Printing – Stenciling – Processes
Reexamination Certificate
2001-04-11
2004-02-24
Yan, Ren (Department: 2854)
Printing
Stenciling
Processes
C123S484000
Reexamination Certificate
active
06694875
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a screen printing method and a screen printing apparatus primarily for printing a print-use paste, such as solder paste or pastes for thick-film circuit formation, onto a board, such as a printed circuit board, for electronic circuit formation.
2. Description of the Related Art
In recent years, screen printing apparatuses have been used for the solder paste printing process in circuit assembly processes of electronic components, or the like. As the board for electronic circuit formation is advanced toward a further fine structure with the miniaturization of electronic equipment, there has been a demand for higher precision of printing with solder paste or the like responsively.
FIG. 16
shows a conventional screen printing apparatus.
A print-object article (article to be printed)
1
is positioned and fixed to a positioning stage
2
that can be ascended and descended by a positioning stage ascent/descent driving means
3
. For printing process, the positioning stage
2
is lifted by the positioning stage ascent/descent driving means
3
to such an extent that the top surface of the print-object article
1
comes in near contact with the bottom surface of a stencil
4
.
A left-squeegee ascent/descent driving means
6
a
and a right-squeegee ascent/descent driving means
6
b
, which are commonly implemented by double-rod air cylinders, have squeegees
5
a,
5
b
attached to their ends. Lower-limit positions of the squeegees
5
a
,
5
b
, (i.e., the push-in strokes to the stencil
4
) are set by positional adjustment of stoppers
7
a
,
7
b.
A drive source for a horizontal reciprocation driving means
8
is commonly an AC servo motor. In the state that the left squeegee
5
a
and the right squeegee
5
b
have descended into contact with the top surface of the stencil
4
, the horizontal reciprocation driving means
8
moves the left squeegee
5
a
and the right squeegee
5
b
horizontally (in the X-direction), so that a print paste
9
, such as solder paste, is moved on the top surface of the stencil
4
by the left squeegee
5
a
and the right squeegee
5
b.
The printing process is carried out as shown in FIG.
17
.
At the first step, the print-object article
1
is positioned and fixed to the positioning stage
2
. At the second step, the print-object article
1
is lifted to a proximity of the bottom surface of the stencil
4
. The left squeegee
5
a
is lowered at the third step, and the left squeegee
5
a
is moved rightward in the X-direction at the fourth step, by which printing is executed.
Thereafter, the print-object article
1
is separated away from the stencil
4
at a low speed (20 mm/sec) at the fifth step, the left squeegee
5
a
is lifted at the sixth step, and the print-object article
1
is removed at the seventh step.
Next, a printing operation on the right squeegee
5
b
side is also executed in the same way as above, and the operation is then alternately repeated.
The drive source for the positioning stage ascent/descent driving means
3
is commonly an air cylinder, pulse motor, AC servo motor, or the like. Among these, the AC servo motor is particularly suitable, because low-speed descent of the driving means
3
allows an easy accomplishment of high-precision printing results with less spread and blurs (i.e., less making indistinct and hazy in outline). The left squeegee
5
a
and the right squeegee
5
b
are given mainly by elastic material, commonly urethane rubber (hardness: Hs 60-90°).
In this way, successful prints free from spread and blurs can be obtained continuously. However, there are issues as shown in
FIGS. 18A
to
18
C.
FIG. 18A
shows a completion state of the fourth step,
FIG. 18B
shows an early-stage state of the fifth step, and
FIG. 18C
shows a last-stage state of the fifth step. In the completion state of the fourth step as shown in
FIG. 18A
, a print pattern
11
a
has been formed with relatively good precision. In this state, the left squeegee
5
a
is curved only by a portion corresponding to the push-in stroke into the stencil
4
.
In
FIG. 18B
, the left squeegee
5
a
pushes down the stencil
4
to an extent of the push-in stroke, so that the stencil
4
is tilted, causing the print pattern to gradually collapse as shown by a print pattern
11
b.
As a result, as shown in
FIG. 18C
, a horn
10
with the print paste
9
lifted is formed at a corner portion of a print pattern
11
c.
The horn
10
of the print paste
9
would gradually bow, and drop onto the print-object article
1
, causing printing faults as an issue.
As an example, in electronic component assembling processes typified by solder paste printing, the solder paste bowed and dropped after the subsequent-process soldering reflow would cause soldering faults such as solder balls and solder bridges.
In recent years, in the fields of screen printing methods and apparatuses therefor, there have increasingly been cases where screen printing is executed on boards on which an area corresponding to a large opening area of a screen metal mask (stencil) and an area corresponding to a minute opening area thereof are mixedly present in circuits. The term “minute opening area” refers to an area where the value of each opening size of openings of the mask along the squeegee's moving direction is smaller than a specified threshold. The term “large opening area” refers to an area where the value of each opening size of openings of the mask along the squeegee's moving direction is not smaller than a specified threshold.
Now a case where a board in which an area corresponding to a large opening area of the mask and an area corresponding to a minute opening area thereof are mixedly present in circuits is screen-printed by a conventional screen printing method and apparatus therefor is described with reference to
FIGS. 19
to
22
.
Referring to
FIG. 19
, which is a perspective view of the screen printing apparatus, reference numeral
101
denotes a screen metal mask;
102
denotes a board;
803
denotes a print head;
104
denotes a print-head-use AC servo motor for driving the print head
803
;
105
denotes a print-head-use ball screw for transferring the driving force of the print-head-use AC servo motor
104
;
106
denotes a print-head-use AC servo driver for driving the print-head-use AC servo motor
104
;
107
denotes a visual recognition camera for recognizing recognition marks of the screen metal mask
101
and the board
102
;
108
denotes a recognition-camera-use AC servo motor for driving the visual recognition camera
107
;
109
denotes a recognition-camera-use ball screw for transferring the driving force of the recognition-camera-use AC servo motor
108
;
110
denotes a recognition-camera-use AC servo driver for driving the recognition-camera-use AC servo motor
108
;
811
denotes a controller for issuing commands to the individual servo motor drivers;
112
denotes a control panel for entering data into the controller
811
;
113
denotes a stage for restricting the board
102
;
114
denotes a stage-use AC servo motor for driving the stage
113
;
115
denotes a stage-use ball screw for transferring the driving force of the stage-use AC servo motor
114
;
116
denotes a stage-use AC servo driver for driving the stage-use AC servo motor
114
;
117
denotes a loader for carrying in the unprinted board
102
;
118
denotes an unloader for carrying out the printed board
102
; and
119
denotes the main unit of the screen printing apparatus.
Referring to
FIG. 20
, which is a plan view of the screen metal mask
101
, reference numeral
120
denotes a large opening area where solder paste in large openings of the mask
101
are printed on the board
102
(e.g. an area corresponding to a chip component area of the board
102
) and
121
denotes a minute opening area, where solder paste in minute openings of the mask
101
are printed on the board
102
(e.g. an area corresponding to a narrow-pitch QFP (Quad Flat Package) area of the board
102
).
The operation of
Chikahisa Naoichi
Kakishima Nobuyuki
Nagafuku Nobuyasu
Nagasawa Yousuke
Naito Takao
Wenderoth , Lind & Ponack, L.L.P.
Yan Ren
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