Note: Descriptions are shown in the official language in which they were submitted.
20~0~79
STENCIL FOR USE IN THE APPLIC~TION OF ~ VISCOUS
SUBSTANCE TO A PRINTXD CIRCUIT BOARD OR T~E ~IKE
Field of the Invention
The invention relates generally to an apparatus
used in the application of a viscous substance to a planar
substrate. More specifically, the invention relates to a
stencil and a method of making the stencil for use in the
application of solder paste, component adhesives and like
substances, to a printed circuit board.
Background of the Invention
A printed circuit board (PCB) is the basic
15 building block of most electronic products. In recent
years a variety of components such as resistors, capacitors
and integrated circuit packages have been developed for
mounting directly on the surface of a PCB. The term
~surface mounted component" is commonly used to generically
describe any type of electronic component which is adapted
for mounting on the surface of a printed circuit board.
The technique now in common use for mounting such
components on the surface of a PCB comprises the steps of
applying a controlled amount of bonding material, typically
a viscous substance such as solder paste, on the metallized
areas on the circuit board, placing the component on the
surface of the circuit board such that the conductive
members thereof are in contact with the solder paste-coated
metallized areas, and heating the solder paste to bond the
conductive members of the component to the metallized
areas.
There are various methods by which solder paste
may be deposited on the metallized pads on a PCB. One
approach is by forcing the paste through openings in a
stencil placed in intimate contact with the circuit board.
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The stencil has openirlgs arranged in the same pattern as
metalllzed areas on the circuit board, so that only the
metallized areas are exposed through the stencil openings.
Solder paste is applied to the stencil and a squeegee blade
is moved across its surface to Eorce the paste into the
stencil openings and onto the metallized areas on the
printed circuit board. All of the metallized areas exposed
through the stencil openings can usually be coated with
solder paste during one or two passes of the squeegee blade
lo across the stencil.
Alternatively, an adhesive may be applied through
openings in a stencil to bond leadless components to a
circuit board. The application of solder paste or
adhesives to PCBs is presently realized using rigid
metallic stencils usually made of brass. These stencils
tend to have several drawbacks in that they are expensive
to manufacture and the duration of tlme required to order,
manufacture and receive finished stencils is often
prohibitive. Such stencils also require specialized
storage space in libraries and are difficult to clean after
usage, the latter requiring the use of environmentally
undesirable chemicals.
It is therefore an object of the invention to
provide an inexpensive stencil which overcomes the
aforementioned limitations.
It is also an ob~ect of the invention to provide a
process for fabricating the improved stencil.
In accordance with one aspect of the invention
there is provided a process of fabricating a stencil for
use in the application of a viscous substance to a planar
substrate, the process comprising the steps of stretching a
sheet of polymer material in a two-dimensional plane and to
a predetermined amount whereby tne sheet is under
2090579
approximately uniform tension and its surface is
substantially flat; securing the stretched polymer sheet on
a rigid frame; and perforating the sheet to form a
plurality of apertures in the sheet for allowing the
passage of the viscous substance.
In accordance with another aspect of the invention
there is provided a stencil for use in the application of a
viscous substance to a planar substrate comprising a sheet
lo of polymer material having predetermined dimensions and
being inert to the viscous substance to be applied to the
substrate, the sheet having a plurality of apertures
defined therein for allowing the viscous substance to pass
therethrough, and means for holding the sheet in a
stretched state.
The stencil of the invention may be rapidly
fabricated at a small fraction of the cost of conventional
metallic stencils and it eliminates the requirement of
harsh environmentally hazardous chemicals currently
required in the etching and cleaning of metallic stencils.
Description ~ the Drawings
An embodiment of the invention will be described
in conjunction with the drawings in which:
FIG. 1 is a top view of a conventional prior art
framed brass stencil used in the application of solder
paste;
FIG. 2 is an oblique broken away view of a hole
punching die and uncut polymer sheet attached to a frame in
accordance with the invention;
FIG. 3 is an oblique view of the polymer stencil
of FIG. 2 attached to the frame and ready for use; and
FIG. 4 is a cross-sectional side view of the
framed stencil of FIG. 3 positioned over a printed circuit
board.
2090579
Detai led Descri~t ion
FIG. 1 illustrates a conventional brass stencil
commonly used in the industry. A brass stencil 10 is shown
attached to a screened fabric 14 which is stretched over an
aluminium frame 16. The manufacturing of such a stencil
comprises the steps of stretching the screened fabric 14
over the frame 16, attaching the screened fabric 14 to the
frame 16, positioning the brass stencil 10 over the
screened fabric 14, gluing the borders of a brass stencil
o 10 to the screened fabric 14, cutting away the screened
fabric 14 overlaying the brass stencil 10 intermediate the
borders to expose the usable portion of the stencil, and
applying an emulsion to the exposed screened fabric to seal
its interstices. Brass stencils require special
manufacturing facilities and are thus usually produced by
niche specialized manufacturers situated remotely from the
PCB manufacturers. An urgently needed stencil must
nevertheless usually follow the route of ordering,
specifying, manufacturing and shipping, often resulting in
lost productivity at the PCB manufacturing facility.
In FIG. 2 and FIG. 3 a polymer sheet film 20 is
shown attached to a rigid frame 22. The film 20 is
stabilized by being stretched and then held in the
stretched position by a series of closely spaced pins 24
located on the frame border mating with complementary
openings defined in the polymer sheet film 20. Distortion
in the film 20 is minimized by using a large number of
evenly spaced pins 24 to secure the film 20. In order to
achieve consistent results, it is preferable to stretch the
film using a mechanized system rather than a manual system
and to avoid undesirable bowing of the film, it is
preferable to attach it in a completely flat manner onto
the frame rather than having edges or ends that bend or
fold over the edge of the frame. A sheet of film can be
suitably stretched by a stretching frame which firmly holds
the film along all four edges and applies a predetermined
` 2090~79
tension in the two normal directions along the x-y axis of
the film 20. In order to ensure that the holes defined in
the film 20 fit snugly on the pins 24, a row of holes is
punched at once by stamping the film onto the pins on the
frame using a die 26. In this way the frame 22 and the
pins 24 serve as the punch, each pin punching its
respective hole. Acceptable results were obtained using
MYLAR~ sheet film having a thickness of approximately 10
mils and stretched to about 50 pounds of tension.
Preferably, for this purpose, each pin 24 is made
of hardened steel ground to a flat end with sharp edges for
punching. The frame 22 may be of a lightweight material
such as aluminium stock provided with holes to receive the
pins 24. Alternatively, the frame may be fabricated from
hardened steel and may have integrally formed pins. In
FIG. 4, each of pins 24 is shown to be of length @~.
Preferably, the pins should be no longer than the thickness
of the printed circuit board 27 so that the stencil may lie
flat against it in the case where the circuit board is
supported on a surface larger than itself.
Of course, it is desirable for the film to be
inexpensive, easy to work with and yet be strong and
dimensionally stable. Rigidity and strength must be
considered when selecting a suitable film for stencilling.
If the film is not rigid enough, the tension may distort
the stencil pattern, especially if there are many holes in
the pattern. A variety of polymer materials have been
tested and have been found to be suitable as stencilling
sheet film. MYLAR~, LEXAN~, CAPTON~, ULTEM~, all
provided the required mechanical properties, however,
MYLAR, was deemed to be the most cost effective film to use
at the time of the tests.
MYLAR is commercially available in varying
thicknesses within the range suitable for solder paste
2090~7~
application. The thickness of MYLAR is generally
consistent to within a 5 per cent variance; it is nominally
73 per cent stronger than brass, but is also about 30 tirnes
more flexible. secause of this increased flexibility, it
is preferable to cut the desired stencil pattern in the
film after it is stretched on the frame.
It is also preferable for the stencilling filrn to
be inert with substances with which it will come into
o contact. MYLAR is known to be inert with most generally-
used solvents except ammonium hydroxide, not typically used
in the solder paste application.
Any stencilling material will be prone to wear
caused by the abrasive action of the solder paste as a
squeegee sweeps it back and forth over the stencil.
Degradation of the surface may result in a significant
reduction of the film thickness or, it may cause a
roughening of the surface, making it difficult to wipe the
stencil clean.
Since MYLAR is a polymer, it is significantly
softer than brass, and thus more prone to scratching;
however, the surface of MYLAR film is also
significantly smoother than brass. This, together with the
inherent lubricating properties exhibited by most polymers,
allows the solder paste or adhesive to slide easily with
minimal abrasion. Thus, the durability and cost of a MYLAR
stencil make it an economical product.
After a stencil is stretched and mounted on a
frame a pattern is cut into the stencil to provide a
plurality of openings or holes 28 through which solder
paste may be applied to a circuit board. A punching press
is the preferable choice of cutting instrument allowing
stencils to be perforated at a moderate cost. Alterna-
tively, an Excimer laser produces excellent cutting results
2090~79
at high speed, but generally at a greater cost. A less
preferable, however less expensive method of cutting holes
in a stencil includes drilling the stencil at predetermined
locations to obtain a desired pattern of holes.
s
In summary, the present invention provides an
improved stencilling apparatus and method of making the
apparatus for applying solder paste or like viscous
substances to a substrate material such as a printed
o circuit board.
Numerous other embodiments may be considered
without varying from the spirit and scope of the invention.
