The present invention relates to a backup material for drilling which consists of a sheet-like laminate and is intended, in relation to a work-table or a similar base, to form a support for a panel-shaped or other pla- nar material when drilling through the material with a drilling tool.

In the drilling of panels, sheets, boards, cards or the like consisting of for instance laminates of resin-impregnated glass fibres with printed electric circuits for making drill holes to permit passing leads, connections etc. therethrough, it is of importance that the material be drilled without damaging the edges of the holes and without leaving cuttings adhering to the material. It is therefore necessary during the drilling operation to support the material, and it is known in the art to use drilling backup materials as a protective support between the work-table and the material in order to prevent the tip of the drill from penetrating down into the work-table and to prevent the formation of copper burrs at the exit side of the hole of the drilled material. It has been proposed to use as drilling backup materials laminates made up of board and glass fibre fabric attached to the board by means of epoxy resin or other resinous binders, the problem being however that the drill tip, when drilling through a stack of glass fibre laminates with printed circuits, backup material and possibly also a drilling top cover, is heated to a relatively high temperature, for instance up to 250°C or more, whereby the drill tip will tend to melt the binder both in the circuit board laminates and in the backup material so that it becomes sticky with an ensuing increased tendency to contamination. In addition, the edges of the drill holes may soften and increase the

tendency to formation of copper burrs. In order to over¬ come these problems it has been suggested to replace the glass fibre fabric with metal foil glued on both sides of a wood fibreboard. The metal foils, usually aluminium foils, were expected to reduce the tendency of the wood fibreboard to warp, and produce a cooling and clean¬ ing effect on the drill tip.

In experiments conducted in connection with the conception of this invention, a vast number of laminates were tested, using cover layers of metal foil, however with disappointing results. It was found that the useful¬ ness of the capacity of the metal foil to abduct heat from the drill tips was negligible in comparison with the substantial adverse effect of the metal foils caus- ing more extensive wear and heating of the drill tips by frictional engagement therewith than did the printed circuit boards.

It was therefore necessary in some way to cool the drilling tools and so it was suggested to replace the wood fibreboard forming the central layer of the drilling backup material with an undulating metal foil and to suck air through the flute ducts and the drill hole along the drilling tool during the extraction thereof. Unfortunately, the drilling tools were su jec- ted to even greater wear and heating when they had to penetrate through an additional metal foil, which to a great extent took away the effect of an otherwise efficient air cooling.

The object of the invention is to solve these problems by providing a drilling backup material which has a wavy or fluted central layer to permit sucking off dust and particles from the drill holes and efficiently cooling the drilling tools, and which additionally causes little wear of the tools, which is highly important. Another essential object of the invention is to provide a drilling backup material that is inexpensive, this being also of great importance, especially since

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the service life of a drilling backup material is rather short.

These objects have now been achieved in that the drilling backup material according to the invention has 5 been given the features stated in claim 1 and optionally the improvements stated in claims 2-5.

The invention will be described in greater detail hereinbelow with reference to the accompanying drawings, in which Fig. 1 schematically shows a drill with a pres- 10 sure foot, a work-table and a drilling backup material according to the invention for sheets or laminates with printed circuits to be drilled. Fig. 2 is a view similar

( _- to that of Fig. 1 but shows the drilling backup material in a section at right angles to that of Fig. 1, i.e. 15 along the flute tubes, and Fig. 3 on a larger scale shows a section of the drilling backup material accord¬ ing to the invention.

In Fig. 1, there is shown part of a drilling machine with a work-table 1, a pressure foot 2 and a 20 drilling tool 3 for drilling a number of sheets or panels 4 with printed electric circuits, for instance copper circuits on ^' glass fibre laminate. The sheets or panels 4 are placed on a drilling backup material 5 according to the invention which itself is placed on 25 the work-table 1. As drilling top cover, there may be disposed on top of the sheets of panels 4 a thin sheet 6 of a suitable material, for instance phenolic resin- impregnated paper if this is desirable or in a special case is deemed advantageous. 30 This stack consisting of sheets or panels 4 with printed circuits, the backup material 5 and the top cover 6 is held together and maintained at the work- table 1 in a conventional fashion by means of a number of stacking pins 7 which extend through holes in the 35 stack of sheets and backup material and into holes in the work-table 1. To perform drilling the pressure foot 2 is applied against the stack.

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The pressure foot 2 comprises a housing which sur¬ rounds the drilling tool 3 (or each drilling tool) and is connected by an ⁴ opening 8 and a tube 9 to a suction source {not shown) for sucking off particles and dust. It has surprisingly been found that the problems described by way of introduction can be overcome in all prior art drilling backup materials simply by using corrugated board, i.e. a paper product consisting of one layer of fluted paper 12 and at least one facing, i.e. single faced corrugated board, or one layer of fluted paper 12 and two facings 11, 14, i.e. double faced corrugated board. The flute tubes 10 can be used as ducts for air through-flow in the drilling backup material and for sucking off dust and particles through the drill hole and by the pressure foot 2 to the dust extractor as soon as the tip of the drilling tool 3 has penetrated through the top facing 11 of the corrugated board.

The air flow from the flute tubes 10 through the drill hole is immediately initiated when the drilling tool 3 punctures the facing 11, and the upward air stream, for instance a helical flow along the twist drill 3, will effectively clean both the drilling tool and the edges of the drill hole and further has the beneficial effect of effectively cooling the drilling tool immediately after a drilling operation has been performed and during the withdrawal of the drilling tool, dust and particles detached from the material and the drilling tool being sucked off during the extraction of the drilling tool.

Such air cooling efficient per se would however not give an overall advantage if the drilling backup material, in order to ensure the air cooling, were to cause an increase in wear and, as a result thereof, increased frictional heating of the drilling tool. The material according to the invention however only slightly affects the drilling tools and additionally is advantage-

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ous in so far as it gives, for _. a desired small total amount of material and weight, a relatively great thickness to provide the desired distance between the material 4 to be drilled and the work-table 1. 5 Particles that are not sucked upwards and out of the housing 2 when the drill tip has penetrated through the top facing 11 of the panel of corrugated board are not entrapped in the drill hole proper, from where they could penetrate in between the sheets 4 and the backup 10 material, but can fall freely into the empty spaces defined by the flute tubes 10.

To sum up, it has been found that heat conducting mJ metal foils only theoretically enhance the cooling effect on the drill tip itself by abduction of heat. 15 whereas they subject the drill tip to increased wear, this being per se a considerable drawback, and that the heating of the tool will thus also increase. The invention rebuts the theory that contact between drill tip and heat-abducting metal is necessary to ensure 20 cooling down to a desired low level, and also the theory that a thin aluminium foil causes little wear, and has demonstrated that corrugated board, strengthened with a suitable resin, will give superior results in every respect. 25 The drilling backup material 5 shown in Fig. 3 consists of resin-coated double faced corrugated board on which an extra outer layer 13 is glued for additional strengthening.

Corrugated board being a paper product, one might 30 believe that dust and fibres would loosen and contaminate or completely ruin the printed circuit boards. Even if the dust extraction as described with reference to Fig. 2 would produce a certain effect, it was believed that the drilling tools in all likelihood would ruin 35 the printed circuit boards by fraying the edges of the drill holes and cause flocculation of loose fibres which by the extraction of the drilling tools are pulled

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6 up into the holes to stick there. Panels of ordinary corrugated board would in addition make an unstable support which might permit movement of one facing with respect to the other and hence make the material use- less as a backup material for drilling. The very rapid, accurate drillings which are performed in the drilling of printed circuit boards and are very similar to punch¬ ing, in fact require an immobile backup material, i.e. backup material which neither is movable laterally nor ^' warps. On the other hand, corrugated board is an inex¬ pensive material which in standard qualities is manufac¬ tured in thicknesses that are suitable and with flute tubes that are sufficiently large to ensure the above- described dust extraction and air cooling. According to the invention, it has been found that ordinary corrugated board of standard quality can be strengthened and improved by a relatively simple and relatively inexpensive treatment, so as to make a superior drilling backup material as compared with facings of metal foil with a sandwiched layer of fluted metal foil. This is achieved by strengthening the corrugated board with resin. Phenolic resin in particu¬ lar has proved to impart the desired strength and stabi¬ lity to corrugated board. Furthermore, this type of resin has the property of making the cover layers or facings relatively brittle so that they can be readily punctured by the drilling tips, giving holes with clean sharp contours. Phenolic resin can readily be applied to the facing or facings by spraying or perfusion, which should and can easily be so performed that the glue joints of the corrugated board are not dissolved but rather, upon curing of the resin, are strengthened at the flute crests and without the flute tubes being clogged and thus obstructed. After application of the phenolic resin, the material is cured in a furnace, for instance for about 40 min. or more at about 170°C de¬ pending on the basis weight of the corrugated board and

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the amount of resin applied.

As basic material for the drilling backup board according to the invention use is preferably made of corrugated board of standard quality with a thickness of at least 1 mm and most preferably between 1.5 and 3

2 mm and a basis weight of between about 150 g/m and 2

280 g/m for both fluted layer and facing before the impregnation with phenolic resin.

As an example of the degree of impregnation, it may be mentioned that corrugated board with a thickness of 2 mm requires phenolic resin with setting agent in an

2 amount of about 0.125 1/m for optimum results and im¬ pregnation on both sides. The production costs for the backup material according to the invention will then amount approximately to only about 10-25 % of the pro¬ duction costs for the drilling backup materials which have hitherto been used and of which the backup material composed of one layer of fluted metal foil and two

2 facings of metal foil will cost about 100 crs/m in manufacture, to be compared with a cost of about 10 crs/m^ for the impregnated and cured material according to the invention as described above. In addition, the backup material according to the invention is an improvement in all respects contemplated herein and, as far as is known, inferior in no other respect. Above all, the material according to the invention is far more gentle with respect to the drill in that the facing or cover layer of paper is hard after impregnation, yet brittle and readily punctured by the drill tips, forming clean hole edges without burrs during drilling. The cured resin will not soften and will entail no tendency to sticking at prevailing temperatures which can be kept relatively low by the reduced penetration work, which will entail less wear and extended tool life. The foregoing description has been concerned with printed electric circuits, circuit boards etc., but it goes without saying that the drilling backup - aterial

according -to the invention is also useful in the drill¬ ing of other materials in which the problems encountered are equivalent to those met with in conjunction with the drilling of boards with printed electric circuits, and in paifticular where it is vital that the temperature of the drilling tools is kept relatively low. Good results are possible to obtain according to the invention also with resinous impregnating agents other than phenolic resin, such as epoxy resin, phenolic resin being however preferred since, among other things, it is relatively . inexpensive. For the same reason, corrugated board of any commercial type is preferred.