GB2163092A

GB2163092A - Plastics film severing tool assembly

Info

Publication number: GB2163092A
Application number: GB08518862A
Authority: GB
Inventors: Ian James Shearer; Gary David Wallington
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1984-07-25
Filing date: 1985-07-25
Publication date: 1986-02-19
Also published as: GB8518862D0; GB8418951D0

Abstract

The tool assembly is used in making photographically reducable artworks for precision masks used in the etch control of circuit boards, for example those used for microwave integrated circuits, where the artworks comprise two films of polyester material stuck together, one of a clear material and one of a coloured material. The tool is used to sever the coloured material in a chosen pattern and comprises a heated slender tool bit (B) capable of being inserted into the coloured material and locally melting it as the tool assembly is traversed around the pattern. The tool has a symmetrical tip (B1) so that it does not have to be reoriented for cutting in different directions, and can replace the stylus in a conventional flat bed x.y plotting machine. The tool is heated by a resistance coil (D), and is carried by a metal housing (A) in electrical and heat insulating collars (C2, C1), being an interference fit in collar (C2) which is axially adjustable, and a sliding fit in collar (C1) which is fixed in the housing (A). <IMAGE>

Description

SPECIFICATION Plastics film severing tool assembly This invention relates to severing tool assemblies suitable for use in making photographically reducable artworks for precision masks used in the etch control of circuit boards, for example those used for microwave integrated circuits.

Such artworks are made by use of a thermoplastic, e.g. polyester, masking film. The film comprises two layers temporarily stuck together, one of a clear material, and the other of a coloured material. The coloured material is preferably bright red to give a strong contrast in the photographic process. Hitherto, a cutting machine, having a precisely controlled cutting edge, has been used to sever the coloured layer in a chosen pattern, after which the unwanted coloured areas of the mask are stripped from the clear material resulting in a high contrast artwork which can be photographically reduced to produce a precision mask for etch control.

A major drawback of this known process is the necessity for the cutting machine which, because the cutting edge is in the form of a blade and is directional, (i.e. it must always be kept aligned with and perpendicular to the direction of cut), is a bulky and expensive piece of equipment.

Thus the present invention has for an objective the replacement of this machine with alternative smaller and less expensive equipment by removing the directional requirement inherent in the cutting -machine.

According to the present invention, a severing tool assembly suitable for mounting in the traversing head of a machine capable of effecting traverse of the head in a given plane and for severing a film of thermoplastic material when so mounted, comprises, a housing adapted for mounting in such a traverse head, a tool bit rigidly carried by the housing having a symmetrically formed tip region for inserting into the film to be severed, and heating means for heating the tool bit to a temperature suitable for locally melting the film on such insertion, the arrangement being such that severing of the film can be readily effected irrespective of the direction of traverse without reorientating the severing tool assembly or traversing head.

Whilst such a severing tool assembly can be fitted to various types of machines, even to the cutting machine previously referred to, because it is essentially- non-directional and therefore requires no re-orientation when severing is to be achieved in differing directions, it can be utilised in the traversing head of a graphical plotter. In this case, the severing tool assembly has its housing adapted to replace the pen or stylus of the plotter. Moreover, since a plotter is designed to operate with the pen offering relatively low drag, the severing tool assembly must do likewise. Thus the tool bit is formed to a fine point; this, together with the melting of the layer, contributes to low drag.

One embodiment of the invention suitable for use with a Hewlett-Packard 9872 Plotter, but not in principle restricted to that type, is described with reference to the accompanying drawings in which: Figure 1 is an end view of a severing tool assembly, and Figure 2 is a sectional view upon line Il - II of Figure 1, that is to say in a plane including the longitudinal axis X - X of the assembly.

The Figures illustrate a severing tool assembly which fulfils the following criteria: (i) that it is compatible with the Hewlett-Packard pen-holding arrangement in the traversing head, (ii) that it has a relatively low mass, (iii) that the tool bit is held rigidly with reference to the housing, and, (iv) that the tool bit is heated to a sufficient temperature to melt the film, without transmitting sufficient heat to damage the pen-holding arrangement or the traversing head.

Thus the severing tool assembly is symmetrically formed about the axis X - X and includes a housing A dimensionally similar to a standard Hewlett Packard 9872 plotter pen. The housing A is machined from aluminium. It has holes Al, fins A2 and, moreover, is black anodised to aid heat dissipation. The housing A is basically cylindrical and inserted within it are two axially spaced reinforced plastics collars C1 and C2 which, being of annular form, provide coaxial location for a tool bit B. The collars are conveniently formed of Tufnol (Registered Trade Mark) which is both an electrical and heat insulator. The tool bit B is conveniently formed of chromium-plated steel. It is an interference fit in the collar referenced C2 and a sliding fit in that referenced C1.Contact between the collars C1 and C2 and the bit B is kept to a minimum to reduce heat transmission.

That collar referenced C2 is a sliding fit in the housing A to allow axial adjustment of the bit B with respect to the housing so that pressure exerted in use on the film to be severed can be varied. A locking screw F provides positive location subsequent to such adjustment.

The bit B is formed with a sharp point B1 which in use is urged into the film to a desired depth.

The bit B is further formed with a length of reduced diameter between the collars C1 and C2 around which is wound a heating coil D of resistance wire. This is electrically connected to twin connector pins E to which power is applied via lightweight multistrand wire, not shown.

The illustrated severing tool assembly is relatively light (around 20 grams) but further weight reduction would reduce strain on the plotter mechanism and reduce inertia leading to more accurate severing. This could be done by forming more holes Al in the housing A.

Stabilisation of the tool bit temperature may, if desired, be achieved by mounting a thermocouple thereon and effecting interruption of the power supply.

In practice, using a Hewlett-Packard 9872 flat bed plotter handling sizes of masking film of the type described up to A3 sheet size has yielded a plot ting accuracy of plus or-minus 0.2% and of deflection pius or minus 0.008" Since a typical microwave integrated circuit (MIC) substrate is about 1"x 1" and the original artwork may be chosen to be 10 times full size, this is found to give a total positional error of about plus or minus 0.03"on the original or plus or minus 0.003"don the final artwork. This is theoretically suf ficientfor the manufacture of MIC for use at operating frequencies up to 35 GHz. However, severing the upper film by the present arrangement gives a slightly ragged edge limiting it to use for the manufacture of MIC for use at frequencies of around 18 GHz. Using a larger original (Al or AO sheet size) and greater reduction would render MIC manufacture by the present arrangement suitable for use at the 35 GHz level.

The present arrangement is suitable for use wherever high contrast artwork is required; it is also applicable to those artworks used for preparing printed circuit boards;

Claims

1. A severing tool assembly suitable for mounting in the traversing head of a machine capable of effecting traverse of the head in a given plane and for severing a film of thermoplastic-material when so mounted, comprising, a housing adapted for mounting in such a traverse head, a tool bit rigidly carried by the housing having a symmetrically formed tip region for inserting into the film to be severed, and heating means for heating the tool bit to a temperature suitable for locally melting the film on such insertion, the arrangement being such that severing of the film can be readily effected irrespective of the direction of traverse without reorientating the severing tool assembly or traversing head.

2. A severing tool assembly according to Claim 1 in which the housing is of generally cylindrical form, the tool bit is of generally rod-like form and lies co-axially within the housing with its tip region protruding therefrom, and wherein there are provided collars of insulating material supporting the tool bit within the housing, a first collar being adjacent the tip region to enhance rigidity.

3. A severing tool assembly according to Claim 2 wherein a second collar is provided-axially spaced from said first collar, and electrical coil heating means are provided between the collars to heat the tool-bit.

4. A severing tool assembly according to Claim 2 in which the tool bit is axially movable for adjustment purposes with respect to the housing.

5. A severing tool assembly according to Claim 3 wherein, to allow said adjustment, the first collar is fixedly attached to the housing and the tool bit is capable of axial sliding movement in said first collar, and the second collar is capable of axial sliding movement in the housing-and the tool bitis fixedly attached in said second collar.

6. A severing tool assembly substantially as hereinbefore described with reference to the accompanying drawings.