| Give Leaks No Chance |
|
Peter Langhof
|
|
Whether in daily manufacturing or the development of new processes and products: surfaces and their properties are playing an increasingly important role in nearly all industrial areas. The surfaces of the materials employed often have to be pre-treated in order that additional properties may be imparted which will qualify them for further application areas. Open-air plasma is a method of producing new surface properties and optimising existing manufacturing processes. The open-air plasma, applied by special nozzles, effects micro-fine cleaning and extensive activation of all kinds of surfaces and leads to optimum adhesion of coatings, paints and adhesives, such as the epoxy adhesives and hotmelts frequently used in electronics.
The process permits continuous in-line application to all or only some parts of the surface at high speeds. If the nozzle is guided by a robot, high-precision localised effects can be obtained and the finest contours cleaned, activated and coated. Pretty often, a material cannot be used until its surface has been custom treated with open-air plasma.
Electrically neutral plasma beam
The systems, which are based on a nozzle principle, work at atmospheric pressure and produce a plasma with the aid of an arc ignited in the nozzle and the air which serves as working gas. The plasma then flows potential-free onto the product to be treated . It possesses sufficient numbers of excited particles to introduce targeted effects onto the surface (Fig. 1).
A special feature is the fact that the emitted plasma beam is electrically neutral, and this greatly extends and simplifies applicability. Its intensity is so high that processing speeds of several 100 m/min are possible. The heating typically undergone by plastic surfaces is less than 20℃ during treatment. The open-air system works in three ways: it activates the surface by targeted oxidation processes, discharging the surface at the same time and leading to micro-fine cleaning. The nozzle systems employed can be integrated in-line into new or existing production lines. The process of surface discharge means that the open-air plasma system offers cleaning effects which far exceed those of conventional systems. Here, the user is benefiting from the high electrostatic discharging effect of a free plasma beam. This effect is additionally boosted by the very high emission speed of the plasma, as a result of which loosely adhering particles are also effectively removed from the surface.
 | Fig. 1. A plastic surface that was pretreated
as a function of distance and speed with
plasma. Treatment renders the surface
polar and the surface tension rises to >72
mN/m with a large processing window. |
Whether of metal or plastic, the geometries of work pieces are becoming increasingly sophisticated. The fact that the tips of the plasma nozzle can be readily adapted to the work piece geometry opens up a broad range of applications. This adaptability is crucial for pre-treat- ment of highly complex, three-dimensional geometries with a large number of undercuts.
Plasma in automotive electronics
Electronic parts play an increasing role in automotive manufacture as most innovations are electronics-based. Major automotive subcontractors use open-air plasma to protect their highly sensitive sensors and control elements. These perform important tasks, such as managing the engine and air-conditioning, regulating the braking and chassis systems and controlling the lighting system. The IEC standard and the IP degrees of protection stipulate that the sensors and control elements must be protected against penetration by moisture and contamination.
Automotive electronics are exposed to various risks, and so the requirements on the integrity of the protective plastic housing are high: the electronics must withstand temperature fluctuations of -40℃ to 140℃ in the test process and all moisture, liquids and chemicals must be kept at bay. With a view to achieving the best-possible seal, it is standard practice to treat the adherends of electronics packaging with plasma before applying the adhesive. This pre-treatment cleans and activates the housing surfaces to the extent that the increased adhesion properties of the subsequent bonded joint guarantee a seal against penetration of harmful influences, and the risk of short-circuits, malfunctioning and possible long-term destruction are virtually eliminated (Fig. 2).
Plasma pre-treatment not only provides for high quality, it can also be used in-line, is controllable and monitorable and the process is reproducible in line with ISO 9000 due to the constancy of the parameters.
 | Fig. 2. Pre-treatment and activation of the
housing adherends with open-air plasma
prior to application of adhesive causes
optimum sealing of the electronics packa_
ging. |
 | Fig. 3. High adhesion after plasma
treatment: LCD with heat-sealing film (right)
and after tearing of the film (remaining film
strip, left). |
Clean displays
Wherever vibrations occur - in cars, car radios, on-board computers, mobile telephones, pocket calculators or computer monitors - displays are coated with a heat-sealed film. The film is the flexible connection between the printed circuit board and the contact surface or display, which usually consists of two thin glass panes. The foil is bonded to the pane, which has been coated with indium tin oxide (ITO). Glass surfaces, however, get dirty easily, whether during packaging, storage, handling or transport. Finger prints or dust on the glass surfaces are difficult to avoid and require additional cleaning.
The short-term or long-term consequence of this contamination is segment errors, which usually manifest themselves by the fact that too many or too few items appear in the display. The reasons are nearly always the same: the film adheres poorly and particles are present which are causing short-circuits. Most manufacturing processes today still employ conventional manual cleaning of the display glass with the aid of a cotton bud and solvents. The average fail rate of 12 % is common. The situation is totally different for those manufacturers who clean the displays by means of pre-treatment with atmospheric plasma: here the fail rate sinks to below 1% (Fig. 3). Added to which, the protective plastic film, ITO layer and pole filters are not damaged by the careful, potential-free treatment and the entire in-line process is environmentally sound, monitorable and reproducible, as is also the case during circuit board cleaning.
 | Fig. 4 Prior to over-moulding with hotmelt,
the entire surfaces of the components are
treated with open-air plasma (left). Subse_
quent sealing with hotmelt protects the printed
circuit board against moisture and mech_
anical damage (right). |
Compliance with protection guidelines
Outdoor electronics require high levels of protection, since they are permanently exposed to the weather. Novar GmbH, Albstadt/Germany, the renowned security systems manufacturer attached to Honeywell Security AG, attaches special importance to open-air plasma treatment during the manufacture of its high-quality alarm systems. Unlike the case for control elements in cars, which must be accessible at any time and therefore are packaged in a sealed housing, the soldering process for alarm system is followed by electronic encapsulation with the aid of a hotmelt adhesive to protect the mounted boards against moisture or mechanical damage (Title picture).Without pre-treatment,however,the adhesive either does not adhere to the substrate or detaches itself after a while. Here, too, cleaning pre-treatment with open-air plasma is the answer. The plasma nozzle leads the jet across each square centimetre of the plate. The outcome of the micro-fine cleaning is that when the hotmelt is subsequently applied it adheres so well that the highest international degrees of protection (IP 68) and even the relatively new IP 69K are attained - the latter is aimed particularly at the stability of electrical components to high pressure jet cleaning (Fig. 4). After completion, the printed circuit board is inserted into an additional housing where it serves as a keyboard in the outer entrance area of a complete, highly complex intruder alarm system. Failure of this keyboard could lead to a malfunction of the security system, which is why Honeywell - going far beyond the requirements of the standard - subjects each individual board to a twelve-hour underwater functionality test.
Summary
Open-air plasma technology is a new process used in the electronics industry to process parts and circuits during production. High levels of protection has become an important concern and form part of the overall package.
|
|
|
|
|
