Television – Special applications – Manufacturing
Reexamination Certificate
1999-11-15
2003-09-16
Kelley, Chris (Department: 2613)
Television
Special applications
Manufacturing
C348S095000, C348S094000, C348S131000, C348S132000
Reexamination Certificate
active
06621517
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to viewing and imaging systems for particular, though not exclusive, application in the field of screen printers for applying solder paste to printed circuit boards. The following description will focus on screen printer applications, but it will be understood that the various aspects of the invention find application in other areas where similar techniques are used.
BACKGROUND
In screen printers for printed circuit boards, the screen is positioned over the circuit board and solder paste is applied to the board through apertures in the screen. To ensure that the solder paste is printed at the correct locations on the board for subsequent component placement, the screen must be aligned with the board prior to printing” To perform the alignment, an imaging device such as a video probe is generally moved between the screen and board to view reference marks, or “fiducials”, at corresponding positions on the screen and board. Images of corresponding fiducials on the screen and board are relayed to vision processing apparatus. The vision processing apparatus determines the relative mis-alignment of the screen and board from the positions of the fiducials in the acquired images, and the relative position of the screen and board is adjusted to achieve alignment to prior to printing.
The general arrangement of one type of screen printer is illustrated in
FIG. 1
of the accompanying drawings. The circuit board
1
to be printed is supported by a base (not shown) of the printer in a generally horizontal plane parallel to the x and y axes shown in the figure. A screen
2
is supported in a frame
2
a
so as to lie above, and generally parallel to, the board
1
. The position of the screen
2
in the xy plane can be adjusted by means of screen positioning motors
3
the operation of which is controlled by a position controller
4
. The support frame
2
a
and screen positioning motors
3
are mounted in a printhead portion (not shown) of the printer which can be pivoted away from the base to allow access when necessary, for example to adjust the board support. The apparatus includes an imaging device
5
which is mounted on an XY table (not shown) for movement in a horizontal plane between the board
1
and screen
2
. The position of the imaging device
5
is also controlled by the position controller
4
.
The surface of the screen
2
facing the board
1
has a plurality of reference marks, or screen fiducials,
6
a
and
6
b
thereon. The surface of the board
1
facing the screen
2
has corresponding board fiducials
7
a
and
7
b
. The positions of the board and screen fiducials are such that when each pair of corresponding fiducials
6
a
,
7
a
and
6
b
,
7
b
are in alignment, the screen is correctly aligned relative to the board for the subsequent printing operation to be performed after simply raising the board into contact with the screen by means of a mechanism provided in the base of the printer.
The imaging device
5
is operated so as to acquire images of the fiducials
6
,
7
and the acquired images are supplied to a vision processor
8
which is programmed to determine the locations of the fiducials in the images. The fiducial location data is then supplied to the position controller
4
for adjusting the position of the screen
2
to bring the corresponding fiducials
6
a
,
7
a
and
6
b
,
7
b
, and hence the board and screen, into alignment. A video monitor
9
a
allows an operator to monitor images supplied to the vision processor
8
. An operator interface
9
b
is connected to the position controller
4
to allow operator control of the apparatus, eg. during preliminary set up procedures.
The general arrangement of a known imaging device
5
is illustrated schematically in
FIG. 2
of the drawings. Here, the imaging device is a video probe comprising a CCD camera
11
and an optical system indicated generally at
12
. The optical system
12
comprises screen and board lighting indicated schematically at
13
and
14
respectively. The screen and board lighting
13
and
14
constitutes “direct lighting” for illuminating the regions A and B of the screen and board with substantially collimated light as indicated by the arrows in the figure. The direct lighting
13
,
14
can be implemented in a number of ways, for example by optical arrangements which receive light from sources at one side thereof and reflect the light upwardly/downwardly towards the screen/board. As indicated by the bold arrows in the figure, light reflected from the screen and board is incident on reflecting means
16
in a return light channel
17
of the system which is indicated schematically by the broken lines in the figure. The reflecting means
16
may be, for example, a prism arrangement which reflects incident light from the screen/board along the return light channel
17
. The return light is transmitted along the channel
17
via various optics (not shown) to the CCD camera
11
. The illuminated regions of the screen and board may be imaged side by side on the CCD array.
While the direct light, which is nominally Perpendicular to the screen and board, is sufficient for viewing fiducials for alignment purposes, if the probe
5
is also to be used for inspection purposes then it is desirable for additional, diffuse lighting to be provided. In particular, if the probe is to be used to inspect the screen
2
for aperture blockage or contamination, or to inspect the board
1
for missing, misaligned or excess print, then the direct light may be insufficient to enable the required features to be distinguished. However, the location of the direct screen and board lighting
13
,
14
above and below the return light channel
17
as shown in
FIG. 2
means that in practice there is inadequate space to provide the required diffuse lighting on the probe if a practical compact design is to be achieved.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided viewing apparatus for location between a screen and board in a screen printer for viewing opposing regions of the screen and board, the apparatus comprising:
reflecting means for reflecting incident light from the screen and board along a return light channel;
beamsplitter means in the return light channel for transmission of incident light from the reflecting means along the return light channel:
lighting means, comprising first and second light sources, for illuminating the beamsplitter means, the beamsplitter means being arranged for reflecting light from the first and second light sources towards the reflecting means and the reflecting means being arranged for further reflecting the light from the first and second light sources towards the screen and board respectively, and
a light barrier, extending between the beamsplitter and reflecting means, for inhibiting cross-talk between light from the first and second light sources reflected by the beamsplitter means.
In embodiments of this aspect of the invention, therefore, the light for illuminating the screen and board is introduced into the return light channel while cross-talk between the screen and board lighting is inhibited by the light barrier. This frees the areas occupied by the screen and board lighting
11
,
14
in the arrangement of
FIG. 2
, so that a compact design can be achieved and the regions occupied by the screen and board lighting
13
,
14
in
FIG. 2
can be used for diffuse lighting means if desired.
While arrangements can be envisaged where the light from the first and second light sources is introduced into the return light channel from above (screen side) and below (board side), it is preferred that the lighting means is located laterally of the return light channel. Further, while arrangements can be envisaged where the light from the first and second light sources is introduced into the return light channel from opposite sides thereof, it is preferred that the lighting means is located on one side only of the return light channel so that light from both light sources is introduced
DEK International GmbH
Kelley Chris
Orrick Herrington & Sutcliffe LLP
Parsons Charles
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