Benutzer:MRMEIER/Technical Cleanliness

Technical Cleanliness

The term technical cleanliness means the sufficiently low contamination of cleanliness-sensitive technical components with harmful particles.

If the unavoidable particle contamination - also known as residual dirt - in a technical system is so low that there are no short-term or long-term functional restrictions and system damage, the system is considered sufficiently clean in terms of technical cleanliness.

In the case of electronic assemblies, according to the ZVEI's “Technical Cleanliness in Electrical Engineering” guideline, the term technical cleanliness is understood to mean the absence of particles (metallic, non-metallic, fibers, etc.) on components that can affect the further manufacturing process or impair or prevent the correct function of the component or assembly.

Clean production

The production of cleanliness-sensitive parts, assemblies and systems in terms of technical cleanliness takes place in the so-called clean production. The areas of production, assembly, personnel, cleaning, packaging, storage and transport are taken into account, along the entire value chain from raw material to end use.

Along the process chain, in order to achieve the defined specifications for technical cleanliness, measures must be taken in each individual process step to avoid or minimize

·        Particle contamination from outside

·        Particle carryover through the process chain

·        Particle formation in the process

The different types of particles must be taken into account both when assessing the potential damage effects on an assembly and when determining appropriate avoidance and minimization measures.

·        Lint

·        Fibers

·        Non-metallic particles

·        Metallic particles

·        abrasive particles such as abrasives (corundum, etc.) or blasting material (sand, glass balls)

Technical cleanliness is relevant for particles in the size range 15–1.000 µm.

Cleanliness area

The environment in which the clean production takes place is called the cleanliness area (according to VDA 19 Part 2). Cleanliness areas are therefore divided into cleanliness levels.

·        unregulated area (SaS0)

·        Clean zone (SaS1)

·        Clean room (SaS2)

·        Clean room (SaS3) (see EN ISO 14644)

Industrial association for technical cleanliness (TecSa)

Due to the technical development in the automotive industry, damage from residual dirt increased by the beginning of the 1990s. For example, anti-lock braking systems or direct injection systems in diesel vehicles were particularly sensitive. As a result, numerous companies from the automotive industry complained about the need for standardization with regard to the definition of the specifications for clean production and methods for verifying compliance with these specifications. This led to the formation of the industrial association, which in summer 2001 was named TecSa - Technical Cleanliness.

A comprehensive set of rules was drawn up between 2001 and 2004. It stipulates how to proceed with cleanliness tests on products in the automotive industry.

Defined are:

·        Extraction procedures

·        Analysis methods

·        Documentation of the results

The set of rules was named „VDA Volume 19 Testing of technical cleanliness - Particle contamination of functionally relevant automotive parts / 1. Edition 2004“. Eine wichtige The Fraunhofer-Institute for production technology and automation in Stuttgart played an important role in the creation of the set of rules. There has been an annual specialist congress since 2009: Technical cleanliness in assembly and production processes which is organized by the "Süddeutsche Verlag Veranstaltungen GmbH". Here, experts from the automotive sector in particular advise and discuss the importance of the technical cleanliness of small and very small parts and exchange their experiences. The congress usually takes place in May of each year, lasts two days and always includes a factory tour on the subject of technical cleanliness.

The ISO 16232 standard was published in 2007. ISO 16232 is the international counterpart to VDA 19. Thanks to the cooperation of the German Spiegelausschuss the two sets of rules are absolutely compatible.

Revision of VDA 19 in the industrial association for technical cleanliness (TecSa 2.0)

Since the development of VDA Volume 19 for testing technical cleanliness in an industrial association, this set of rules has been in use for a good ten years. During these years, working with VDA Volume 19 enabled new knowledge and important experience to be gained. At the same time, however, the needs and requirements of industry have changed over the years.

In 2012, the need to revise VDA 19 was queried in an industrial workshop and the relevant topics were categorized and prioritized. Subsequently, based on these results, the TecSa 2.0 industrial alliance was launched at the end of 2012, in which over 40 companies from the industry prepare the relevant topics in working groups.

The revised VDA Volume 19 Part 1 (short form: VDA 19.1) is available since May 2015. A large part of the approximately one hundred entries for yellow printing were adopted.

Industrial association assembly cleanliness (MontSa)

The industrial association MontSa Montage Cleanliness was founded - again under the “leadership” of the Fraunhofer Institute IPA. The objective was to create a guide for planning or optimizing processes and procedures in cleanliness-sensitive assembly areas and their surroundings. This should prevent contamination by particles along the entire process chain. The target group for the guideline should be production planners and quality managers. After two years of work, the guideline named VDA Band 19 Teil 2, Technische Sauberkeit in der Montage – Umgebung, Logistik, Personal und Montageeinrichtungen, 1. Edition 2010, was published.

Technical cleanliness in electronics production

The current trend in electronics is towards smaller circuits with low power consumption and the longest possible service life. To reduce the power consumption of electronic assemblies, for example, active components with higher input impedances are installed. In addition to the advantages of these assemblies when used in battery or accumulator-operated systems, the use of these high-resistance components offers the opportunity to stop the trend of the increasing discrepancy between the electricity generated and the forecast power requirement. The disadvantage of using these economical active components is that the reduced power consumption is associated with very low signal currents and, as a result, there is an increased sensitivity to external interference, such as leakage currents. These leakage currents can be moisture-induced (reduction of the insulation resistance of the assembly surfaces and especially the solder masks, especially in combination with hygroscopic contamination) or they can also be caused by existing particle or fiber contamination between open contacts or conductor tracks. In the case of moisture-related leakage currents due to hygroscopic (i.e. water-absorbing) impurities and particles or fibers, the reduction in the self-drying potential of the circuit also plays a decisive role. The self-drying potential is largely determined by the power-induced heat loss.

In addition to the shortening of the air and creepage distances, the following further possible errors can occur if the particle cleanliness on electronic assemblies is insufficient:

·        Leakage current

·        Flashover

·        Electrical insulation for contacts

·        Electrochemical migration (ECM) through hygroscopic particles

Due to the aforementioned possible causes of errors in the event of particle contamination, an electronics production process should be planned and operated with the aim of keeping the number of particles of potentially harmful size or type so low that no disruptions occur during the production process or later operation of the assembly. Both conductive (metallic) particles and non-conductive (non-metallic) particles or fibers can have a damaging effect:

1. Risk potential of metallic particles or chips:

·        As aging progresses, oxidation occurs on these particles or chips

·        the resulting oxide layer leads to a decrease in the conductivity of these particles or chips

·        Stress breakdowns can be estimated based on the so-called wetting voltage

2. Risk potential of non-metallic particles or fibers:

·        As aging progresses, the polarity of the surface of these particles or fibers increases

·        the polar surface leads to an increase in hygroscopicity and thus to an increase in the conductivity of these particles or fibers in the presence of moisture

·        in case of condensation, electrochemical migration (ECM) can occur

Due to the increased use of components with low power consumption and the associated low switching currents, it is necessary to evaluate materials and processes with regard to potential risks from contamination (particles, chips, fibers, organic films, etc.). This requires an approach to risk assessment that holistically records ionic, filmic and particulate contamination.

In addition to the risk assessment, (particle) contamination also represents a major challenge in predicting the service life of assemblies. Besides small changes in the circuit design (circuit layout / replacement of components), it is mainly impurities on the assembly that change the moisture resistance of the circuit significantly. A first approach to calculating the service life are prediction models based on statistical values.

Guidelines and standards for electronics manufacturing

ZVEI guideline “Technical cleanliness in electrical engineering“

The terminology of technical cleanliness and the associated test procedures and documentation steps are common in numerous industries. In addition to the classic application area of mechanical engineering, the topic of particle contamination on circuit boards and assemblies is also increasingly coming into focus in the electronics industry. Even small amounts of particle contamination can significantly increase the risk of failure of manufactured electronic assemblies and thus of the entire product.

The approach and the methodology, as described in VDA 19 Part 1 and Part 2, are kept so general that they can be applied to the entire range of materials and processes in the automotive industry. The guideline "Technical cleanliness in electrical engineering" published in 2013 by the ZVEI (Central Association of the Electrical and Electronic Manufacturers' Association) deals specifically with component cleanliness testing and the planning of production areas for printed circuit boards and electronic assemblies. This guideline gives recommendations for testing, measuring and evaluating particles and particle contamination on assemblies. The cleanliness test in accordance with VDA 19 as well as the issues dealt with in Part 2 (VDA 19 Part 2) on the planning and optimization of cleanliness-relevant production areas are examined and specified from the perspective of the production of electrical, electronic and electromechanical components as well as circuit boards and electronic assemblies.

The advantage of this coordination and definition of the cleanliness test procedure, which is specialized in the field of electronics production, is that the comparability of analysis results of parts and components from electronics production is significantly increased. This shows how these results of the cleanliness analyzes can be classified and interpreted statistically, which leads to targeted information about the contamination risks in the respective production steps. Due to the large number of combinations of possible particles (in material and shape) and assembly layouts, the definition of general limit values for a maximum particle load was kept off. Therefore it is advisable to carry out a separate risk assessment for each individual case, which can serve as a basis for setting limit values between supplier and customer.

The main topics dealt with in the ZVEI guidelines on technical cleanliness in electrical engineering can be summarized as follows:

- A detailing of the VDA 19

- A definition of particles and fibers

- A recommendation on how cleanliness analyzes should be carried out and how the results have to be presented

- A consideration of the results of cleanliness analyzes from a statistical point of view

- A representation of the actual state with regard to particle pollution in the production of electrical, electronic and electromechanical components, circuit boards and electronic assemblies

- A consideration of possible particle sources within processes

- The presentation of (design) recommendations for the reduction of particles

- Instructions for transport and logistics

High-voltage guideline for power electronics

In the automotive industry - especially against the background of the use of power electronics in electromobility - the issue of technical cleanliness is currently attracting increased attention. For this reason, in 2014 the industrial association "TecSa" added the delivery conditions "Technical cleanliness for high-voltage components" to the ZVEI guideline "Technical cleanliness in electrical engineering". This so-called high-voltage guideline for power electronics specifies, for example, aids for setting particle limit values and minimum distances between electrical components. The following conditions are used to establish particle limit values:

- Electrical distances: The expansion of conductive particles should be less than half of the smallest electrical distance.

- Clearances and creepage distances: Electrical safety distances must not be undercut, taking into account the size of conductive particles.

- Quantity limits for size classes must be estimated at the beginning of the project and verified during the series process implementation.

- Non-metallic particles and fibers are to be assessed with regard to their risk.

Possible consequences: insulation faults, mechanical blockage of contacts, optical weakening / interruption of light barriers / light guides, etc.

Particularly with regard to the particular risk of air and creepage distances with the high field strengths in the high-voltage components in combination with mechanical and electromechanical components, a holistic approach should be pursued in the case of particle contamination. Critical particles can be introduced into the process due to the large number of components (metallic / non-metallic components, electronics, packaging materials, etc.) and also arise directly in the process during production steps. High-voltage flashovers and short circuits can be caused particularly by conductive particles. The increasingly complex circuits and the design of the power electronics increase this risk even more.

The holistic approach to minimizing the risk from particle contamination within the framework of the high-voltage directive includes the entire value chain from development to manufacture of the high-voltage component, including the supply chain. The attempt was made to orient the measures to implement technical cleanliness to what is technically feasible and economically sensible. To achieve particle cleanliness, methodical approaches are shown which aim to avoid particles during the entire manufacturing process and in logistics. In addition to the strategy of particle avoidance, cleaning off the particles at the end of production is also a possible way of achieving previously defined particle cleanliness requirements.

Qualification measures

Cleanliness analysis according to VDA volume 19

Factors influencing assembly cleanliness according to VDA 19.2

To consolidate the subject of technical cleanliness in practice, the responsible personnel should be made aware of cleanliness aspects. The topic can be divided into two different questions:

·        Cleanliness analysis of components

·        Factors influencing the cleanliness in the assembly area

·        A qualification of the executing TecSa laboratory is ensured by so-called particle standards. By using a particle standard, the rate of loss and the quality of the TecSa analysis can be ensured (gravimetric), as well as via the optical evaluation using a stereo microscope

Methods to ensure technical cleanliness in electronics production

In order to achieve technical cleanliness on electronic components or assemblies as well as high-voltage components that corresponds to the defined requirements, two different approaches can be followed.

One approach is to avoid contamination of the components or assemblies along the entire process chain. This begins with the first planning for the layout of the assembly and must be taken into account from the purchase of supplier parts to the transport to production and the passage through various processing processes in production to packaging and shipping to the end customer. To ensure this, it may be necessary to carry out the entire production in a clean area or clean room and to align the necessary logistics (locks, transport systems, protective clothing for employees, etc.) as well as the training and qualification of employees with the goal of avoiding particles. In order to qualify a production for technical cleanliness, an audit to determine the current cleanliness status makes sense. In the next step, after the risk assessment of the particle contamination, the first measures to improve the particle cleanliness during production, purchasing or logistics can be determined.

The second approach pursues the strategy of removing the particles created during production and the previously carried out value chain as well as logistics by a cleaning process at the end of production. This possibility can only be considered if it is guaranteed that particles that are created or entered within a process chain do not lead to problems within this process chain.

Another possibility is to combine both strategies - particle avoidance and particle cleaning. This option should be considered if very high requirements are placed on technical cleanliness and thus on reliability and service life.

In addition to cleaning the assembly (low-voltage or high-voltage components), it may also be necessary to clean the production systems itself at certain steps within the process in order to be able to achieve the respective cleanliness requirements for the end product. Especially if assembly production is designed as a no-clean process, which means cleaning of the finished assemblies is not provided, it is essential to minimize the particle entry through the production systems. For example, the soldering process itself can also be a source of particles if soldering frames and ovens are not cleaned regularly. For example, burnt-in flux residues form a solid layer on the soldering frame, which flakes off through mechanical action (movement by conveyor chains or manually by the production staff) and releases its particles.