maintenance costs. An increase in the electronics reliability inside the inverter will help achieve longer lifetimes of the device and an increase in profitability and customer satisfaction, while reducing system degradation and lowering operating and maintenance costs.
Sustainable assembly materials
The sustainable approach to inverter manufacturing should cover at least four key perspectives: energy, water, waste and toxicity.
It is recommendable that the solder paste used in the assembly of PCBs inside an inverter feature a halogen-free and halide-free flux medium that will increase electrical reliability of finished assemblies upon the elimination of ionic compounds. The flux medium of the solder paste should also be non-toxic, reducing the risks for both the material supplier as well as the circuit board manufacturer.
In order to avoid corrosivity, the flux residues of the solder paste should remain inert after soldering. This is assessed by several tests, the most common methods are Surface Insulation Resistance (SIR) and Electrochemical Migration Resistance (ECM/EMR), to measure and quantify the improvement in electro-chemical corrosion. The bono test (Figure 2) is the best technique to differentiate the nature of solder paste residues. It also becomes a very good parameter for assessing the compatibility between the solder paste residues and the conformal coating. Chemical reliability is even more critical when the PCBAs are not cleaned before coating.
Inverter PTH process reliability and sustainability will increase by using VOC-free fluxes with low residues bringing less pollution on the wave system and lowering cleaning frequency of the wave carriers. Residues should also be chemically inert as described for the solder pastes.
For reliability purposes, inverter manufacturing could also implement the use of automotive grade soldering materials. Meeting the requirements of under the hood components that face working temperatures from -40°C to 125°C would give the solar inverter producer more confidence in the durability of the final product.
For the cleaning of stencils and wave carriers, use of environmental friendly chemistries should be used, with no risk to the operators, the process and the environment.
The PCBA of a solar inverter is usually not cleaned, as no clean soldering agents are commonly used. However, defluxing of the PCBA could improve the reliability of the finished device. No clean fluxes can be removed. The easiness of residue removal depends on the thermal profile used, the type of component, the flux formulation, the cleaning chemistry and the process, including the cleaning system. Water or solvent based solutions? The key consideration is to respect environmental regulations and look at the full picture and all the variables involved: use of water, energy, time, toxicity, emissions, manpower, etc.
To protect the PCBA of the inverter from environmental conditions, the use of conformal coating is required. This provides the protection to significantly extend the life of the components and circuitry. The choice of coating material should take into consideration performance aspects such as adhesion, chemical resistance, application process, curing process, compatibility, etc; but also environmental and safety aspects: VOC-free, low VOC, flammability, solvent-free, toluene and isocyanates-free. These features are also critical during the repair operations: for the removal of conformal coatings, non-toxic cleaning chemistries should be considered.
Conclusion
Chemical assembly materials used in the manufacturing of inverters PCBA play a key role for the reliability and sustainability of the device. Solar and wind inverter manufacturers still have opportunities to improve not only the PCBA materials, but also the practices and methods while meeting and going beyond environmental requirements.