Screen printing

Details Screen printing Screen printing

2002-04-04

2004-06-08

Parker, Fred J. (Department: 1762)

Coating processes

Electrical product produced

Integrated circuit, printed circuit, or circuit board

C427S282000, C427S288000

Reexamination Certificate

active

06746710

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to screen printing and particularly to screen printing in which pasty product to be printed is contained within a screen printing head and delivered through the printing head by applied pressure.
BACKGROUND OF THE INVENTION
It is an established technique in the assembly of printed circuit boards to deposit solder paste where connections are to be made with components, place the components on the paste deposits, and then heat the assembly to re-flow the paste and complete the connections. Screen printing machines have been used to deposit solder paste onto printed circuit boards through the apertures of a stencil or screen.
Solder paste consists of metallic microspheres of solder joined by an organic material or flux. The metallic content of such solder paste typically makes up 50% of the volume, and up to 90% of the weight of the paste. The viscous flux consists of rheologic agents, adhesive agents and cleaning agents some of which are thixotropic and others of which are volatile solvents. The thixotropic property of the solder paste has the effect that relative movement of regions within the paste causes a process of shear thinning to locally reduce the viscosity of the paste.
A typical print will comprise a multiplicity of small blocks of solder paste, and for consistent quality it is essential that each block contains the same proportions of each constituent material. This requires a consistent homogeneous distribution of the materials within the solder paste.
In one traditional screen printing technique, as illustrated in
FIG. 1
, an inclined squeegee
1
is used to push a volume of a pasty product
2
over a stencil
3
which includes apertures
4
and is located above a circuit board
5
, thereby filling the apertures
4
in the stencil
3
and providing a deposit on the circuit board
5
. Forward movement of the squeegee with a horizontal force F
1
, causes a downward Force F
2
to be applied to the pasty product
2
. This downward force F
2
forces the pasty product
2
into the apertures
4
in the stencil
3
, and in conjunction with the adhesion of the pasty product
2
to the stencil
3
causes the pasty product
2
to roll across the stencil
3
as depicted by arrow
6
, thereby shear thinning the pasty product
2
.
There are many problems associated with this screen printing technique. One problem is that exposure of the pasty product
2
to the atmosphere results in evaporation of the solvents of the pasty product
2
and hence drying of the pasty product
2
. Another problem is that increasing the speed of the squeegee
1
to increase the downward force F
2
which forces the pasty product
2
into the apertures
4
of the stencil
3
, not only reduces the time available to fill the apertures
4
, but can also cause the pasty product
2
to slide across the stencil
3
, thereby reducing the rolling effect and hence the shear thinning.
Screen printing heads have been proposed, for example as disclosed in U.S. Pat. No. 4,622,239, which enclose the pasty product to overcome the problems of evaporation, but these printing heads have not addressed the problem of setting a suitable print speed.
WO-A-96/20088 discloses a screen printing head which attempts to overcome both the problems of evaporation and the setting of the print speed by applying a pressure directly to the pasty product. However, this printing head does not provide for a rolling action of the pasty product and hence shear thinning of the same. Furthermore, this printing head requires a very high pressure to be applied to the pasty product. This high pressure can result in the separation of the metallic and flux components of solder pastes which results in inconsistent printing.
WO-A-98/16387 discloses a screen printing head which has been developed partially in response to the known problems of evaporation and the setting of the print speed. As illustrated in
FIG. 2
, this printing head comprises a main body
10
, first and second wiper blades
11
,
12
, which contact a stencil
13
and together with the main body
10
define a chamber
15
containing a pasty product
16
, a grille
17
located at the lower end of the main body
10
, and a piston
18
for applying a downward force F
2
on the pasty product
16
. The stencil
13
, which includes a plurality of apertures
19
, is located above a circuit board
20
onto which deposits of the pasty product
16
are to be printed. In use, the printing head is moved in one of two opposite printing directions, with a horizontal force F
1
, which causes the wiper blades
11
,
12
, which are pressed against the stencil
13
by the force imparted on the pasty product
16
by the piston
18
, to act to lift the pasty product
16
from the region above the stencil
13
and cause the pasty product
16
to pass upwardly through the grille
17
, which pasty product
16
is subsequently forced back downwardly through the grille
17
by the action of the pressure developed by the piston
18
. This rolling action of the pasty product
16
, as depicted by arrows
21
, shear thins the pasty product
16
and thereby enables the pressure F
2
applied by the piston
18
to be maintained at a low level and also prevents separation of the components of the pasty product
16
by the mixing effect of the rolling action. Further, the pressure F
2
applied to the pasty product
16
is independent of the speed of movement of the printing head.
Whilst this printing head provides for much improved screen printing, it has been established that in some circumstances this printing head does not provide for sufficient shear thinning of the pasty product
16
as necessary for a perfect print.
In the screen printing process, as illustrated in
FIG. 3
a
, incomplete tilling of the stencil apertures
19
can result where the pasty product
16
is not sufficiently thinned. Where the stencil apertures
19
are incompletely filled, the action of the trailing wiper blade
11
can shear the pasty product
16
over the stencil apertures
19
, with the result that the pasty product
16
in the apertures
19
is pushed to one, the forward, end of the apertures
19
, as illustrated in
FIG. 3
b
, resulting in only a partial print. In an extreme case, the remaining pasty product
16
in the stencil apertures
19
may not be in sufficient contact with the circuit board
20
such that when the circuit board
20
and the stencil
13
are separated, the pasty product
16
is insufficiently adhered to the circuit board
20
and remains in the stencil
13
, as shown in
FIG. 3
c
, resulting in virtually no print at all. Furthermore, the retention of pasty product
16
in apertures
19
of the stencil
13
can itself lead to problems with subsequent prints since, as mentioned hereinabove, the pasty product
16
is prone to drying out and the drying out of pasty product
16
in the apertures
19
will give rise to printing problems. These problems have been made worse by recent trends in miniaturization which have led to the use of smaller stencil apertures
19
, which miniaturization reduces the area of pasty product to circuit board contact relative to the area of pasty product to aperture wall contact.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a screen printing head for applying a pasty product to a printing screen, comprising: a main body; wiper blades disposed to the main body for contacting a printing screen; a first chamber providing a reservoir for containing a supply of pasty product, the first chamber being defined at least in part by the main body and including at least one outlet opening through which pasty product is in use forced under pressure; a second chamber in fluid communication with the at least one outlet opening, the second chamber being defined in part by the main body and the wiper blades and being in use in communication with the printing screen; and a flow director disposed in the second chamber and configured such as in use to cause a circulatory flow of pasty product contained therein which passes

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