Dust-free in space

Even the most minute particles of contamination ignored during the manufacture of payloads destined for space can result in eventual degradation of performance or even circuit failures and the possible loss of a mission. An engineer, of course, cannot be on hand to fix it. So EADS Astrium, part of the European Aeronautic Defence and Space Company, chose the accurate monitoring and reporting capabilities of the latest Fluke handheld particle counter for use in its recently revamped cleanroom at its Portsmouth site.

EADS Astrium designs, manufactures, assembles and tests the essential electronic payloads, which make up the heart of satellites and space probes. It is a leading designer and manufacturer of satellite sys-tems and a wide range of avionic and space equipment. Satellites produced by Astrium include those for the Galileo GPS system, and Immarsat-4 for telecommunications. It strictly adheres to the requirements of the European Space Agency (ESA) in the classification of its cleanrooms during the assembly and test stages of those modules.

At Portsmouth, specialist wafer dies are assembled in the company’s hybrid cleanroom (a Class 2, M5.5 (10,000) cleanroom). The larger, recently-revamped production facility is in a Class 3 M6.5 (100,000) cleanroom. Assembly, integration and testing of electronic modules and sub-assemblies takes place here. At various stages, client personnel can carry out visual checks in laminar flow cabinets. Even with these rigorous inspection and testing procedures, it is essential that no particulate contamination is in the environment that might lodge anywhere out of sight. The forces and vibrations that the payload undergoes during launch is typically a time when any debris can get dislodged and move around, possibly to cause circuit degradation or failure.

Jonathan Roe, manufacturing engineer at EADS Astrium, explained the decision to purchase Fluke 983 Particle Counters for the cleanrooms on the site. “Originally we used a single trolley-based particle counting system but it had many drawbacks,” he said. “Its lack of mobility meant it was difficult to monitor exact locations and its output of data was all in printed form. We had to read the data and then manually enter it into a PC for reporting purposes. We realised the new handheld Fluke 983 could be the ideal solution. In trials it proved just that.”

The Fluke 983 Particle Counter has rechargeable batteries and can be held in the hand. Chris Jeffery, process control manager at EADS Astrium, explained, “The size and portability of the Fluke 983 means we can sample air quality at any location within the cleanrooms. So we can take measurements, for example, up high, near air conditioning vents to check the filtration of the incoming air, at locations hidden behind equipment or benches and within laminar flow cabinets. The 983 also electronically records ambient temperature and relative humidity so we have been able to do away with the old fixed position polar charting devices with values we then had to read and transfer manually.”

Personnel wear the requisite clothing to protect against their hair, skin, clothing fibres or make-up causing contamination. In the redesigned cleanroom, sequential production methods have been introduced so that the company can continue to be at the forefront in offering efficient satellite production facilities. The new workbenches have been equipped with computers with LCD monitors to display assembly instructions and technical diagrams to avoid the introduction of paper into the cleanrooms. Over 1000m2 of new, charge-dissipative tiling has replaced the vaulted flooring. The filtered air conditioning systems always maintain the air pressure slightly above atmospheric in the rooms to discourage ingress of contaminated air when people enter or exit.

Pick-and-place surface mount assembly and reflow soldering of circuit boards and tiles takes place – although due to the specialised nature of the work, throughput volumes are low. ATE facilities are available for assembly testing. Each functional module typically consists of circuit boards and components securely bonded to gold-plated aluminium or ceramic plates. The production personnel connect the circuits using gold wire of one-third the diameter of a human hair. Manipulation and soldering is achieved with the aid of bench microscopes.

Special testing sites for the modules and sub-assemblies have been designed and built and are ready in situ in the cleanroom. The extreme hot and cold temperatures that the payload may experience in space are also simulated as part of the module and sub-assembly testing procedures. The sub-assemblies are typically packed in containers with silica gel and temperature and relative humidity indicators, purged with nitrogen and vacuum-sealed. They are transferred to the final assembly cleanroom where they are fixed to the panels that make up the payload and then covered in insulating blankets. These are shipped to the Toulouse integration facility where the payload is readied for eventual mounting on a spacecraft.

Typical daily procedures for checking air quality include regular readings with the Fluke 983 at designated locations around the cleanrooms, though it will also be an ideal tool for locating exact sources of contamination should the event ever arise. The 983 is capable of storing up to 5,000 sampling results, based on time, a set number of counts, or with a programmable delay. For each sample time, it records date, time, counts, relative humidity, temperature, sample volumes, any alarm signals, and a location label. The measurements are classified in particle size distribution steps between 0.3µm and 10.0µm in diameter.

Chris Jeffery concluded: “The Fluke 983 is intuitive to use. Readings are displayed on the LCD screen, and the software provided with it enables the data to be downloaded to a PC back in my office. So I can easily import it into a spreadsheet for analysis and regular reporting to meet ESA requirements. And by storing the information on a database, it is easy to interrogate later for purposes of traceability and statistical process control.”