GB2292115A

GB2292115A - Solder stencil mounting frame

Info

Publication number: GB2292115A
Application number: GB9519608A
Authority: GB
Inventors: Nicholas Mark Baker
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-11-17
Filing date: 1995-09-26
Publication date: 1996-02-14
Anticipated expiration: 2015-09-26
Also published as: GB9519608D0; GB2292115B

Abstract

A frame for mounting a metal stencil for use in the printing of solder paste on to printed circuit boards, comprises a rectangular frame (46) having along each side thereof a mounting bar (44, 53) having a plurality of attachment means (80) for attaching an edge portion of a stencil (82) thereto, at least two adjacent mounting bars being slidably mounted to move outwardly and inwardly of the frame, and the frame including manually-operable means (40, 41, 48 - 50) for moving the movable mounting bars outwardly of the frame together, whereby the stencil may be tensioned in two directions. Pivotal levers (41) may be operated to urge the opposed bars apart by respective rotatable cam shafts (40) and axially movable cam shafts (49). Alternative actuating mechanisms are disclosed. <IMAGE>

Description

SOLDER STENCIL MOUNTING FRAME Field of the Invention This invention relates to a frame for mounting a stencil for use in printing a solder paste pattern on to a printed electrical circuit board, and to a stencil used with the frame.

Background to the Invention In electrical printed circuit boards used for surface mounting of components, rather than mounting in holes formed in the board, pads of solder paste neea to be provided on the circuit elements where components are to be mounted.

This is achieved by screen printing the solder paste on to the board using a metal stencil with apertures corresponding to the positions and shapes of the solder pads.

The stencil is held in a frame during this operation.

Conventionally, a solid or hollow box section aluminium frame has a flexible plastics mesh stretched across it, to which the stencil is attached by adhesive, the mesh on the working surface of the stencil then being cut away. This system requires the use of a separate frame for each stencil, increasing the cost of the stencil and presenting a problem with storage if a number of different stencils are used.

In order to try to overcome these disadvantages, various different types of frame have been proposed. In US-A-3 769 908 there is disclosed a screcn-printing frame for use in the initial manufacture of printed circuit boards, having a mesh stretched over the frame with an apertured metal stencil overlying it. The edges of the screen are bent over the frame and locked in grooves surrounding the frame by means of locking strips. WO 92/08616 discloses a frame for use in solder paste printing, in which the stencil is mounted across an aperture in the frame, the mounting on one side being linked to the frame through an inflatable flexible member in order to pull the one side of the stencil relative to the first when the member is inflated. The frame only tensions in one direction, and is relatively complex to install and use.

GB-A-2 264 460 discloses a system in which the stencil is bent over a frame so as to be held along two opposed edges out of the plane of the body of the stencil. The stencil is provided with a large number of elongate slots along the opposed edges, and these are engaged by teeth in comb-like members on the frame which are rotatable to apply tension to the stencil. Positioning of the stencil on the frame requires care, since the elongate slots increase the likelihood of distortion of the edges of the stencil. The side edges of the stencil may require stiffening to stabilise them.

Summary of the Invention According to the invention there is provided a frame for mounting a metal stencil for use in the printing of solder paste on to printed circuit boards, comprising a rectangular frame having along each side thereof a mounting bar having a plurality of attachment means for attaching an edge portion of a stencil thereto, at least two adjacent mounting bars being slidably mounted to move outwardly and inwardly of the frame, and the frame including manually-operable means for moving the movable mounting bars outwardly of the frame together, whereby the stencil may be tensioned in two directions.

All of the mounting bars may be movable, with the manually-operable means being provided at two opposite corners of the frame to move the two adjacent bars together. Alternatively, two adjacent bars may be fixed, and the other two movable, the manually-operable means being operable between them.

The manually-operable means may comprise a lever rotating a cam acting on the bar or on projections therefrom, the ends of the cam member acting on the adjacent bars by way of a screw-thread action. Return springs may be provided to bias the bars inwardly of the frame. The springs may be helical springs acting on the bar directly or on the projections engaged by the cam. Alternatively, a slidable cam rod may be used to urge at least one of the movable bars outwardly, the rcturn movement being again accomplished by means of springs.

In order to assist in tensioning of the stencil in the frame, it may be desirable to provide regions of reduced strength along the edges of the stencil, to facili tate stretching. These regions may be formed by etching or machining to produce a pattern of metal of reduced thickness, for example, or a pattern of holes through the stencil.

Preferably the stencil is provided with spaced apertures along each edge to fit on to mounting pins on the bars. Five mounting pins are suitably located along each bar, each pin being elongate in plan longitudinally of the bar, and more preferably presenting a straight surface outwardly of the frame to engage a corresponding straight edge of the aperture in the stencil. Each pin is preferably formed with a slightly enlarged head thereon to help retain the stencil thereon. It will be appreciated that fewer or more mounting pins may be used, depending on the size of the frame, for example.

Advantages of the frame and stencil of the invention include the avoidance of any bending of the plate, ensuring a flat stencilling surface and obviating the need for additional stiffening of the sides of the stencil as in the conventional single-direction tensioning systems, a larger stencilling area available, the removal of any risk of solder paste falling through the sides of the stencils, and the reduction of the risk of damage to the stencils when fitting into or removing from the frame. Tensioning in two directions also ensures that the stencilling surface is flat and free from distortion.

Brief Description of the Drawings In the drawings, which illustrate exemplary cmbodimcnts of the invention: Figure 1 is a plan view of the frame according to a first embodiment of the invention, before fixing of the stencil; Figure 2 is an enlarged sectional view of one of the side bars of the frame shown in Figure 1, showing the stencil fitted thereto; Figure 3 is an enlarged plan view of a corner portion of the frame of Figure 1; Figure 4 is an enlarged plan view of a corner portion of a frame according to a second embodiment of the invention; Figure 5 is an enlarged plan view of a portion of one side of the frame of Figure 4, with the mounting bar removed; - ~ Figure 6 is a cross-sectional view on line A-A of Figure 5, with the mounting bar shown;; Figure 7 is an enlarged plan view of a portion of one other side of the frame, again with the mounting bar removed; Figure 8 is an enlarged perspective view of a pin on one of the mounting bars of the embodiment of Figures 4 to 7; Figure 9 is an enlarged plan view of the pin of Figure 8, showing how the stencil fits on to the frame; Figure 10 is a cross-sectional view of a portion of one side of a frame according to an alternative embodiment; and Figure 11 is an enlarged plan view of a corner portion of a frame according to another embodiment of the invention.

Detailed Description of the Illustrated Embodiments Referring first to Figure 1, the square frame 1 is suitably cut from a single piece of aluminium or an alloy thereof, but may also be fabricated from separate components. Each side of the frame has a mounting bar 2 slidably mounted thereon by means of screws 3 passing through elongate slots 4 in the bars 2. Each mounting bar has a number (here shown as five merely as an example) of mounting pins 5 projecting therefrom. The pins will be described hereinafter in more detail with reference to Figure 2. In two opposite sides la and Ib of the frame 1 a channel 6 is formed, in which is mounted an elongate cam member 7 having at the two ends thereof oppositely-handed screw threads 8 engaging in threaded bores in slidable blocks 9.A handle 10 at one end of each cam member 7 permits the cam member to be rotated through approximatcly 90 degrees. Rotation of the cam member 7 drives the blocks 9 inwardly or outwardly simultaneously, because of the opposite screw threads 8. When the blocks move outwardly, they engage the mounting bars 2a on the other two sides ic and id of the frame and force these bars 2a outwards, as may be seen from Figure 3.

Referring to Figure 2, the mounting bars 2b on the sides la and ib of the frame are provided with spaced cam-engaging pins 11 directed downwardly into the frame side members la and lb, in which suitable apertures 12 (indicated by broken lines) are formed communicating with the respective channel 6. The pins 11 thus project into the channel 6 when the bar 2b is in its innermost position relative to the frame, and are urged back into the apertures 12 when the cam member 7 is rotated towards the position shown in Figure 2, thus driving the bar 2b outwardly of the frame at the same time as the bars 2a are urged outwardly by the action of the screw threads 8 on the blocks 9.A saucer type of spring 13 is located in each aperture 12 to bear against the pin 11 to bias it, and therefore the bar 2b, inwardly to assist in removing the stencil after use, when the tension is released.

The other bars 2a may also be provide with return springs, or the blocks 9 may be attached to the bars so that, as the cam members are rotated to their release position, the bars 2a are necessarily drawn inwards with them.

The shape of the mounting pins 5 may be seen from Figures 2 and 3. In plan, the pins 5 each have a straight outwardly-directed side 5a and a convex inwardly-directed side 5b. The outwardly-directed side 5a has a lip Sc to help retain the stencil 14 thereon prior to tensioning. The stencil 14 has apertures 15 along each edge corresponding in position to the mounting pins 5, each aperture also having a shape similar to the plan of the pins 5, but slightly larger all round to facilitate location on to the pins.

The frame 1 is provided with a circumferential raised edge 16 whose upper surface lies in the same plane as the upper surfaces of the mounting pins 5, to provide a uniform supporting surface separate from the pins.

Figures 4 to 9 illustrate an alternative arrangement, in which two adjacent sides of the frame are fixed and the other two sides of the frame are movable by means of a single lever. A cam shaft 40 is directly fixed to the handle 41 and, as may be seen from Figures 5 and 6, rotation of the handle causes cams 42 set at intervals along the side of the frame to bear against pins 43 projecting downwardly from the mounting bar 44 and to move them and the bar outwardly. The pins 43 run in grooves 45 machined in the body of the frame 46, and each groove 45 contains a helical compression spring 47 to ensure that, as the handle is rotated to the release position, the mounting bars move inwardly again to permit the stencil to be removed.

The cam shaft 40 also mounts a secondary cam 48 which acts against the end of a slidable cam rod 49 running in a channel 50 extending along the other side of the frame. Referring to Figure 7, the cam rod 49 has a series of oblique camming surfaces 51 along its length, corresponding to the positions of pins 52 extending from the second movable bar 53 into grooves 54 machined into the frame side. As with the first movable mounting bar, helical compression springs 55 urge the pins 52, and hence the mounting bar 53, towards the release position. As the handle 41 is rotated from left to right, relative to the view of Figure 4, the secondary cam 48 drives the cam rod 49 along its channel 50, and the camming surfaces 51 drive the pins 52 outwardly to move the mounting bars outwardly and tension the stencil mounted thereon.The cam rod 49 is provided with a return spring at its end remote from the secondary cam 48 to return the cam rod when the handle 41 is moved to the release position as illustrated in Figure 4.

Figures 8 and 9 illustrate how the stencil is fitted on to the mounting bars on the frame. Disposed at intervals along the mounting bars, as may also be seen from Figure 4, are mounting pins 80 which are generally D-shaped in plan and, as may be seen particularly from Figure 8, are provided with undercut portions 81 at each side thereof. The stencil 82 (Figure 9) is provide with a series of keyholeshaped apertures 83 along each edge, the larger part of each of which just fits over the-head of the respective pin 80, while the narrower part corresponds to the width of the undercut portion 81 of the pin, to hold the stencil firmly in place. It will be seen that a straight edge of the aperture 83 engages a straight edge of the pin 80, to ensure even transfer of the tensioning force, thereby avoiding the risk of distortion or damage to the stencil.A clamping plate, not shown, consisting of a thin strip of metal with keyhole slots corresponding in position to the pins 80, is preferably located over the stencil on each edge thereof, engaging with the pins to hold the edge portion of the stencil flat and lock it in place during fixing and tensioning.

This further minimises the risk of damage to the edge of the stencil when tensioned.

An alternative design of frame is illustrated in Figure 10. The innermost edge 100 of the frame is raised slightly above the level of the surface of the mounting bars, to lic substantially flush with the top surface of the mounting pins 80.

Thus, when the stencil is installed, it is caused to deflect over the edge 100, ensuring that, in use, the lowermost surface (the frame is shown inverted from the position in which it is used) is the surface of the stencil itself, and not the pins.

Figure 11 shows an alternative arrangement to that of Figure 4. The main cam shaft 110 attached to the handle 111 is substantially the same as in Figure 4, except that, at the end adjacent to the handle, a bevel gear 112 is provided, meshing with a second bevel gear 113 on a cam shaft 114 extending along the adja cent side of the frame. This cam shaft 114 drives the movable mounting bar with which it is associated in the same way as the main cam shaft 110.

It will be appreciated that, although the frame is illustrated and desaibed with the stencil uppermost, since this is the orientation in which the fitting of the stencil will be carried out, in use the frame will be inverted and the stencil will be lowermost. It will be seen that, apart from the thickness of the lips on the pins 5, there will be nothing projecting downwardly in use below the plane of the stencil, thus ensuring that its use is not impeded by the frame.

Claims

1. A frame for mounting a metal stencil for use in the printing of solder paste on to printed circuit boards, comprising a rectangular frame having along each side thereof a mounting bar having a plurality of attachment means for attaching an edge portion of a stencil thereto, at least two adjacent mounting bars being slidably mounted to move outwardly and inwardly of the frame, and the frame including manually-operable means for moving the movable mounting bars outwardly of the frame together, whereby the stencil may be tensioned in two directions.

2. A frame according to Claim 1, wherein all of the mounting bars are movable, and the manually-operable means are provided at two opposite corners of the frame to move the two adjacent bars together.

3. A frame according to Claim 1, wherein two adjacent mounting bars are fixed, and the other two mounting bars are movable, the manually-operable means being operable between them.

4. A frame according to Claim 1, 2 or 3, wherein the manually-operable means comprise a lever rotating a cam acting on the movable mounting bar, or on a projection therefrom.

5. A frame according to any preceding claim, wherein return springs are provided to bias the movable bars inwardly of the frame.

6. A frame according to Claim 5, wherein the springs are helical springs acting directly on the bars, or on projections therefrom.

7. A frame for mounting a metal stencil for use in the printing of solder paste on to printed circuit boards, substantially as described with refercnce to, or as shown in, the drawings.

8. A metal stencil for use in the printing of solder paste on to printed circuit boards, the stencil being mounted in a frame according to any preceding claim.

9. A stencil according to Claim 8, provided with regions of reduced strength adjacent to the edges thereof.

10. A stencil according to Claim 9, wherein said regions comprise areas of reduced thickness.

11. A stencil according to Claim 9, wherein said regions comprise a pattern of holes through the stencil.

12. A stencil according to any of Claims 8 to 11, comprising a plurality of mounting holes along each edge thereof engaged on mounting pins along the mounting bars.