GB2349590A

GB2349590A - Adhesive coated conductive foil

Info

Publication number: GB2349590A
Application number: GB9910372A
Authority: GB
Inventors: Graham G Skelhorne; Colum F Dickson; Ian M Lancaster
Original assignee: Rexam Custom Ltd
Current assignee: Rexam CFP Ltd
Priority date: 1999-05-05
Filing date: 1999-05-05
Publication date: 2000-11-08
Also published as: GB9910372D0

Abstract

A method for producing adhesive coated foil, typically for use in the manufacture of printed circuit boards, comprises the steps of: <SL> <LI>(a) coating a surface of a conductive foil with an adhesive layer comprising thermally curable adhesive material and including some adhesive material which is curable by ultra-violet (u-v) radiation; and <LI>(b) curing or semi-curing the u-v curable component of the adhesive layer. </SL> The flow properties of the adhesive layer may be controlled by selection of the relative amount of u-v curable adhesive material in the adhesive layer and/or the degree to which the u-v curable component is cured. The surface of the adhesive layer may then be applied to a substrate which may be used to form a printed circuit board, and the thermally curable component may be cured after application to a substrate.

Description

METHOD FOR PRODUCING ADHESIVE COATED FOIL This invention relates to a method for producing adhesive coated foil, typically for use in the manufacture of printed circuit boards.

The production of multilayer printed circuit boards is described in US patents 5,557,843 and 5,362,534 by McKenney et al. The processes described involve the production of laminates by first curing an adhesive on a conductive foil, applying and semi curing a second adhesive to the first cured adhesive and subsequently applying a second substrate over the second adhesive.

Although this method of manufacture of multilayer circuit boards has some advantages in terms of having a guaranteed dielectric layer thickness due to the cured first pass adhesive layer, the application of two separate adhesive layers can be time-consuming.

Furthermore the adhesive technology described in these patents and known in the art of manufacture of printed circuit boards is typically that based on thermally curable epoxy adhesive. Although thermally curable epoxy adhesive gives many advantageous properties that are required in printed circuit board manufacture, unfortunately the curing process is rather slow and as such limits the production speeds achievable thus resulting in higher manufacturing costs.

It has been found that a more economic route to manufacturing such coated foil products can be employed by use of a different type of adhesive system. In particular, it is possible to utilise a one layer adhesive system and control the dielectric layer thickness by good control of the adhesive flow properties and the press lamination conditions.

One aspect of the present invention provides a method for producing adhesive coated foil comprising the steps of : (a) coating a surface of a conductive foil with an adhesive layer comprising thermally curable adhesive material and including some adhesive material which is curable by ultra-violet (u-v) radiation ; and (b) curing or semi-curing the u-v curable component of the adhesive layer.

The expression"curing"refers to the curing of a component of the adhesive material as fully as is practical under given circumstances. In practice this may not always result in a 100% cure but nevertheless the component is substantially cured. By contrast,"semi-curing"refers to deliberately curing the material to a degree which represents less than a 100% cure.

The flow properties of the adhesive layer may be controlled by selection of the relative amount of u-v curable adhesive material in the adhesive layer and/or the degree to which the u-v curable component is cured.

The surface of the adhesive layer may then be applied to a substrate which may be used to form a printed circuit board. The thermally curable component may be cured after application to a substrate.

If the u-v curable component is semi-cured in step (b) of the process, it may be subsequently fully cured thermally. This will normally require the addition of a thermal initiator to the formulation, such as a peroxide compound.

This technique enables the semi-cured u-v curable component to be fully cured even after application to a substrate, when this would not otherwise be possible due to the difficulties in exposing the adhesive layer to u-v radiation at that stage in the process.

Another aspect of the invention provides an adhesive coated foil comprising : a layer of conductive foil; a layer of adhesive material disposed on the surface of the foil comprising thermally curable adhesive material and including some cured u-v curable adhesive material.

A further aspect of the invention provides a foil coated substrate comprising: a substrate ; a layer of adhesive material disposed on a surface of the substrate comprising cured thermally curable adhesive material and including some cured u-v curable material; and a layer of conductive foil disposed on the surface of the adhesive layer.

In an advantageous embodiment, the method according to the invention can be used to obtain a significant improvement in production speeds compared with known methods, resulting in a reduction in manufacturing costs.

The full or partial curing of the u-v curable component of the adhesive layer enables the properties of the adhesive layer to be controlled, e. g. its subsequent tendency to flow under the influence of temperature and pressure.

The desired properties of the adhesive layer may be achieved by controlling the relative proportion of the u-v curable component in the adhesive material formulation. Alternatively or additionally, the degree to which the u-v curable component is cured may be used to control the properties of the adhesive layer.

In all aspects of the invention discussed above, the thermally curable adhesive material and the u-v curable adhesive material are mixed before being applied to the conductive foil, rather than being applied in separate layers.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows in diagrammatic form one example of a method according to the invention at each stage of the process; and Figure 2 illustrates schematically a typical production line for producing an adhesive coated foil according to the invention.

Referring to Figure 1, initially a layer of adhesive la (solvated in an organic solvent) is accurately applied to a conductive foil 3 by means of a coating technique (for example, reverse roll coating or slot die coating). The adhesive layer comprises some thermally curable adhesive material and at least some u-v curable adhesive material, and is typically of a thickness between 10 and 150 microns. The foil 3 is typically between 5 and 40 microns thick. The coated foil is then passed through a drying tunnel to remove the solvent and the coated foil then passes under one or more u-v lamps to cure or semi-cure the u-v component of the adhesive formulation. The resultant adhesive layer lb may be tack free at this stage or alternatively may have some residual level of tack. In the latter case an interleaf film may be applied to the tacky surface to prevent blocking of the adhesive layer to the back of the conductive foil when it is wound into a reel or stacked in sheets. The interleaf layer is typically a plastic film such as polyethylene, polypropylene or polyester. It may also be a siliconised film or paper.

If the adhesive system described above were not to contain the u-v curable component to control the subsequent flow characteristics of the adhesive, then the flow would need to be controlled by the'B'staging or semi-curing of the thermally cured adhesive component. Materials known in the art for use in printed circuit board manufacture are epoxy compounds and these materials require relatively long times at elevated temperatures in order to induce the curing process.

With known coating processes, typical line speeds required to semi-thermally cure ('B'stage) an epoxy coating utilising a 20 metre drying tunnel at a temperature of 150 C are around 3-5 metres/minute.

However, using a preferred embodiment of the present invention, it is possible to carry out the u-v curing of the modified adhesive at a speed of 10-60 metres/minute (typically 30 metres/minute). This means that the coating operation can be carried out much more quickly than a typical operation using a semi-cured thermally curable adhesive layer.

The adhesive coated foil may be applied to a substrate 2, and the thermally curable component of the adhesive layer may then be cured to form a cured adhesive layer lc.

Figure 2 shows a production line comprising a reel 21 from which a roll of foil can be unwound, a coater 22, a drying tunnel 23, typically between 10 and 40 metres in length, u-v lamps 24 and a second reel 25 for rewinding the coated foil.

The u-v lamps used are typically medium pressure mercury lamps with a power of between 100 and 450 watt/inch, preferably 300 watt/inch.

The dryer temperature for the removal of solvent from the adhesive layer is typically 50-130 C.

By way of example of the advantages of the invention the following examples are given: The u-v curable adhesive material used to modify the thermally curable adhesive material is preferably an epoxy acrylate or epoxy novolac acrylate e. g. Croda Chemicals UVE140, and the material used as the thermally curable adhesive component is typically an epoxy compound and preferably a brominated epoxy compound e. g. Dow Chemical DER592A80.

A typical formulation for the modified adhesive layer is: Parts by weight: Croda Chemical UVE140\80 2.5 Irgacure 500 0.1 Dow DER592A80 20.0 DICY (20% in DMF) 5.6 MI (4% in DMF) 1.5 A typical thermally curable adhesive formulation known in the art would be: Parts by weight: Dow DER592A80 43.4 DICY (20% in DMF) 5.6 MI (4% in DMF) 1. 0 Irgacure 500 is the photoinitiator which initiates the cure of the u-v curable component of the modified adhesive layer.

DICY is a crosslinking agent for the thermally cured epoxy component.

The MI is 2-methylimidazole.

In the laboratory, the modified adhesive system was coated onto 12 micron copper foil to give a dry (solventless) adhesive coat weight of 80gsm. The coated foil was then left in an air circulation oven at 100C for two minutes to remove the solvent. The dried coated foil was then passed under two medium pressure mercury lamps at 30 metres per minute to cure the u-v curable component of the adhesive formulation. The dwell time under the lamps was less than one second. The adhesive coated foil produced by this process was tack free.

In comparison, the typical thermally curable adhesive material described above was coated at the same dry thickness onto 12 micron copper foil.

The coated foil was then left in an air circulating oven at 100C for two minutes to remove the solvent. The oven temperature was then increased to 150C and the adhesive layer was semi-cured by leaving for a further 3 minutes at this temperature. The coated foil produced by this process was also tack free.

It can be seen from the above that the modified adhesive system could be used to produce a coated foil in a much more cost effective manner due to the much shorter time frame required to cure this modified adhesive compared to the standard thermally cured system.

The final product can be press bonded to a further copper foil at 180C for 50 minutes under a pressure of 200psi to cure the thermally curable adhesive and give a multilayered construction for use in printed circuit board manufacture.

In order to compare the flow properties of the modified adhesive system and a typical adhesive system, the following test was carried out using pieces of copper foil respectively coated using the two example formulations and methods described above.

Two pieces of the coated copper were cut with a 4"by 4"square cutter, and their combined weight recorded (wl). The samples were placed coated face to coated face between two pieces of tedlar film and two 6"by 6"steel caul plates. The construction was then press laminated at 171 degrees C and 200 psi pressure for 10 minutes.

The construction was removed from the press and the copper laminate separated from the tedlar film. From the centre of the laminate a 2.82" by 2.82" (8 square inches) piece was cut and weighed (w2). Also, a 4"by 4"piece of uncoated copper was cut and weighed (w0).

The percentage flow was calculated as: wl-2 x w2 x 100 wl-2xw0 Using the typical thermally curable adhesive material, the percentage flow was 36%, whereas the modified adhesive system gave rise to a percentage flow of 39. 3%.

It can be seen from these results that the method according to a preferred embodiment of the invention can be used to produce an adhesive layer with similar flow properties to those resulting from a conventional method, whilst allowing the adhesive coated foil to be produced more quickly and economically.

In order to demonstrate the effect of the press lamination conditions on the flow of the adhesive material, and hence on the thickness of the dielectric layer, the following test was carried out.

Using the same method as in the previous examples, 18 micron copper foil was coated with an adhesive layer, to give a 100gsm dry coating after drying and curing. The formulation for the adhesive layer was: Parts by weight: DER592A80 17.4 DER560 (80% in DMF) 2.25 DICY (20% in DMF) 2.6 MI (4% in DMF) 0.5 UVE 140/80 2.5 IRGACURE500 0.3 [DER560 is a thermally curable brominated epoxy used to improve flame retartant properties] The percentage resin flow of the system was measured, using the method described previously, under various press lamination conditions, and the results were as follows: Press Temperature Pressure Percentage Flow 170C 200psi 24.6 150C 50psi 3.0 150C 400psi 54.1 190C 50psi 3.2 190C 400psi 62.9

Claims

Claims: 1. A method for producing adhesive coated foil comprising the steps of: (a) coating a surface of a conductive foil with an adhesive layer comprising thermally curable adhesive material and including some adhesive material which is curable by ultra-violet (u-v) radiation ; and (b) curing or semi-curing the u-v curable component of the adhesive layer.
2. A method according to claim 1, wherein the amount of u-v curable material is such that after curing or semi-curing, the adhesive layer is tack free.
3. A method according to claim 1, wherein the amount of u-v curable material is such that after curing or semi-curing, the adhesive layer has a residual level of tack.
4. A method according to any preceding claim, wherein the adhesive materials are applied in solvated form.
5. A method according to claim 4, wherein the solvent is removed by passing the coated foil through a drying tunnel.
6. A method according to claim 5, wherein the drying tunnel is between 10 and 40 metres in length.
7. A method according to claim 4,5 or 6, wherein the solvent is removed at a temperature of between 50 and 130 C.
8. A method according to any preceding claim, wherein the u-v curable material of the adhesive layer is cured or semi-cured by moving the coated foil past at least one ultra-violet lamp.
9. A method according to claim 8, wherein the coated foil is moved past the lamp (s) at a speed of 10-60 metres/minute.
10. A method according to claim 9, wherein the coated foil is moved past the lamp (s) at a speed of 30 metres/minute.
11. A method according to any of claims 8 to 10, wherein the lamp (s) have a power output of 100-450 watt/inch.
12. A method according to claim 11, wherein the lamp (s) have a power output of 300 watt/inch.
13. A method according to any preceding claim further comprising the step of applying the surface of the adhesive layer to a substrate.
14. A method according to claim 13, further comprising the step of curing the thermally curable component of the adhesive layer.
15. An adhesive coated foil comprising: a layer of conductive foil; a layer of adhesive material disposed on the surface of the foil comprising thermally curable adhesive material and including some cured or semicured u-v curable adhesive material.
16. An adhesive coated foil according to claim 15, wherein the foil layer is made of copper.
17. An adhesive coated foil according to claim 15 or 16, wherein the thickness of the foil layer is between 5 and 40 microns.
18. An adhesive coated foil according to any of claims 15 to 17, whereinthe thickness of the adhesive layer is between 10 and 150 microns.
19. An adhesive coated foil according to any of claims 15 to 18, wherein the u-v curable adhesive material comprises an epoxy acrylate or epoxy novolac acrylate.
20. An adhesive coated foil according to any of claims 15 to 19, wherein the thermally curable adhesive material comprises a brominated epoxy compound.
21. A foil coated substrate comprising: a substrate; a layer of adhesive material disposed on a surface of the substrate comprising cured thermally curable adhesive material and including some cured u-v curable material; and a layer of conductive foil disposed on the surface of the adhesive layer.
22. A method for producing adhesive coated foil substantially as hereinbefore described with reference to the accompanying drawing.
23. An adhesive coated foil substantially as hereinbefore described with reference to the accompanying drawing.
24. A foil coated substrate substantially as hereinbefore described with reference to the accompanying drawing.