Marking of Computer Keyboards by Means of Excimer Lasers
G. Ricciardi (2). M. Cirntello (2), G. Savant Aira, IsiiilItn RTM, Vico Canavese (TO), ItJly Received on January 11,1996
An excimer laser-based technique for marking computer keyboards has been developed with the aim of achieving high speed and flexibility on the production line and of avoiding the use of toxic solutions for printing and varnishing. The marking process was developed by investigating contrast and depth of colour change as a function of laser parameters as well as of polymer composition. The laser-printed symbols on various optimised polymeric materials (polystyrene, PC. ABS), both dark and light grey, passed all computer keyboard standards tests This article also describes the prototype system developed for industrial pilot production
Kevwords: Excimer laser, Processing. Plastic
In the field of industrial marking of polymer materials used for casings, cable insulators, medical instruments and a variety of other applications, high production rates, combined with good contrast, geometrical definition and wear resistance of printed symbols, are normal requirements. The most common technique employed IS based on ink printing, often followed by lacquer spraying. Despite the high processing speeds, this technique entails a number of serious problems associated with low system flexibility, frequent shutdown time, and environmental pollution involved in the handling of colour ink and lacquers. An alternative technology, based on the use of CO,, Nd- YAG or excimer lasers, offers several advantages over conventional printing methods [ 1). As a non-contact technique, laser marking does not involve the introduction of any foreign material or exert any mechanical stress on the workpiece. The marks, which may also be produced on a moving part, are permanent and cannot be washed off or easily abraded, since they are produced by etching or colour changing of the material itself The possibility of achieving a high level of automation is also worthy of consideration The particular advantage of excimer lasers in marking processes is due to the short laser waveiength and pulse duration High photon energy can induce photochemical reactions in materials, resulting in colour change with negligible thermal side effects. The short UV wavelengths favour resolution of an optical system, so that micron- sized features can be produced. Poor coherence of free- running excimer lasers makes them suitable for projection marking, whilst the broad beam cross section with relatively good spatial uniformity increases throughput. Moreover, the possibility of marking the finished article can drastically simplify production flows and reduce inventories.
2. PARAMETERS AND STANDARDS FOR PC KEYBOARDS
The contrast of a mark is determined by the reflectivity of the background and foreground, which in turn also depends on the illuminating wavelength. The reflectivity can be measured either for each single wavelength or as an average value over the visible spectral range. For both cases, it is possible to define the following:
Contrast: C = (Rb-Rf) I Rb Ratio. R = R b l Rf
where Rb and Rf are background and foreground reflectivity measured over the spectral range of interest. Total colour difference (dE) between the mark and the background, according to the Norma ASTM D2244. is also used to characterise the mark:
dE = J(dL)2 +(da) +(db)
L = 0 corresponds to black L = 100 corresponds to white da > 0 means colour shifted towards red da < 0 means colour shifted towards green db > 0 means colour shifted towards yellow db < 0 means colour shifted towards blue
. . . - . . .-
2.1 Product Reauirements
The ratio between background and mark reflectivity must be higher than 3.0 for the central (light grey) keys and over 2.2 for the peripheral (dark grey) keys.
The change of the total colour difference dE between mark and background after 7 500 000 keystrokes, which correspond to 3 500 duty cycles of the testing apparatus, must be less than 2C% After the standard sun test (100 hours) the total background colour change (dE)b should not exceed 1 0.
Annals of the ClRP Vol. 45/1/1996 191
3. EXCIMER LASER-BASED TECHNIQUE
3.1 Theoretical backqround
Titanium dioxide is a filler commonly added to mar.y plastic materials as a pigment for aesthetic and protective purposes High-power radiation from an excimer laser can induce photochemical changes in white 1:tanium dioxide by removing some oxygen (2).
TiO, -+ TiO,., , O c n c 0 . 5 The resulting non-stoichiometric material poorly reflects visible light, while the overall reflectivity of the laser- affected area depends on the pigment transformation rate. The absorption coefficient of pure TiO, has a maximum at around 300 nm. In previous investigations it was shown that marking by induced colour change, albeit with less efficiency, can also be achieved at laser wavelengths outside the UV This highlights the fact that the process is not a purely photochemical one, but that also some thermal effects play a role in TiO, transformation. There have been very few studies on the mechanism of laser-induced colour change in polymers, whereas ablation processes of organic materials have been extensively investigated. S.W. Williams  demonstrated experimentally that when the energy density of excimer laser radiation is raised above the minimum value necessary for inducing a detectable colour change up to a limit of saturation, mark contrast also increases Further increments in laser intensity do not induce any additional changes until the threshold for ablation is reached. The threshold values for both phenomena depend on laser wavelength, pulse duration, and material properties (3.41. Working with low-absorbing polymers and using longer wavelengths requires a much higher intensity to achieve sufficient absorbed energy for ablation. According to the dynamic model, if the energy absorbed by the sample during the first laser pulse is not sufficient to reach the ablation condition, a number of laser pulses are then necessary in order to accumulate a sufficient volume of gaseous fragments beneath the interface (51
The first marking experiments were carried out on ABS materials normally used by Olivetti for Personal Computer keys, and on polycarbonate (PC) used for computer and peripheral casings The experiments were conducted on standard Olivetti both light- and dark- coloured materials. As the best overall results were achieved on PC material, which also has better mechanical properties but is too costly to be acceptable for the specific application, subsequent research was oriented towards marking and optimisation of a PClABS mixture and polystyrene.
3.3 Laboratory setup
A Lambda Physik LPX 315 excimer laser was used at the wavelengths of 248 nm and 351 nm, with a pulse duration of 25 ns. The maximum available energy was 800 mJ at 248 nm and 450 mJ at 351 nm The beam
cross sectOon was 30x10 rnrn? with nearly top flat intensity distribution The background and forec-ound reflectivity was measured with a standard Gamma Scientific photometer on the 0 4-mm diameter spo!s Energy variation through the optical system was obtained by means of a variable attenuator mounted after the laser output coupler The energy density on the substra:e was deduced by measuring the power trarsmiCad by :he imaging lens and the affected area on the sample surface Surface roughness was anaiysed with a RODENSTOCK 2000 optical profiler
4. RESULTS AND DISCUSSION
4.1 Markina of ABS and PC w lv rne rs of standard composition
The samples of ABS and PC were marked using the laser at an operating wavelength of 351 nm and with an energy density of 0.75 Jlcm (the maximum one available to achieve a mark of the required dimensions) and varying the number of laser pulses. For all rhe samples examined, the first laser pulse caused a surface lifting of between 2 pm and 4 pm. as well as an increase in surface roughness. The subsequent pulses induced ablation of the material (Fig. 1). and consequently a lowering of the characteristic mark ratio (contrast). From microscopic examinations of the vertical section of the mark, with x500 magnification, it was seen that in a thin layer of a few micron beneath the lifted surface the material took on a spongy structure. This effect can be explained as the trapping of small molecular products of polymer-laser interaction, which causes foaming i f the local temperature reaches the glass-transition or melting point . When marking was performed with a single laser shot and an energy density of 0 75 Jlcm, the depth of colour change was higher for PC samples, whilst dark ABS material was marked to a depth of only 6 pm. Wear resistance of the mark on the light ABS (18 pm deep) was just on the limit of acceptability. The marks on PC samples presented an interesting behaviour at the wear test, after which it was found that the measured mark ratio and dE became dightly higher. This phenomenon can be explained try the rather deep colour change and by the improved surface smoothness after wear, which reduced the scattering of light
n = 5
- Fig. 1 Surface profile of ABS light-coloured material exposed to 1, 2 and 5 laser pulses at 0.75 Jlcm.