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

1. INTRODUCTION

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)’

where

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 [2] 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

3.2 Materials

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 [6]. 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 =1

JJ

n = 5

- Fig. 1 Surface profile of ABS light-coloured material exposed to 1, 2 and 5 laser pulses at 0.75 Jlcm’.

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As the PC materials were marked with sufficiently high contrast and depth of colour change, the energy density was reduced to 0.60 Jlcm' in order to see the effects on surface foaming and contrast In this case the rat13 was more than 10% higher owing to the reduced surface foaming, and the mark depth remained sufficiently high and resistant to wear.

4.2 Compound characterisation

On the basis of the results obtained in the first experimental stage, the experiments proceeded with marking of PClABS. with the application of a single laser pulse to avoid ablation of the material. The aim set was to find the main factors contributing to surface foaming under laser radiation and to optimise the materials in order to eliminate this effect at the energy densities necessary to produce a sufficiently dark and deep mark. Investigation of the marking phenomena was carried out in the following steps: 1) Markmg of the basic PC/ABS polymer containing only the standard agents. such as UV stabilisers. lubricants. antioxidants. antistatics etc., without any pigment fillers. In this case the areas irradiated by applying one laser pulse and an energy density of 0.75 Jlcm? were not "covered" with the reflective layer This result was very important as it meant that the surface foaming was not inherent to the basic polymer, but caused by the presence of the pigments. 2) Determination of mark contrast and depth as a function of titanium dioxide concentration (from 0.5% to 5.0%)). Both parameters increased significantly as the content of TiO, decreased, as is depicted in Figs. 2 and 3. Optimum values are below 1%. 3) Foaming of surface as a function of TiO, and carbon black additives. Foaming is very sensitive to carbon black content. To achieve a reasonably large mark, PClABS containing 0.5Oh of TiO, can be marked, with no foaming effect on material if the concentration of carbon black is lower than 0.7% (see Figs 4 and 5). As the concentration in the Olivetti standard dark grey samples was 0 025%. it is obvious that the amount of TiO, added to the material is a determining factor for mark quality in terms of low reflectivity and significant depth of colour change

d [uml

3 1 '. ... 1. -.... I 9.. .. , . _ ! a 2 TC?(%) 5

Fig.2 Mark reflectivity as a function of TiO, % for PClABS samples mthout CB, marked at 0.75 Jlcm'

Fig.3 Depth of mark produced with one pulse at 0 75 Jlcm' for different TiO, % in PClABS devoid of CB

4.3 Optimised PClABS Compound

According to the results obtained, samples of PClABS with only 0 5% of titanium dioxide were prepared. Carbon black and some other colorants that are non-absorbing in the UV, were also added in the quantities required to meet the standard Olivetti light and dark colours. Marking was carried out with a single laser pulse and an energy density of 0.75 Jlcm'. The characteristic ratio obtained was over 4 0 on the light plastic materials and over 2.4 on the dark, i.e., above the values required by the computer key standard. The ratio measured both before and after material optimisation, together with a lower limit value, is shown in Figs. 6 and 7. No surface foaming was observed, and the roughness, which was checked with the optical surface profiler, remained unchanged after exposure to laser radiation. Mark depth was (100fl0) pm on both light and dark materials, so that the mark colour change d(dE) after the wear test proved negligible. The dark samples also passed the sun test successfully, whilst the light samples exhibited a background colour change (dE)b slightly above the value of 1.0 set by the standard. This effect is caused by the reduced amount of titanium dioxide, which is normally present in concentrations of a few per cent in light polymer materials. a fact that also plays a role in UV polymer stability. Marking was also carried out on PClABS samples of a standard light blue-grey colour used for all the keys of the Olivetti notebook computer keyboards. Also these samples contained 0.5% of titanium dioxide. The results of marking were similar to those obtained on the light- grey PC/ABS material. Reflectivity of mark and background was measured with a Gamma Scientific photometer on areas of a diameter of 0.4 mrn. For each sample the measurement was made in three points, and a slight variation in contrast, i.e.. the point-to-point ratio, was observed. In the 0.65 to 0.75 Jlcm' energy density range, the ratio had a constant value of 4.220.2 After exposure to an energy density of 0.58 Jlcm', the ratio measured had a value of 4.0+0.2. This means that any variations in laser output energy of up to +7% have no influence on mark quality, this being a conservative

I

10

Fig.4 Threshold for surface foaming as a function of TQ% for PClABS samples free of carbon black.

10 Ixi 9.1

05:

04

8

1

0 ' ...T ""' ' '

'I rnl 0 01 a 3 1

Fig.5 Threshold for surface foaming on PClABS having 0 5 % of TiO, and varying carbon black content

193

... .. -~ . ... .

i PCllBS intcrlighl !

,.--.. .,, I I - a ._. .

I

5

2 ; '

Lnknpth lnml 350 r50 550 6M 750

e i . , Figs. 6 and 7

dark samples before and after material optimisation; measured over the entire visible range ( 400 -750 nm)

Mark contrast ratio for PClABS light and

estimate as it neglects possible errors in measuring energy density. The depth of colour change was (100?10) pm, and the difference in background colour, in terms of dE, before and after the sun test was only 0.41, so that the material can be considered stable under long-term UV exposure.

4.4 Markina of computer kevs

The computer keys were made of PClABS material, containing 0.5% of titanium dioxide. in a light blue-grey colour used for the keyboards of Olivetti notebook computers. Marking was done using a single pulse at a laser energy density of (0 7020 7) Jlcm'. The first set of keyboards marked had a very rough surface, with Ra = 4.6 pm, measured to an accuracy of ?5%, whereas the plane samples used in previous experiments had Ra = 0.2 pm Surface preparation proved to be an important factor for mark quality as the ratio obtained on the keycap was only 3.0k0.1, as compared to the ratio of 4.250.2 obtained in the marking of plane samples of the same material. The second set

Fig. 8 Computer key marked with one pulse per symbol at an energy density of 0.70 Jlcm'

of keys marked wi!h the same laser parameters showed an improved smoothness with Ra = 1 5 pm The backgroundhark reflectivity ratio in this case was 3 2t0 2. i e , still much lower than that measured on the smoo!h surface This difference was not caused by changes in mark reflectivity. but by background reflectivity. which is very sensitive to surface quality Fig. 8 shows a photcgraph of a key marked with two laser shots. one per symbol

5. EXCIMER MARKING CELL 5.1 ConceDtual desian

A preliminary study was carried out in order to optimise the system architecture of the marking cell, with the aim of achieving high modular design, an easily reconfigurable system, and availability for further upgrades. The study took into consideration two possible architectures with the purpose of achieving a better compromise between industrial requirements (in terms of productivity, quality, flexibility, reliability and costs) and the performance levels that the marking system must have For both solutions, the excimer laser beam was first homogenised. then shaped by a mask and projected onto the working plane The basic difference between the two solutions consisted in the mask unit device. In the first setup (see Fig. 10). the mask unit consisted of a series of thin disks made of a dielectric layer on a quartz substrate. All the symbols needed for marking the various keyboard layouts were set out on the outer rim of the disks. During the marking process the disk rotated and the laser beam intercepted the symbols, which were thus reproduced on the keys. The second solution involved the use of a rectangular mask holder having the same dimensions as the keyboards The mask holder bore the plates, which were also made of a dielectric layer on a quartz substrate and which made up the board pattern (Fig. 11) The mask was moved by means of an X Y high-precision positioning stage for intersecting the beam. The motion of the keyboard was synchronised with the mask-holder unit by means of an appropriate control system The first solution. i.e., the one based on rotating masks, is interesting from the productivity standpoint because it could enable "in parallel" marking of more than one board by using an appropriate optical device which splits the laser beam A further advantage of this architecture is that it should afford the possibility of changing the masks or carrying out maintenance of some subsystems without suspending the marking process However. upon closer analysis, this architecture proved to be technically very risky. On account of the high speeds involved, it is very difficult to build a control system that is capable of real- time management of all the process parameters Furthermore, the very high speeds of rotation and angular accelerations are to be expected to induce some dangerous stresses in the material (quartz) of the masks. This fact introduces an additional critical parameter into system reliability For the above reasons, it was decided to reject the first solution and retain the second.

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5.2 System Architecture ACKNOWLEDGEMENTS

\Ne gratefully acknowledge the support provided by the EC Commission for this research, which was partially funded under the BRITE Project No 5541 "High-quality laser marking of polyveric materials by means of an excimer source"

Starting from the conceptual layout defined during the foregoing phase, the final configuration of the marking cell presents the following main features (Fig. 12).

Modular configuration of various subsystems High-speed processing: 50 mlmin. with 1 g acceleration Large markable area: 600x600 mm' High accuracy: maximum tracking error of 0.02 m m Marking time of 15 seconds for a complete keyboard On-line measurement of mark contrast Automatic loadinglunloading of workpieces

The marking cell consists of two Cartesian 2-DOF robots working in parallel. The working area of each robot is set on two parallel planes. The robot working on the upper plane moves the mask in co-ordination with the lower robot which has the task of moving and positioning the workpiece. The cell structure is made up of light sections having the appropriate characteristics for guaranteeing high overall machine stiffness The laser fires at maximum repetition rate, while the mask and the workpiece move counter to one another The control system synchronises the motion of the axes and the laser pulses. By means of a high-speed vision algorithm, another computer checks the quality of the marks at a maximum rate of 32 samples per second A dedicated end-effector automatically loads and unloads the keyboards onto/from a conveyor line. Figs 13 and 14 show the marking cells for marking a desktop PC keyboard during pre-production tests at the RTM Institute

6. CONCLUSIONS

An excimer laser process for producing symbols on computer keys with the quality required by standard specifications has been described The material chosen to obtain an optimal (mark quality)/price ratio was a PC/ABS mixture. Foaming of the polymer surface under laser radiation was also studied, and the main factors involved in this process were determined It was shown that foaming effects reduce the mark contrast and that this problem can be overcome by keeping the titanium dioxide concentration at low levels. In this way also the depth of colour change IS improved. The contribution of another pigment to the foaming phenomenon, i.e., carbon black, which is also used to obtain the right colouring of plastic materials, becomes significant only at high concentrations, which are not of interest in computer key production. Grade of surface preparation also plays a role in marking quality, since background reflectivity determines the mark contrast. A laser marking cell which is able to meet all the requirements, with a nominal throughput o f 1 keyboard every 15 seconds, has been developed and successfully

REFERENCES

!1] G. D. Paulin. P A. Eisele. T A. Zn'otins. "Advances in excimer laser materials processing: an update", SPlE Vol 1023 Excimer Lasers and Applications p.202. (1 988).

[2] S. W. Williams. Optics and Laser Technology Dept., Sowerby Research Centre. British Aerospace PIC "Excimer laser printing of aircraft cables", unpublished paper.

[3] V Srinivasan. M A Smrtic. S. V. Babu, "Excimer laser etching of polymers". J Appl. Phys. 59 ( l l ) , p 3861, (1986).

[4] S Lazare. V. Granier. "Ultraviolet laser photoablation of polymers: a review and recent results", Laser Chem. Vol. 10, p 25, (1989).

' ;5] E Sutcliffe. R. Srinivasan, "Dynamics of UV laser ablation of organic polymer surfaces", J. Appl Phys. 60 (9), p.3315, (1986).

[6] J H. O'Donnel. "Chemistry of Radiation Degradation of Polymers", p.403. (19 .).

[7] Summary Report BE5541, "High-quality laser marking of polymeric materials by means of excimer source", (1 9. )

Fig. 9 A sample keyboard marked with the excimer tested marking cell

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Fig. 12 Final System Architecture

196