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BEFORE VARNISHING PRINTED CIRCUIT BOARDS Atmospheric plasma ensures secure adhesion Conductive inks applied by the screen printing process adhere better when the surfaces of sensor circuit boards are pretreated with atmospheric plasma. This process has been successfully used by a sensor manufacturer for some years now. The throughput has tripled thanks to this system. Adhesion problems have been reduced to zero. Sensors feel temperatures and pressures, they measure speeds and motions, positions and angles. What they detect is mostly converted by them into electrical signals. One of their most essential components is the printed circuit board. For potentiometric position transducers and rotary sensors, the circuit boards are printed with electrically conductive ink using the screen printing process. Given that innumerable automation processes in industry rely on the quality of such a coated circuit board, secure adhesion of the conductive ink must be ensured by the manufacturers. To achieve good adhesion of the ink, the substratum must be cleaned and activated. Various methods are available here. One of the still frequently used methods is the pretreatment with solventcontaining chemicals. This method, however, is detrimental to the environment and often The rotation jet has two atmospheric plasma sources for inline cleaning and neutralization of the circuit board while activating the material surface so strongly that it becomes receptive to all coating, printing or adhesive bonding processes. associated with high waste disposal costs. Novotechnik in Ostfildern, a manufacturer of potentiometric and non-contacting position transducers and rotary sensors, already focused on environmentallyfriendly production in the 90ies when the circuit boards were pretreated with lowpressure plasma in a vacuum chamber. This method was indeed efficient. But while vacuum chambers are perfectly suited for batch processes, they are less suited for the pretreatment of large quantities. The process times used to be too long. What's more: Integration into the existing screen printing lines was not possible either. Adding to this was the very labor-intensive operation since one person had to equip
Position transducers mainly operate under harsh environmental conditions and at high positioning speeds. and another one had to empty the low-pressure chamber. When Novotechnik decided in 2000 to increase production, the company looked into an alternative process. The solution was again a plasma process but this time without the above limitations namely the Openair process developed by Plasmatreat. The circuit boards undergo a continuous process from the equipment via the plasma system through to the screen printing plant. Novotechnik's space-saving plasma plant serves four screen printing lines. Plasma without vacuum chamber This method is based on a nozzle principle for the most varied component geometries. In contrast with the complex plasma low-pressure systems with vacuum chamber, the plant is now integrated into the production line and the continuous process flow and operates under normal atmospheric conditions. The jets are driven solely by compressed air and high voltage. The figure shows a plastic surface that was pretreated with plasma. Treatment renders the surface polar and the surface energy rises to over 72 mj/m 2
The plasma system is characterized by a threefold action: It activates, discharges and cleans the surface to a microfine level. By adding a precursor material, this technique was developed further for nanometer thick coatings in close collaboration with the Fraunhofer Institute for Manufacturing and Advanced Materials (IFAM). Today this process is also used for coating deposition and the functionalization of surfaces. Strong surface activation on plastics The most important objective in the pretreatment of circuit board surfaces is the activation, i.e. the increase in surface energy, which is the most essential criterion for assessing the probable adhesive strength of an adhesive layer or coating. Plastic materials usually provide a low surface energy between 28 and 40 mj/m2. But good adhesion conditions can only be obtained from 38 to 42 mj/m2 onward. Plasma treatment combined with a correspondingly strong activation of the material surface however, can bring about a distinct increase in surface energy. Trials at Plasmatreat have revealed that values up to over 72 mj/m2 are achievable for Dr. Tobias Eckert, Head of the Potentiometer Technology Center at Novotechnik, sees the use of the atmospheric plasma technology as a "milestone" for sensor production. the majority of plastic materials. Not only previously incompatible substrates can be bonded with this method but also adhesion of water-based adhesive or painting systems on very adhesive-resistant surfaces such as unpolar plastics becomes possible in most cases. Pretreating the circuit boards by means of the Openair process makes virtually all adhesive bonding, printing and coating processes feasible in a continuous process flow on both plastic and metal surfaces. Smooth integration Novotechnik sensors find their application not only in the machine building industry but also in automotives. A typical field of application is the motor management where they measure, for example, the throttle valve position or the pedal position. The circuit boards of these sensors are manufactured with a specifically developed conductive ink using the screen printing process. After the Swabian manufacturer had been made aware of the atmospheric plasma process by customers in the year 2000, the potential of the new technology soon became obvious. "The system could be easily integrated into our process and the throughput increased by saving labor at the same time," reports Tobias Eckert, Head of the Potentiometer Technology Center at Novotechnik. Initial tests were firstly performed over two to three months and included, among others, testing of the surface activation and testing of the adhesion (cross-cut). Comprehensive service life tests for quality assurance were furthermore conducted to simulate the load types and the load cycles which can occur during the service life of the product. One customer, for example, demanded evidence for a throttle valve product that the component is capable of withstanding ten
million cycles with no significant changes in its electrical properties. Activation and cleaning within a matter of seconds The Openair process was used by Novotechnik for the manufacture of FR4 circuit boards at the end of 2000 for the very first time. These circuit boards are of glassfiber reinforced epoxy material that is processed in a clean room environment. The material is prestructured and received with etched conductive track structures. Treatment with plasma is necessary to ensure secure wetting of the substratum in the screen printing process as well as good adhesion on the substratum. The plasma flowing onto the surface at almost ultrasonic speed furthermore removes particles adhering to the surface and neutralizes at the same time the electrostatic charge of the circuit boards generated by unpacking. The circuit boards are activated in less than 1 second. They are subsequently transported to the screen printing plant and printed with conductive ink. The flow behavior of the conductive ink is quite likely to change during the screen printing process, and this has an influence on the electrical properties of the finished The equipped circuit board blanks are stoved and cured in the furnace after printing. Each single circuit board, once separated, is subjected to a visual and electrical inspection. product. This is why the screen printing process must be controlled. To this end the wet film thickness is determined by taking samples at random throughout production and by measuring the relevant electrical parameters of the already burned-in components. In this way the process is adaptable while an order is in progress and the electrical properties of the product can be optimized. Each single component, after having undergone the curing process in the furnace, is subjected to an electrical and visual inspection on the computer. Thereafter, the components are either mounted into a housing together with a wiper or delivered as individual components to customers. Triplication of the throughput Apart from components for the automotive industry, Novotechnik also manufactures sensor circuit boards for ship control systems or wind turbine gearboxes with the Openair system. Thanks to the new plasma process, the throughput could be tripled at the works in Ostfildern. This process proved to be safe, fast and cost-effective. It has become obvious that one plasma plant is capable of serving several screen printing lines. The system comes up with minimal susceptibility to failure and with a correspondingly high availability. Since the introduction of the Plasmatreat technology more than 50 million activated individual components have been produced and delivered. There is not one single case where adhesion problems have arisen with the coating. Inès A. Melamies Kontakt: Peter Langhof, Plasmatreat GmbH, Steinhagen,Tel. 05204 9960-0, peter.langhof@plasmatreat.de, www.plasmatreat.de; Dr. Tobias Eckert, Novotechnik Messwertaufnehmer OHG, Ostfildern, Tel. 0711 4489-0, eckert@novotechnik.de, www.novotechnik.de