FOD Detection and Control

In most basic terms, a cleanroom is used to prevent deposition of unwanted particulate contamination on a product or patient. Particle contamination, whether from manufacturing, environmental, or human sources is all but inevitable. Even in a cleanroom, we do not always understand the likelihood of particles or micro-organisms to deposit on critical surfaces. 

In traditional cleanrooms (ISO Class 1 to 4), airborne particles < 5 µm are removed by air flow. However, in cleanrooms with turbulent-flow or high-bay cleanrooms (ISO 5-9) the average airflow velocities are much lower than those used in unidirectional airflow systems. In these cleanrooms, large particles (e.g., foreign object debris, FOD; ≥ 5 µm) divert from slower-moving airflow streams more readily and deposit on surfaces and work environments. They remain there until removed by manual cleaning or are stirred up by normal personnel movements, placing critical products at risk. In these environments, traditional aerosol particle counters struggle to collect a representative sample of these larger particles, leaving your critical assemblies and processes at risk of contamination. 

KILLER PARTICLES

A killer particle is any contaminant that attaches to the surface of a component. For example, in automotive manufacturing and finishing, a killer particle can impact the surface quality of the part, like a small particle causing a paint bubble or plating defect and undermine the protective properties of the coating. However, other particles can be more innocuous – like oil and grease compounds which get distributed via aerosolized particulate from pneumatic tools used within the workspace. Tiny contaminants can impact film absorption and coatings on critical optical surfaces and make them more susceptible to damage from lasers and other instrumentation in the optical assembly. Such damage can impact large optical surface quality on components in space and satellite component manufacturing, but they can also have significant impact in smaller optics like those used in instrumentation for microscopy, spectrophotometry, and similar applications. Finally, these particles can also have abrasive qualities and scratch surfaces – giving rise to the “killer particle” moniker – driving down production yield and component life. 

The first step in controlling FOD and killer particles is understanding their sources, and central to that, is knowing when and where particles are generated or disturbed and when to clean and eliminate them. 

FOD SOURCES

Personnel - Human Hygiene

Personnel are the biggest threat and highest source of contaminant material, accounting for ~75-80% of particles in a cleanroom. People introduce particles through skin flakes, oil, hair, yeast and movement, and bring in external particles from cosmetics, perfume, deodorants, lotions, and clothing. As a result, gowning protocols and related compliance are important to ensuring a clean space – even in ISO 5-8 cleanrooms. 

Equipment, Tools, Raw Materials and Supplies

Manufacturing and other equipment and process tools can generate gas, lubricants, emissions and airborne particles during use. Raw materials used in manufacturing processes can generate glass flakes, cleanroom debris and aluminum particles that can be deposited on a work surface and can disturbed and relocated when people move in the room. Further, contaminants can originate from everyday tools like cleaning products, and writing/record-keeping supplies. Critical attention must be paid when installing new equipment or bringing in new supplies. Ensuring cleanliness before moving equipment into the clean space, understanding equipment exhaust filtration are critical to maintaining your clean space. Raw materials, locally stored components, can also bring in contaminants and should be cleaned and prepared prior to entering the space. Similarly, regularly used supplies like pen, paper, labels, tapes and other items must be compliant with the environment. 

Gowning protocols and compliance

Different industries and ISO classifications require different cleanliness levels. ISO 5 requires complete body coverage and careful donning procedures. Whereas industrial applications (ISO 6-8), have less strict requirements and procedures. 

Studies by 3M show that over 80% of contamination enters the cleanroom through entrances and exits, mostly at the floor level. To combat this problem, many cleanroom entrances include “sticky” polymer mats that attract and capture particles from shoes and wheels as people and equipment enter the clean space. Such mats may contain electrostatic or adhesive coatings that capture particles until the flooring is cleaned or the mat surface is changed. Placement and maintenance of these surfaces is critical to their effectiveness as a tool to minimize particle transmission into the cleanroom. 

The selection of cleanroom classification and gowning procedures should be carefully considered and based upon a complete risk assessment of the processes, equipment, materials to be used and protected and human/machine impact on all these factors. Donning practices and frequency of change should be part of a regular educational and process audit protocol to ensure both effectiveness and participant compliance. As discussed earlier, human movement causes contamination, and protective clothing such as hats, cleanroom suits and face masks are basic items of contamination control. The suit covers the wearer to prevent skin and hair being shed into a clean environment. But if the employee in an ISO 8 cleanroom doesn’t change their lab coat at the required interval, what impact can that have on critical surfaces and processes? 

The clean space is intended to minimize or eliminate outside contaminants, but what if that doesn’t always work?

FOD SOLUTIONS

Planning and Education

Lean Manufacturing and Six Sigma 5/7S methodologies (sort, straighten, shine, standardize, sustain and for 7S, safety and sprint) underpin good manufacturing and production practices that improve productivity and quality. These methodologies also drive good FOD practices in the “sort, straighten, and shine” aspects by driving the importance of routine cleaning. Certainly, employee training and education is important to ensure the effectiveness of such, but careful design and layout of production and process areas also play a critical role. FOD risk assessment is the first step to effective management – understanding FOD risk and mitigating it by design and layout of work and risk areas is essential. Employee education, FOD awareness, signage and even barriers in some cases can help minimize contamination. 

These programs emphasize ensuring “a place for everything, and everything in its place”; in these programs, loose items are often considered FOD when stored incorrectly. 

Awareness and Education

FOD and killer particles present a challenging risk in ISO 5-8 cleanrooms because they are often large spaces. Within the large space, there are often areas that require different protocols and protections. Many manufacturing organizations utilize a standard three-layered system of signage for designating storage and work areas, and reminding staff to be mindful of identifying and eliminating debris: 

Signs and even floor markings can augment employee training and education. With any program, what gets measured, gets improved. 

Education and retraining should be a key component in protecting and maintaining your cleanroom, regardless of classification. Best practices include required and documented training for new employees and employees who need access to the cleanroom. The training protocol should include gowning protocol, information on the awareness strategies employed (floor markings, signs, etc.), and basic cleanroom behavior – focusing on slow deliberate movements to minimize disruption to air flow – and practices for introducing materials into the cleanroom. 

Cleaning

At SentrySciences, we know that cleaning and validation is key to protecting your critical assembly areas. Our portfolio of solutions also includes portable inspection tools for cleaning verification to ensure that as your process needs change and move throughout the cleanroom, your procedures can be qualified before you put critical products at risk. 

But, are your cleaning processes effective?

The PartSens 4.0 Surface Particle Counter is a portable measuring system with a wireless measuring head for fast and direct quantification of surface particle contamination. The PartSens+ 4.0 can even differentiate particle sizes and counts of metallic and non-metallic particles and fibers on surfaces and is both ISO 16232 and VDA 19 Parts 1 and 2 compliant and can be used to perform Percent Area Covered (PAC) calculations. For surfaces that cannot be measured directly (curved or rough surfaces), both products are compatible with particle trap strips, collection trays and tape lift pads that can be used in the gowning area and throughout the cleanroom for periodic cleaning verification. 

Monitoring

Robust solutions to cleanroom FOD and killer particles include measurement of the Particle Deposition Rate (PDR) at critical locations within your cleanroom. Globally, across industries using ISO 5-8 cleanrooms and controlled environments, PDR has been measured between 3 to 3,000 particles > 15 µm per dm2. The impact of this FOD is dependent upon where they deposit and the effectiveness of the protocols and processes to mitigate and eliminate them. SentrySciences has products to enhance your contamination control strategy and both continuously and automatically monitor large particle deposition at your greatest points of risk. These tools can be permanently installed at critical locations or moved throughout your cleanroom to assess large particle deposition over time at different locations. Whether in permanent or temporary locations, our solution can be configured to trigger alarms in real-time when your PDR, raw particle count or when an ISO 14644-9 or -17 limit has been breached. Up to six sensors can be installed on a single system, facilitating real-time notification of changes at those locations, and your prompt response to solving an excursion before catastrophic damage to critical products or processes.       

SentrySciences personnel can help to tackle FOD and surface particle challenges with unique, purpose-built solutions that help to control your environment and protect critical products and assemblies.