Electric heating – Metal heating – By arc
Patent
1997-02-18
1998-05-05
Walberg, Teresa J.
Electric heating
Metal heating
By arc
21912169, 21912173, 349 2, B23K 2606
Patent
active
057477728
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of laser marking wherein a marking pattern is marked onto an object such as an IC by means of a laser beam passing through a liquid crystal mask, by raster-scanning the laser beam over this liquid crystal mask on which a desired marking pattern is displayed, and more particularly to a method of reducing image unevenness by decreasing the variation of the transmissivity of the liquid crystal.
BACKGROUND ART
In a laser marking system using a liquid crystal mask, a required marking pattern is displayed on the liquid crystal mask and this marking pattern is marked onto an object by irradiating the object with the laser beam through the liquid crystal mask. The laser beam is made to scan over the liquid crystal mask by so-called raster scanning produced by main scanning and auxiliary scanning.
Japanese Patent Publication No. H. 2-268988 discloses a conventional laser marking device of this type.
In this conventional art, as shown in FIG. 8, the liquid crystal mask is divided into a plurality of blocks and the time required for the laser beam to perform scanning from the starting point to the end point is measured beforehand for each block, and re-writing of the marking pattern is performed in unit of each division by detecting the lapsed time after the commencement of scanning by means of a timer etc. For example, if the liquid crystal mask is divided into two divisions, laser scanning of the lower division is performed after completion of laser scanning of the upper division and the marking pattern displayed in the upper division is concurrently switched over to the subsequent marking pattern.
In this way, re-writing of the liquid crystal mask and laser scanning are carried out in parallel, so the time required for the marking process can be shortened compared with the case where changeover of the display pattern of the liquid crystal mask to the subsequent marking pattern is only effected after completing the laser scanning of the entire liquid crystal.
However, in the conventional art described above, the divided block units are of comparatively large area so variation in the transmissivity and temperature occurs between liquid crystal pixels in the same divided block, causing unevenness of the image produced by marking.
In more detail, even within the same block, there is a considerable difference between the upper region and the lower region in the time that elapses from the time-point of changeover of the liquid crystal until scanning is performed; this time difference causes unevenness of transmissivity. For example, in FIG. 8 referred to above, considering liquid crystal pixels a and b of block A, although the time-point at which the marking pattern of these pixels a, b is switched over is the same, since the laser is scanned in raster scanning from the top left to the bottom right, the time-point of the laser scanning in respect of pixel a is considerably earlier than the time-point of the laser scanning in respect of pixel b. This therefore results in a large difference between pixels a and b as regards the lapsed times from the time-point of liquid crystal changeover until laser scanning is performed; this causes variation of the transmissivity.
FIG. 9 shows the liquid crystal transmissivity characteristic. As can be seen from this Figure, the liquid crystal transmissivity reaches a prescribed transmissivity on the lapse of a prescribed time after the drive voltage is turned on, but thereafter gradually rises as the time for which the voltage is kept on gets longer.
Thus, by making the time for which the liquid crystal is maintained in the on condition after the voltage being turned on fixed as far as possible, the transmissivity of the liquid crystal can be fixed.
With the foregoing in view, it is an object of the present invention to provide a laser marking method wherein the variation of transmissivity of the liquid crystal is reduced so as to enable marking to be performed with little unevenness of the image.
DISCLOSURE OF THE INVENT
REFERENCES:
patent: 5231263 (1993-07-01), Kuwabara et al.
patent: 5309273 (1994-05-01), Mori et al.
patent: 5587094 (1996-12-01), Yoshida et al.
patent: 5605641 (1997-02-01), Chiba et al.
patent: 5608563 (1997-03-01), Matsumura et al.
Matsumura Yukinori
Tanaka Hirokazu
Komatsu Ltd.
Mills Gregory L.
Walberg Teresa J.
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