Coating apparatus – Solid applicator contacting work – With work-handling or work-supporting
Reexamination Certificate
2002-04-11
2004-11-09
Edwards, Laura (Department: 1734)
Coating apparatus
Solid applicator contacting work
With work-handling or work-supporting
C118S406000, C118S504000, C118S505000, C101S129000, C228S039000
Reexamination Certificate
active
06814804
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus and a method for printing and applying solder paste onto surfaces of circuit boards.
In the prior art of manufacturing electronic circuit boards, solder paste has been primarily used to solder electronic components such as chips or the like onto printed boards. A solder paste printing apparatus has been employed to print and apply the solder paste in a required pattern.
A squeegee head loaded onto a conventional solder paste printing apparatus
100
is shown in FIG.
20
. Generally, a squeegee head
102
moves from left to right and from right to left in
FIG. 20
, alternately for each printed board
5
, to print the solder paste. A right squeegee
101
a
is used for printing in the right direction, namely, from left to right in
FIG. 20
, and a left squeegee
101
b
is used for printing in the opposite left direction.
Operation of the conventional solder paste printing apparatus
100
will be described with reference to
FIGS. 20-22
. In
FIGS. 20-22
, reference numbers respectively indicate:
3
a mask having openings
4
formed in a required pattern;
5
a printed board;
6
a land to which a solder paste
7
is printed; and
8
a solder resist. The above-mentioned required pattern of the mask
3
is a pattern in which the openings
4
are formed corresponding to the lands
6
on the printed board
5
.
Firstly, during printing in the right direction, the printed board
5
is positioned and overlapped with the mask
3
so that the openings
4
align with the lands
6
. Thereafter, while the left squeegee
101
b
is kept raised, the right squeegee
101
a
is lowered to bring a squeegee front end part
103
in touch (contact) with a surface
3
a
of the mask
3
with a suitable printing pressure. In this state, the right squeegee
101
a
is linearly moved in the right direction to thereby fill the solder paste
7
provided at the surface
3
a
of the mask
3
into the openings
4
of the mask
3
. The printed board
5
is separated from the mask
3
after the right squeegee
101
a
is moved to the right end of the mask
3
, when the printing is finished. Meanwhile, in the case of printing in the left direction, the printed board
5
is positioned and overlapped with the mask
3
, which is similar to printing in the right direction. Then, the left squeegee
101
b
is lowered, while the right squeegee
101
a
is held up, to bring the squeegee front end part
103
in touch with the mask
3
. The operation afterwards is the same as in the case of printing in the right direction. By repeating the operations in the right direction and left direction alternately for every printed board
5
, the solder paste
7
is continuously printed and applied onto the lands
6
of every printed board
5
via the mask
3
.
the conventional solder paste printing apparatus
100
, the squeegee
101
a
or
101
b
is moved while the front end part
103
of the squeegee is kept in touch with the surface
3
a
of the mask
3
with a suitable printing pressure. Each of the conventional squeegees
101
a
,
101
b
executes two kinds of operations (i.e., a scraping operation of scraping the solder paste
7
at the surface
3
a
of the mask
3
and a filling operation of filling the solder paste
7
into the openings
4
of the mask
3
). These operations will be described with reference to
FIGS. 21 and 22
.
FIGS. 21 and 22
are enlarged views of the printing squeegee
101
a
, etc. when the printing is carried out in the right direction. As shown in
FIG. 21
, when the right squeegee
101
a
is lowered to bring the front end part
103
in touch with the surface
3
a
of the mask
3
and moved linearly in the right direction, the right squeegee
101
a
, which touches the solder paste
7
fed onto the surface
3
a
of the mask
3
, moves while scraping the solder paste
7
. The scraped solder paste
7
flows in a rotary direction called rolling as indicated by an arrow I in FIG.
22
. At this time, a fluid pressure is generated inside the solder paste
7
. When the right squeegee
101
a
moves further right to reach the opening
4
of the mask
3
in the above condition, the solder paste
7
is pressed into the openings
4
because of the above-described fluid pressure. In other words, the solder paste
7
is filled in the openings
4
. A pressure with which the solder paste
7
is pressed into the openings
4
will be denoted as a “filling pressure” hereinbelow.
A coordinate system is shown in
FIG. 23
with the printing squeegee
101
a
. A viscosity of the solder paste
7
is &eegr;. An angle of the surface
3
a
of the mask
104
of the printing squeegee
101
a
facing the surface
3
a
(referred to as a “squeegee angle” hereinafter) is &agr;. A speed of the moving squeegee
101
a
(referred to as a “squeegee speed”) is v. It is known that a fluid pressure p produced inside the solder paste
7
is expressed by an equation below:
p=
(2&thgr;
v/r
)*(
A
sin &thgr;+
B
cos &thgr;) (1)
wherein r is a radial position in a polar coordinate system of
FIG. 23
, &thgr; is an angle of the surface
3
a
of the mask
3
to the above r, A=sin
2
&agr;/(&agr;
2
−sin
2
&agr;) and B=(&agr;−sin &agr;* cos &agr;)/(&agr;
2
−sin
2
&agr;).
From the above expression (1), a fluid pressure distribution in the solder paste
7
and a pressure distribution at the face
104
of the printing squeegee
101
a
are as indicated in FIG.
24
. Specifically, a shaded part
105
in
FIG. 24
is where the fluid pressure p (adequate filling pressure) is generated.
Print time in the printing process is reduced in order to improve productivity. However, if the squeegee speed v is made faster in order to shorten the print time in the conventional solder paste printing apparatus
100
, the amount of the solder paste
7
filled in the openings
4
of the mask
3
becomes less as indicated in FIG.
25
. This results in printing failures such as non-filled parts
9
(unfilled parts) and hinders stable printing. The non-filled parts
9
are not generally brought about, for example, if ink is used for screen printing, but are created when a paste-like substance of a mixture of a high viscosity flux with powder solder (e.g., the solder paste
7
or the like is used). The non-filled parts
9
are caused by using the powder solder. In other words, it is impossible to realize a reduction of the print time through an increase of the squeegee speed v in the conventional solder paste printing apparatus
100
.
How the non-filled part
9
results from an increase of the squeegee speed v is described as follows. When the squeegee speed v is increased (more than in the prior art), a time for the front end part
103
of the printing squeegee
101
a
to pass over the opening
4
is reduced. Consequently, a time for the solder paste
7
to be filled in the opening
4
(referred to as a “fill time”) is naturally shortened. The filling pressure becomes maximum when r=0 ( i.e., at a contact point between the front end part
103
of the printing squeegee
101
a
and the surface
3
a
of the mask
3
), as is understood from the expression (1) and
FIGS. 23 and 24
. Although P
r
=p
&thgr;
=∞ is theoretically held when r=0, the contact point is practically a stagnation point and accordingly is at a maximum value.
While the filling pressure itself is raised when the squeegee speed v is increased, a high pressure range is narrow as is seen from the pressure distribution of the shaded part
105
of FIG.
24
. In addition, since the front end part
103
passes over the opening
4
instantaneously, the fill time cannot be long enough. As a result of this, the non-filled part
9
is formed.
In order to prevent an occurrence of the non-filled part
9
, from the expression (1), it can be considered that the squeegee angle &agr; should be reduced and at the same time the filling pressure should be increased, whereby the filling is completed even with a short fill time. However, since the conventional printing squeegee
Higashida Takaaki
Kabeshita Akira
Naito Takao
Onishi Hiroaki
Sato Shoji
Edwards Laura
Matsushita Electric - Industrial Co., Ltd.
Wenderoth , Lind & Ponack, L.L.P.
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