By Michael Kövi, Senior Process Engineer, Zestron Europe
The term technical cleanliness is most strongly associated with the automotive industry, but cleanliness testing in accordance with inspection standard VDA 19 is also used in other sectors such as hydraulics, mechanical engineering and medical technology. While frequently neglected, particle contamination is also being increasingly associated with PCBs in the electronics industry. The increasingly smaller gaps between contacts and higher power densities mean that the risk of failure can drastically skyrocket in the presence of even just a small quantity of particles, which means that customers’ expectations and demands concerning electronics products have risen yet further. The following article provides an overview of the risk of particles on electronic PCBs, how they are measured and existing standards as well as presenting a cleaning method for reducing particle loads.
1. Technical cleanliness: The risk of electronic assembly particle contamination
“Component cleanliness is defined as the absence of particles on components that could impact on any upstream production processes or the component or PCB’s proper operation”
(Source: ZVEI Guideline – Technical Cleanliness in Electrical Engineering)
This definition from the ZVEI Guideline (German Electrical and Electronic Manufacturers’ Association) already hints at the fact that it is not technically feasible or economically meaningful to produce a component or surface in such a way that it would be completely free from particles. And that the concentration of particles of a potentially damaging size or nature should instead be kept sufficiently low to ensure that they cannot cause any disruptions during the manufacturing process and the PCB’s operation. Particles that are potentially damaging include electrically conductive particles that could connect two contacts on a circuit board to one another, cause a short circuit and could hence cause the PCB to fail. A conductive particle between two contacts that doesn’t touch either of them can still cause an electrical breakdown or tracking as a result of reducing the air gap and creepage distance (see Figure 1).
Nonconductive particles inside connectors and mechanical components such as, e.g. relays and switches, on the other hand, can have an insulating effect and impact on the performance of photo-optical components such as a photo transistor.
However, there is also always a risk that particles that are not initially conductive but hygroscopic¹ and become conductive by absorbing water molecules (e.g. paper components of packaging, fibres of fabric gloves or ESD garments). Vice versa, metal particles can become non-conductive over time as a result of oxidation and hence cease to pose a hazard to the electrical functions of a PCB.
2. Technical cleanliness standards and testing
Following field failures of gearbox and injection systems caused by particles in the 1990s, an industrial association was formed in 2001 in Germany that published a set of specifications for technical cleanliness testing as VDA Volume 19 in 2004. This volume, the latest version of which was published as VDA Volume 19.1 in 2015, defines the methods for extracting particles from component surfaces, the analytical methods for measuring particle contamination and for documenting the inspection results. These specifications were matched at international level with the publication of ISO 16232 in 2007. The second volume, VDA 19.2 “Technical cleanliness in assembly”, was published in 2010 with the intention of providing manufacturers with guidance on the design of new processes or optimisation of processes and process sequences with the aim of preventing particle contamination along the process chain.
These publications are relevant to the technical cleanliness of the entire range of components found in the automotive industry. The ZVEI Guideline “Technical Cleanliness in Electrical Engineering” published in 2013 is the first set of guidelines ever to refer to component cleanliness testing and the design of production areas, especially for circuit boards and electronic PCBs. The latest and second edition of this Guideline recommends removing particles from components for technical cleanliness testing or analysis by spraying². The extraction liquid generated during this process is subsequently passed through a filter membrane to capture the particles. Once this filter has been dried, the particles can be automatically measured and quantified using light microscopy (see Figure 2). The particles’ reflective properties are used to distinguish metallic from non-metallic particles.
3. How can technical cleanliness be achieved?
There are currently two different approaches to achieving the required level of technical cleanliness within the industrial sector (see Figure 3).
One of these comprises elaborate supply chain management processes aimed at preventing components from becoming contaminated with particles along the entire process chain – starting with the purchase of vendor parts, their transport to the production facility, the various production processes they are subject to, right up to when they are packaged and shipped to the end customer. In some manufacturing situations, the only way to ensure a high level of technical cleanliness is to perform all production processes in cleanrooms. When dealing with production processes that are not overly sensitive to the presence of particles, another option is to remove the particles at the end of the production process using a cleaning process. In situations where technical cleanliness is of utmost importance, the surest way to assure cleanliness may be to combine both of the above approaches.
Cleaning processes for circuit boards tend to predominantly focus on removing flux residues. These cleaning processes involve first cleaning, then rinsing and then drying the circuit boards. The cleaning part of this process does not just remove flux residues but also particles. If the cleaning system used for this process is properly configured, it can be used to effectively contain the contaminants brought into the system and hence prevent them from spreading back to the cleaned item. Amongst other things, this includes constantly filtering the wash and rinse media. This can be achieved with a filtration unit comprising two filter cartridges connected in series, fitted inside the recirculation circuit. In addition to the washing media, the drying media “air” should also be filtered before entering the chamber to remove any dust from the production area and surrounding environment. This can be achieved with a HEPA filter (High Efficiency Particulate Air Filter) (see Figure 4).
In a study performed by ZESTRON Europe, the use of a single-chamber spray-in-air cleaning machine led to a significant reduction of the particle concentration on double sided PCBs (see Table 1).
In this example, the use of two filter cartridges connected in series with pore sizes of 10 μm and 5 μm made it possible to remove all of the metallic particles 400 μm from the PCBs. This led to a significant reduction in the concentration of size E, F, G and H particles. The cleaning process also led to a reduction in the surface cleanliness index (SCI3), which is a single numerical value that reflects the results of the analysis for all of the relevant particle size category particles (based on the total number of particles, by 88%, and for metallic particles by 79%).
Optimising the filtration and rinsing system can help manufacturers meet end customers’ frequent requirement that products be free of any metallic particles larger than 200 μm. In order to avoid re-contamination, the environmental conditions and packaging materials have to meet the associated particle purity requirements. That means that it may be necessary to perform the cleaning process and any subsequent processing steps inside a cleanroom.
A lot of assembly manufacturers are still in the process of defining limit values for their respective PCBs. In that case, manufacturers might perhaps consider using IPC Standard 9202 for validating their PCBs and defining their particle number and size limit values, because it is not just filmic and ionic contamination that can affect surface resistance, but so can particles. A change in surface resistance caused by a defined level of particle contamination (see Figure 5) can be measured with a test board.
4. Conclusion and outlook
The automotive industry is making technical cleanliness an increasingly important subject matter. In 2014, one of the German automotive manufacturers’ industrial associations published a guideline on “Technical cleanliness for high-voltage components” by way of a supplement to the ZVEI Guideline and hence an approach for high-voltage electromobility components. This guideline is to be turned into a standard that, amongst others, will define how to specify particle limit values such as, for example, by ensuring that any electrically conductive particles do not expand by more than 50% of the smallest electrical gap on the PCB.
The second edition of the ZVEI Guideline published in October 2018 provides more information on how to use these kind of limit values. In particular by analysing the risk of short circuits caused by particles in order to derive individual specification values and to make it easier to discuss suppliers and customers specific requirements. One of the ways manufacturers can meet current and future cleanliness requirements is to reduce critical particle loads with the help of cleaning processes as shown in section 3 of this article.
¹Hygroscopic: Capacity to attract and hold water molecules from the surrounding environment, including at humidity levels below
water saturation (humidity < 100 %)
²Available at www.zvei.org