In miniaturised technology, the elimination of tiny particles while fastening screws during component assembly is a new feature of quality. If, when a circuit board is being assembled, a microscopic piece of steel – invisible to the naked eye – escapes, a disaster is sure to happen. A short-circuit will occur if the tiny fragment accidentally connects two of the conducting tracks.
As assembly parts become ever smaller even microscopic particles, measuring a mere 500, 200, or just 100µm, can cause a malfunction. Which is why what has become known in joining technology as “technical cleanliness” is of increasing importance in the design and building of screw fastening assembly systems.
The technical requirements are becoming stricter, according to Jürgen Hierold, sales manager, for Deprag Schulz, a specialist in screwdriving technology and plant engineering based in Amberg, Germany. As early as the 1980s, Deprag was successfully tackling this problem. The machine tool builder was supplying renowned US manufacturers with screwdriving systems for diskette drive assembly units. “Assembly conditions corresponded more or less to sterile room requirements,” recalls Hierold. “That was a challenge for us as machine tool makers.”
The fact that all the components could be harmonised with one another because they all came from a single source, proved to be an advantage. The one-stop-shop company convinced its customers with its high quality, well organised departments, all of which had a single aim: to prevent any abrasion as the fasteners were fed into the screwdriving system and during the actual fastening process.
When cleanliness during assembly is required, the entire screwdriving process must be put under the microscope. “Every step from planning, to production and assembly must be carefully examined,” says Hierold.
The first thing is to obtain an accurate definition of the cleanliness requirements for the product in question. Only with the co-operation of the design engineers, the production, logistics and quality management departments is it possible to achieve the highest possible level of technical cleanliness and thus reliability.
Trained staff are just as important as a clean production environment. In the main it has proved to be unnecessary to set up expensive sterile-room conditions with the air conditioning and particle filtering systems that this entails. The crucial point is that the cleanroom should be maintained in a non-critical state of cleanliness. The principal thing is to prevent disruptive particles from being brought in by people, materials or transport systems.
Designing out contamination
Contamination of components by particles of dirt can be prevented as early as the design stage. In their designs, engineers should avoid corners and edges where dirt can be trapped, and which are hard to clean. Rounded edges are easier to keep free of particles. Similarly at the screw location, drilled-through holes instead of blind holes are easier to keep clean.
“Equally, appropriate cleaning processes are the foundation of preparing the cleanroom, such as using specially treated fasteners,” says Hierold. For example, with its “Cleancon” service, the established manufacturer of threaded fasteners, Arnold, offers fasteners that, in terms of their manufacturing process, cleaning and packaging, are designed specifically to meet the demands of technical cleanliness in the screwdriving process.
But investigations show that the screwdriving assembly process itself can also produce hazardous particles through friction. According to Hierold, alternative materials and processes can counteract this. “If uncoated aluminium or untreated steel is used for the assembly unit, it is simply opening the door to particle contamination caused by friction or corrosion. Choosing alternative materials such as polished stainless steel and eloxated aluminium will reduce the number of such particles and each component of the assembly system is much easier to clean.”
If a pneumatic screwdriver is required in the production plant design, that too must meet the cleanliness criteria. Deprag’s standard range contains pneumatic screwdrivers suitable for sterile rooms, with polished stainless steel housings and controlled exhaust air extraction, which the company recommends for assembling screws inside a cleanroom.
“Such high-quality industrial screwdrivers are ideally adapted to the high cleanliness requirements, and have proven themselves many times,” says Hierold.
Likewise, the number of particles created as the fasteners are separated from one another can also be greatly reduced by choosing the right technology.
Standard vibration feeder/ conveyors move the screws by creating a throwing motion inside the conveyor hopper. The screws knock against one another and harmful particles can be created by this abrasive action. “If technical cleanliness is paramount Deprag recommends that parts should be fed gently using a feed system that employs an oscillating rail segment (Sword Feeder),” says Hierold.
If the screw is fed to a position directly above the screw hole in the component, contamination by particles is not necessarily prevented. “It is better to look for alternatives,” he says.
The feeder system works cleanly and reliably by having the screwdriver travel to a specified position above a vacuum source. A blast of air shoots the required screw into position for the next screwdriving operation. Any particles of contamination that may occur during the process are picked up by the vacuum. Only then does the screwdriver travel back to the “screwing position” and the screw is fitted.
“Vacuum sources at every relevant position increase cleanliness and are recommended at every point where abrasion or friction can occur,” says Hierold.
And Hierold has other assembly process quality issues in his sights. If the screwdriver bit does not mate accurately into the screw drive, unwanted particles are caused by the abrasion. The Deprag Minimat-EC-Servo range of screwdrivers reduce their speed as they engage with the screw. The integral sensor system assists with recognising the precise position of the screwdriver and ensures that the bit engages properly into the screw drive head.
Only once the bit is correctly engaged into the head does the speed ramp up to carry out the fastening action. This prevents bit slippage and particle contamination.
Using cover stencils and screw templates during the production process reduces the chance that hazardous particles make their way to the assembly part and also assist with Electrostatic discharge (ESD) protection.
ESD generated on assembly components during the process is conducted away and hazardous contamination particles are captured by the template cover.
“By bundling this complete package of different countermeasures the build-up of particles is minimised,” says Hierold.
Products can be supplied to meet the specific demands of the screw fastening process in terms of their manufacturing process, cleaning and packaging
He recommends that when plant engineers need to create cleanroom production facilities. they should choose a machine tool builder that offers every key aspect of screwdriving systems from their own range of manufactured tools.
“It’s the best way of ensuring that each component and each process, such as feeding, positioning, and screwdriving is harmonised to the others. The entire process can be examined, evaluated and enhanced with a view to technical cleanliness.”
The miniaturisation of technology is moving at a pace. Technical cleanliness in the production process is a hot topic and has become a quality characteristic.
What is now standard for the more sensitive products in the automotive and computer sectors, is now concerning other industries and suppliers.
Hierold believes those companies that can comply with the high cleanliness requirements will have the best market opportunities.“The specialists at Deprag are ready for the future and can produce the solution,” he says.
This article was first published on-line in Process and Control Today on 28.3.12.
