Method for Producing Adhesive Coated FoilThis 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 inUS 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.
The adhesive technology described in these patents and known in the art of manufacture of printed circuit boards is typically that based on epoxy chemistry. Although epoxy chemistry 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 surprisingly found that by using a different type of adhesive system to produce the cured layer described above that a significant improvement in production speeds can be obtained, thereby reducing manufacturing cost.
One aspect of the present invention provides a method for producing adhesive coated foil comprising the steps :(a) coating a surface of a conductive foil with a first adhesive layer including at least some adhesive material which is curable by ultra-violet (u-v) radiation;(b) coating the surface of the first adhesive layer with a second adhesive layer including thermally curable adhesive material; and(c) semi-curing the thermally curable material of the second adhesive layer.
The surface of the semi-cured second adhesive layer may then be applied to a substrate which may be used to form a printed circuit board.
Preferably, the first adhesive layer contains some thermally curable adhesive material, in order to promote adhesion between the two layers.
In one embodiment of the invention, at least some of the material of the first adhesive layer is semi-cured before the application of the second adhesive layer. However, the second layer may be applied to the first layer before the material (s) of the first layer are cured at all or even dried to remove solvent.
In the case where the first layer is subject to a semi-curing operation before application of the second layer this can be by irradiation with u-v radiation or, if a thermally curing component is present, by the application of heat. If the first layer includes u-v and thermally curable adhesive material, both may be semicured before the application of the second layer. Then the u-v component at least is left semi-cured and is fully cured through the second layer at a later stage.
Alternatively the u-v component may be fully cured and the thermal component semi-cured. If the first layer contains u-v curable adhesive material as the only adhesive material, the material may be semi or fully cured before the second layer is applied.
Another aspect of the invention provides an adhesive coated foil comprising :a layer of conductive foil ;a cured layer of adhesive material disposed on a first surface of the foil including at least some cured u-v curable adhesive material; anda semi-cured adhesive layer disposed over the cured adhesive layer and including thermally curable adhesive material.
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.
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; andFigure 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 u-v curable adhesive 1 a (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 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 semi-cure the adhesive la to form a semi-cured adhesive layer lb. The power output of the lamp (s) and the line speed (time under the lamp (s)) can be adjusted to control the cure level of this first pass operation. Optimum economic conditions are with maximum output of the lamps and running at the maximum speed consistent with the desired cure level of the adhesive material. The cure may be controlled to a level whereby the first pass coating i. e. first adhesive layer is tack free and can be readily wound into a reel without subsequent blocking. The material can then be wound up as a roll and then further processed through the coating operation.
This time a second adhesive layer 2a consisting of a thermally cured epoxy system is applied onto the first semi-cured adhesive layer I b, again coated out of a solvent solution. The coated foil again passes through a drying tunnel to remove the solvent. This time the temperature and dwell time in the drying tunnel are increased in order to semi-cure the second epoxy based adhesive material 2a to form a semi-cured layer 2b. On leaving the drying tunnel the coated foil passes under one or more u-v lamps which fully cures the u-v curable material of the first applied adhesive layer Ib 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.
Typically the conductive foil is copper with a thickness of 5-40 microns, preferably 12-18 microns.
Typically the adhesive layers are from 12-75 micron in thickness, preferably 25-40 microns.
With known coating processes, typical line speeds required to fully thermally cure an epoxy coating utilizing a 20 metre drying tunnel at a temperature of 150 C are around 1 metre/minute, and to semi-cure stages 3-15 metres/minute, preferably 3-5 metres/minute.
However, using a preferred embodiment of the present invention, it is possible to carry out the semi-curing of the u-v curable coating at a speed of 1060 metres/minute (typically 30 metres/minute). This means that the first pass operation (i. e. application of first layer and u-v semi-curing) can be carried out much more quickly than a typical first pass operation using a fully cured thermally curable adhesive layer. This in turn has the advantage that the two passes may be carried out consecutively using the same production line, in a greatly reduced time, reducing costs.
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 u-v curable first adhesive coating is typically 50-130 C.
The material used for the first u-v cured adhesive layer is preferably an epoxy acrylate or epoxy novolac acrylate e. g. Croda Chemicals UVE140, and the material used for the second thermally cured epoxy adhesive layer is typically an epoxy compound and preferably a brominated epoxy compound e. g. Dow Chemical DER592A80.
Typical formulations are:FORMULATION 1: 1 st u-v curable adhesive layer :Croda UVE140 (epoxy novolac acrylate) 42.6Irgacure 500 (Photoinitiator, Ciba Geigy) 2.7Dow DER592A80 (brominated epoxy-80% in acetone) 15.0 Pentaerythritoltetramercaptoproprionate (PTMP) 5.0Methyl ethyl ketone 22.0Dicyandiamide (DICY-20% in dimethylformamide) 2.5FORMULATION 2: 2nd thermally curable adhesive layer:Dow DER592A80 43. 4DICY (20% in DMF) 5.6MI (4% in DMF) 1.0PTMP 3.2Morton 98354 (polyester resin) 5.0 FC430 (3M-flow aid) 0.2In the above example, the first adhesive layer contains some thermally curable epoxy material of the second layer as well as u-v curable material in order to promote adhesion between the two adhesive layers and impart flame retardancy properties to the first layer.
In the above formulations, PTMP is added as an adhesion promoting additive.
Irgacure 500 is the photoinitiator which initiates the cure of the u-v curable component of the first adhesive layer.
DICY is a crosslinking agent for the thermally cured epoxy component.
The polyester resin component of the second adhesive is a flexibilising resin.
The MI is 2-methylimidazole.
In a preferred embodiment, formulation 1 is coated onto 12 micron copper foil and dried at 60 C for one minute to remove the solvent. The coated foil typically has a dry (solventless) adhesive coat weight of 25gsm. The coated foil is then passed under one 300 watt/inch medium pressure mercury lamp at a speed of 30 metres/minute to produce a tack free semi-cured adhesive layer.
The coated foil is then further coated with formulation 2 and dried for one minute at 60 C to remove the solvent. The material is then left in an oven at 150 C for three minutes to semi-cure the second adhesive layer. Other temperatures at which the semi-curing of the second adhesive layer may be carried out are between 120 and 200 C.
The foil, coated with the two layers, is then passed under one 300 watt/inch medium pressure mercury lamps at a speed of 5 metres/minute to fully cure the first adhesive layer.
The final product can be press bonded to a further copper foil at 180 C for 50 minutes under a pressure of 200 psi to give a multilayered construction for use in printed circuit board manufacture.
Using the suggested formulations above it is possible to partially cure the thermally curable material of the first layer with heat instead of partially u-v curing the u-v curable material, before the application of the second adhesive layer 2a.
Other suitable formulations for this alternative method are as follows:FORMULATION 1-The first adhesive layerDER592A80 21. 7DMF 10. 0 DICY (20%) 2.8MI (4%) 0.5 UVE140 (80% in MEK) 21.7IRGACURE500 0.1FORMULATION 2: 2nd thermally curable adhesive layer:DER592A80 43. 4DMF 20.0DICY (20%) 5.6MI (4%) 1.0PTMP 2.5In a preferred embodiment of this alternative method, the first layer is coated on to copper foil to give a'dry' (solventless) coating weight of 25 gsm.
The solvent is removed by drying at 100 C for 30 seconds followed by thermally semi curing the adhesive for 90 seconds at 150 C. The semi cured adhesive may be tacky at room temperature and may require an interleaf to be laminated to the adhesive surface to allow the foil to be wound up into reel form. The interleaf may be a siliconised polyester film. In a drying tunnel, thermal semi-curing could be carried out at a speed of 5-15 m/min, preferably 10 m/min.
The interleaf is removed prior to coating the second adhesive layer on to the first adhesive layer.
After coating, the second adhesive layer (again 25 gsm dry) is dried for 30s at 100 C and semi cured for 2 mins. at 15t C followed by u-v curing under one 300 watt/inch u-v curing lamp.
In a further embodiment of the invention, using the same adhesive formulations as above, the second layer 2a may be applied immediately to the first layer la without drying or curing the first layer. In a test process, the total construction was then dried for one minute at l 00 C to remove solvent and cured for two and a half minutes at 150 C to semi-cure the thermally curable portion of the formulations in both layers la, 2a. The final product was then passed under one u-v lamp at 10 metres/minute to fully cure the u-v curable adhesive material of the first layer.
The advantage here would be a"one pass"manufacturing process which is only achieved by virtue of the fact that two different types of adhesive are being used.
This last mentioned process may make use of a special type of coating method known as"dual slot die coating"which lays down one (wet) layer on top of another (wet) layer.