GB1583544A

GB1583544A - Metal-clad laminates

Info

Publication number: GB1583544A
Application number: GB31042/76A
Authority: GB
Original assignee: UOP LLC
Current assignee: Honeywell UOP LLC
Priority date: 1977-07-25
Filing date: 1977-07-25
Publication date: 1981-01-28

Description

(54) METAL-CLAD LAMINATES (71) We, UOP INC, a corporation, organized under the laws of the State of Delaware United States of America, of Ten UOP Plaza, Algonquin & Mt. Prospect Roads, Des Plaines, Illinois, 60016, United States of America, do hereby'declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to metal-clad laminates and their preparation. More specifically it relates to the preparation of laminates in which the metal layer has a thickness from 1 to 20 microns and is electrically conductive.

Metal-clad laminates, and particularly copper-clad laminates, are widely used in the electrical industry in the preparation of printed electric circuit boards. In these electric circuit boards the copper or other metal which is deposited on the surface of the base member must be bonded to the base member so that subsequent handling and usage thereof will not affect the electrical properties of the board. In addition to the necessary bonding or adhesion of the metal to the base member, it is also preferred that the printed circuit boards be prepared comprising metal-clad laminates in which the metal is as thin as possible. By utilizing a relatively thin layer of metal such as copper, i.e. a layer which is from I to 20 microns in thickness, it becomes possible to reduce the etching time greatly with the concomitant advantage that less etching solution is needed and therefore smaller amounts of spent etchant have to be disposed of. Also, there is a reduced rejection rate due to defects in the copper or other metal surface defects. In addition. less undercutting occurs, and there is a finer line definition with finer or narrower circuit lines and closer spacing, which permits greater circuit density. An added advantage of the use of a thin layer is that it avoids the difficulties associated with the availability of hard-to-obtain thick copper foil.

Metal clad laminates for use in various electrical apparatus have been formed by various means in the prior art. For example. one method of preparing a copper-clad laminate has been to press a resin bonded to a filler with a metal component. a thin film of thermoplastic polymeric material being deposited between the resin and the metal component. Another prior art method is to utilize various base members such as glass, fiber. paper or phenolic resins, onto which a metal layer may be deposited. the surface of the base member being treated in various ways before bonding the metal thereto. Yet another method of preparing metal-clad laminates is to apply a plurality of resins to a sheet of fibrous material to produce a smooth surfaced sheet member and thereafter to apply a resinous adhesive coating to the smooth surface. followed by heat treatment and application of a metal foil. followed in turn by pressing to produce the desired metal-clad laminate. However. in these prior art metalclad laminates. the metal foil which is utilized as the coating or conductive layer is relatively thick due to the inability of the present methods of operation to handle a relatively thin foil.

Currently. the limiting factor in utilizing thinner foils is the inability to handle any metal foil such as copper which is thinner than 1/2 ounce per square foot by ordinary manual or mechanical means.

In addition. it is also known that a thin film of electrically conductive metal may be applied temporarily to a carrier metal. the temporary carrier being of such material that it can be thrown away after one use. The thickness of this temporary carrier will depend upon the stiffness of the material which is used. After preparing the conductive metal film on the temporary carrier. the whole unit is then bonded to a laminate base to form a clad laminate which can then be caused to undergo a number of manufacturing operations involved in printed circuitry, for example drilling, punching or etching. However, before the printed circuit board can be processed further the temporary carrier must be removed by chemical or mechanical means and, due to the fact that it is still present when the various operational steps of drilling, punching etc. are performed. the temporary carrier is not reusable when it is removed by physical means. It is also necessary that the adhesion between the temporary carrier and the conductive metal be kept within certain limits which will ensure an easy removal of the carrier after the conductive metal has been bonded to the laminate base.

However, certain factors are present in the lamination process which can give rise to a metal-clad laminate whose temporary carrier cannot readily be removed from the face of the conductive metal. One such factor could be removal of resin from beneath the conductive metal coating and the forcing of the conductive metal coating into the temporary carrier by the glass fiber weave of the laminate base. This can happen when high spots in the glass fiber mat cause relatively large localized pressures which tend, at the elevated temperatures necessary for lamination, to weld the conductive metal (such as copper) to the temporary metallic carrier (such as aluminum). This welding will then cause some difficulty in stripping the temporary carrier from the finished laminate. It then becomes necessary to keep the dwell time to a minimum in order to obtain laminates which will permit the ready stripping of the temporary carrier from laminate. The dwell time may be defined as the time during which the resin is in a liquid or semi-liquid phase and is free to move against the mat. The longer the time period exists that the resin is in a fluid state, the more the resin will flow to the edge of the pack in order to remove or relieve the pressure and therefore the more resin that is removed from between the mat and the conductive metal, the greater will be the difficulty in removing the temporary carrier. In commercial operations. it is extremely difficult to achieve a relatively short dwell time in a laminating operation. In addition, another difficulty is present when a double-clad laminate is to be prepared inasmuch as there is. in most pre-preg materials containing a low percentage of resins, a disparity between the amount of the resins between the two surfaces of the material. In many instances there is a rough side and a smooth side of the pre-preg. the rough side tending to be deficient in resin and the smooth side having a surplus thereof. Therefore, in the case of double-sided laminates one side may be easily stripped from the supporting carrier while the other side may be difficult to strip of the supporting material.

The present invention. therefore. seeks to provide an improved method which can be used to prepare metal-clad laminates in which the metal layer has a thickness ranging from 1 to 20 microns.

According to the present invention there is provided a method of preparing a metal-clad laminate which comprises depositing a layer of an electrically conductive metal having a thickness of from I to 20 microns on a surface of a carrier bearing a release agent, treating the upper side of the metal layer to improve its adhesive properties with respect to the bonding of a base member. bonding the metal layer to a base member and separating the resulting metal-clad laminate by removing the carrier bearing release agent.

A specific embodiment of a metal-clad laminate according to the invention comprises a layer of copper possessing a thickness of from I to 20 microns supported on a base member and prepared by depositing a layer of copper on a surface of a stainless steel carrier which has been treated with a silane release agent. thereafter treating the upper side of the copper to improve its adhesive properties relative to bonding to a base member. bonding the copper to a base member. e.g. comprising a glass-reinforced epoxy resin. and removing the stainless steel carrier and adhering release agent.

Preferably the upper side of the copper layer has been treated by subjecting it to a high current density and oxidizing the surface by the application of heat. and thereafter the oxidized surface has been coated with a bonding agent. e.g. a silane bonding agent, prior to bonding it to a base member. e.g. a glass-reinforced epoxy resin base member.

The metal-clad laminates which are prepared according to the process of this invention find use as circuit boards in the electrical and electronics industries. for example in radios, televisions and computcrs.

In the first step of preparing metal-clad laminates of the present invention. a thin coat of copper or other electrically conductive metal. for example nickel. tin or gold. is deposited on a surface of a carrier which has been pretreated with a release agent. This carrier may be metallic or non-metallic. metallic carriers comprising for example steel and aluminum and non-metallic carriers comprising for example plastics. e.g. polyethylene. polypropylene and epoxy resins. The carrier is pretreated with a material which will ensure the formation of a relatively poor bond between it and the deposited metal coating. namely a release agent. The release agent. which may also be referred to as a parting agent or slip agent. forms a film which prevents or reduces the adhesion between the surface of the metal coating and the carrier. Examples of release agents which may be utilized include calcium fluoride. alkoxy silanes. polysiloxancs and silica colloids. Silane release agents are generally preferred. One method of applying the release agent to the surface of the carrier is to apply it from a water solution and then to remove excess release agent from the carrier surface, for example by washing or wiping the surface. Following this, the coating of the electrically conductive metal (e.g. copper) is depositied.

In one embodiment applicable when the carrier itself is electrically conductive, i.e.

metallic, the coating is deposited electrolytically. For example, a uniform relatively thin copper coating may be obtained by electroplating using a copper pyrophosphate plating bath which contains copper pyrophosphate, suitably together with nitrate, ammonium and orthophosphate ions. In such a bath the weight ratio of pyrophosphate ion to copper ion is desirably maintained in a range of from 7:1 to 8:1. Plating using such a solution is usually effected at a pH in the range of from 8.1 to 8.8 at a temperature of from 1200 to 140"F. The electrical parameters for optimum electrolytic deposition of copper from such a bath onto the carrier generally include a voltage of from 1.4 to 4 volts, a cathode current density of from 16 to 80 amps per square foot and an anode current density of from 20 to 40 amps per square foot. Alternatively, a copper sulfate plating bath may be used in the presence of sulfuric acid and any addition agents which may be required. Optimum operating conditions for such bath generally include a temperature in the range of from 65 " to 1250F., a cathode current density of from 20 to 50 amps per square foot and an anode current density of from 15 to 45 amps per square foot. During plating of the copper from either the copper pyrophosphate or copper sulfate bath the bath is preferably stirred by means of an air current or preferably a nitrogen current flowing into the bath. Various additives may be present in the copper sulfate bath to improve the deposit of copper on the substrate, if desired. Other baths which may be utilized to effect an electrolytic plating of the conductive metal on the substrate include acid copper fluoroborate, alkaline gold cyanine, acid gold, tin-nickel, nickel sulfamate, rhodium sulfate and silver cyanine. As in the case of the various copper plating baths hereinbefore set forth, it is to be understood that the composition and operating conditions f the various conductive metal baths appropriate for optimum results will vary as to pH, temperature voltage, as well as cathode and anode current densities. However, these variables are well known in the plating art. It is to be understood that the aforementioned metals are only representative of the electrically conductive metals which may be used.

The electrically conductive metal may alternatively be deposited on the carried in an electroless manner, especially when the carrier is non-metallic. This type of deposition may be accomplished, for example, by immersing the pretreated carrier initially in a bath comprising stannous chloride, removing it, washing it with water, dipping it in a solution of slighly acidic palladium chloride, and then dipping it in an electroless bath such as, for example, copper sulfate-formaldehyde which may contain other additives. If so desired; the substrate may be first dipped in a bath comprising a mixture of stannous chloride and palladium chloride, there being a colloidal suspension of the palladium metal in the bath, and then washed and immersed in a bath of metal which is to be deposited on the carrier.

Irrespective of the coating method used, when the metal coating on the treated substrate has reached the desired thickness, that is, from 1 to 20 microns thick, the deposition is terminated.

Next, the upper surface of the metal coating is then treated in order to provide a surface which possesses increased adhesive properties vis-a-vis bonding to a base member. In preferred embodiments this treatment involves the use of high current density and/ or surface oxidation. Preferably the upper surface is first subjected to a high current density with no agitation or stirring of the plating solution. This high current density is preferably achieved by increasing the amperage of the bath from about 18 amps per square foot up to 105 or more amps per square foot. The result of this increased or high current density in the absence of agitation or stirring will be a normally undesirable condition in which the upper surface is slightly roughened. said roughening resulting in a greater surface area for bonding the exposed surface of the metal to the laminate in a subsequent step. Preferably, upon completion of the step of subjecting the metal coating to high current density, the exposed roughened surface of the metal is subjected to the action of an oxidizing agent, e.g. oxygen or hydrogen peroxide, suitably at elevated temperature, this step resulting in the preparation of a surface which is better able to interact with a bonding agent such as one containing an aliphatic amine -group of a type hereinafter set forth.

Following the oxidation of the upper surface of the metal coating, the surface is preferably treated with a bonding agent. Any type of bonding agent known in the art may be used which acts as an interface between the surface ofthe metal and the base member to be applied by forming a resin bond, but the preferred bonding agent comprises a silane, a particularly applicable silane being a gamma-aminopropyltriethoxysilane.

The thus treated metal coating containing the bonding agent thereon in a relatively thin film is then bonded to a base member. Conventional bonding methods such as the application of heat and pressure may be used. the bonding step being preferably performed at an elevated temperature in the range of from 250 to 5000F. or more and at a pressure ranging from 200 to 1000 pounds per square inch or more. The time during which the metal coating is being bonded to the laminate may vary over a relatively wide range of from 0.5 up to 2 hours or more, the optimum bonding time being dependent upon the other parameters of the step such as temperature, pressure and resin composition. The treatment of the upper surface of the metal utilizing a high current density, an oxidation, followed by application of a thin film of a bonding agent, the bonding agent being preferably present in a thickness of 1 molecule, differs from the prior art which teaches that relatively thick coats of bonding agents are utilized to improve the adhesion. Furthermore, the oxidation of the surface has not been proposed in the prior art, which teaches a deoxygenation of the surface followed by polymerization of the bonding agent such as a silane on the surface of the metal in a thick film.

Examples of base member which may be used to form the laminates according to the invention and may be coated with electrically conductive metal on one or both sides thereof preferably are thermosetting resins. The thermosetting resins are preferably impregnated on a base material which may, in a preferred embodiment of the present invention, include paper (which imparts a good mechanical strength combined with low cost), glass fiber (which may be used in either low pressure or high pressure laminates, especially where electrical properties are important, as well as when low moisture absorption, high tensile, flexural and compreheisve strengths are required), fabrics, lignin, asbestos, or synthetic fibers, for example rayon or nylon. The aforementioned base material is impregnated with a thermosetting resin such as a phenolic resin, melamine resin, epoxy resin, silicone, polyimide, acrylic resin, polyester etc. It is to be understood that the aforementioned base materials and thermosetting resins are only representative of the classes of materials which may be used for the base member.

The bonding of the metal-coated carrier to the base member may be accomplished by hot pressing the metal-coated carrier the upper surface of which has been treated in a manner hereinbefore set forth, to the base member, suitably at a temperature and pressure within the range hereinbefore set forth, and the metal-coated carrier may be applied to one face or a pair of opposed faces. Upon completion of the desired residence time at the temperature and pressure previously selected, the metal-clad laminate in which the metal is on one or both sides thereof may be removed from the press. The carrier is thereafter removed from the metal, this removal being facilitated by the presence of the release agent on the surface of the carrier. The carrier which still bears the release agent in a thin film thereon is then ready for reuse in further depositing and bonding cycles to make further metal-clad laminate, it being necessary to refinish the carrier after every cycle.

By utilizing the process of the present invention it is possible to produce metal-clad laminate which will possess many desirable physical characteristics including a relatively thin electrically conductive metal coating from I to 20 microns thick. This coating can be prepared to possess peel strenghs sufficient to meet the requirements placed upon the completed printed circuits. a feat not otherwise easily achieved without the use of certain special procedures currently utilized on commercial foils used in laminate cladding. The following Examples show the effect which is obtained by utilizing the preferred features of the invention. especially subjecting the conductive metal coating to a high current density without agitation followed by oxidation and treatment with a bonding agent.

EXAMPLE I A stainless steel caul plate of the type normally used in the forming of high pressure laminates was treated with a silane known in the trade as A-151 sold by Union Carbide Company. The treatment was effected by applying the silane as a water solution which had been buffered to a pH of about 2 followed by washing the surface to effect the removal of excess silane therefrom. Following this. the steel plate was copper plated by immersion in a copper sulfate-sulfuric acid bath. the bath containing 1980 grams of copper sulfate, 900 grams of sulfuric acid and 12 liters of water. The plating was effected at room temperature and a current of 18 amps per square foot. the bath being stirred by nitrogen bubbling while the anodes were separated by filter paper supported by Plexiglass (Registered Trade Mark) plates which were drilled so as to be provided with 1" diameter holes. When the desired thickness of copper had been plated on the stainless steel plate, stirring was discontinued by halting the nitrogen input and increasing the current density by raising the charge to 5 volts for a period of 1 5 seconds. Further treatment of the copper plate comprised placing the copper plated steel caul plate in an oven at a temperature of from 200 to 300"F. for a period ranging from several minutes to several hours. ideally until a noticeable. but firmly adherent, oxide coating was formed on the exposed surface of the metal film. An alternative method of oxidizing the exposed surface of the copper was to subject the aforesaid copper plated steel plate to the action of hydrogen peroxide. said hydrogen peroxide being added in a strength of 1% to 30C/c concentration of hydrogen peroxide for a period of time such that a noticeable oxide film was observed on the metal surface. the duration of the hydrogen peroxide treatment being dependent upon the strength of the solution.

The final step in treating the exposed surface of the copper was to add a bonding agent, said bonding agent comprising a silane known as A-1 100 and sold by Union Carbide Company.

This silane comprised, as hereinbefore set forth, a gamma-aminopropyltriethoxysilane. The laminate was then prepared for pressing by laying up suitable prepregs with the metal-clad caul plate and pressing the composite for a period of 1 hour at a pressure of 1000 pounds per square inch while maintaining the temperature of the pressure apparatus in a range of from 340" to 350"F.

The metal-clad laminate was then separated from the caul plate by removal of the flow (the excess resin which was squeezed from the prepregs by the pressure applied during curing), and the copper which coated the edges of the caul plate was removed by grinding or cutting the material and removing the caul plate from the laminate. The thickness of the copper coating on the laminate was measured with a micrometer and was found to be in a range of from 5 to 12 microns. Other samples were then prepared in a similar manner with the exception that the thickness of the copper coating was allowed to reach a range of from 15 to 20 microns for use in a peel value test.

In addition other metal-clad laminates were also prepared for use in a peel value test.

However, certain of the steps of the preferred embodiments of the present process were omitted during the preparation thereof. For example, the high current density step was omitted with some laminates, in other laminates the oxidation step was omitted while in a third series of laminates the treatment with the bonding agent was omitted. The results of these tests are set forth in Table I below.

TABLE I Laminate High Current Oxidation Bonding Peel value Density Treatment Treatment Agent Ibs/inch A 5 volts for 15 sec. --- --- 4.82 B --- Yes --- 5.35 C --- --- Silane 1.50 D --- Yes Silane 3.69 E --- --- Silane 1.90 F --- Yes Silane 2.85 G --- --- Silane 4.49 H 5 volts for 15 sec. Yes Silane 8.62 I 5 volts for 15 sec. Yes Silane 8.00 From the above table which discloses the peel value of a copper strip which had an average width normallized to I inch and an average thickness of about 1.5 mils it is evident that the copper which was subjected to the three preferred steps of the present process. namely high current density. oxidation and the addition of a bonding agent. possessed peel values which were greatly in excess over the peel value of those strips which were not subjected to all of the preferred steps of the process. When more severe treatments such as subjecting the coated substrate to a high current density of 105 amps per square foot for a longer period of time are used. it is possible to attain a peel value of 14 pounds per inch.

EXAMPLE 11 In like manner a thin coating of gold is plated on a steel caul plate by treating said caul plate with a release agent comprising a vinyl silane. The plating is accomplished by placing said caul plate in an acid gold bath at a pH in the range of from 3.5 to 4.5 while maintaining the temperature of the bath in a range of from 80" to. 120"F. utilizing a voltage of about 2 volts and a cathode current density of about 10 amps per square foot. Upon reaching the required thickness of gold on the steel caul plate. that is. a thickness of I to 20 microns. agitation is discontinued and the voltage is raised to provide a high current density treatment. Following this the exposed surface of the gold coating is subjected to an oxidation treatment by heating in an oven or under a flame at a temperature of about '300 F. Alternatively the oxidation may be accomplished by an anodic oxidation in a suitable electrlyte. Following this bonding agent comprising a silane similar in nature to that set forth in Example I above is placed on the surface of the gold and the thus treated gold coating is pressed to a base member comprising a glass epoxy resin which is in a B stage prepreg condition. The composite is subjected to a pressure of I 1000 pounds per square inch at a temperature of about 35() F. until the desired stage of curing is realized. Following this the gold-coated laminate is then separated from the caul plate. etched and recovered. Other gold-coated laminates are prepared according to the above process with the exception of a thicker coat of gold and subjected to a peel value test. It will be determined that the peel value of the gold coating which has been prepared according to the above process will be greater than that of gold coating which has been pressed on a glass epoxy resin in a process whereby one or more of the above steps in the preparation have been omitted.

EXAMPLE 111 In like manner a nickel coating having a thickness of from I to 20 microns is plated on a steel caul plate which has been treated with a release agent comprising a vinyl silane. Upon rcaching the desired thickness plating is discontinued, the agitation of the plating bath is also discontinued and the voltage is increased so that the exposed surface of the nickel is treated with a high current density. Following this, the nickel plated caul plate is removed from the bath, washed and treated with hydrogen peroxide in the oxidation or second step of the process. Thereafter a bonding agent is coated on the exposed surface of the nickel and the nickel coating is laminated to a glass polyimide resin base, the pressing of the nickel coating to the base being accomplished in a manner similar to that set forth in Example I above. When the pressing is completed, the caul plate is removed from the metal-clad laminate. Other laminates are prepared according to the above process possessing a slightly thicker nickel coating, these laminates then being etched and the nickel coated on the laiminate is subjected to a peel value test. The peel value of the nickel coating on the laminate which was prepared according to the above process is greater than the peel value which is possessed by a nickel coating pressed on a laminate which has not undergone the treatment comprising the steps of high current density, oxidation, and the placing of a bonding agent thereon.

EXAMPLE IV In this example a tin coating is plated on a steel caul plate in a manner similar to that set forth in Example I above. The caul plate comprises a stainless steel plate which has been prctreated with a release agent such as a vinyl silane. The release agent acts as a point of fracturc. By utilizing the vinyl silane, the formation of the release agent on both surfaces is prevented and the fracture which occurs later in the process between the conductive metal and the stainless steel occurs between said metal and the silane release agent. Upon completion of the plating operation in which the tin coating reaches a thickness of from I to 20 microns the plating step is terminated. agitation is discontinued. and the voltage is increased so that the exposed surface of the tin is treated with a high current density. Thereafter the tin coated steel caul plate is removed and oxidized in an oven or by treatment with a hydrogen peroxide solution in a manner similar to that hereinbefore set forth in Example I. Thereafter a bonding agent comprising a gamma-aminopropyltriethoxysilane is placed on the exposed surface and the tin coating is bonded to a paper phenolic resin which is in a prepreg state. The bonding is accomplished by pressing at a pressure of about 1000 pounds per square inch and an elevated temperature of about 350"F. for a period of time sufficient to realize the desired stage of cure. When this stage of cure is reached. pressure and heat are removed and the caul plate is separated from the tin-clad paper phenolic r

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

to the above process will be greater than that of gold coating which has been pressed on a glass epoxy resin in a process whereby one or more of the above steps in the preparation have been omitted.

EXAMPLE 111 In like manner a nickel coating having a thickness of from I to 20 microns is plated on a steel caul plate which has been treated with a release agent comprising a vinyl silane. Upon rcaching the desired thickness plating is discontinued, the agitation of the plating bath is also discontinued and the voltage is increased so that the exposed surface of the nickel is treated with a high current density. Following this, the nickel plated caul plate is removed from the bath, washed and treated with hydrogen peroxide in the oxidation or second step of the process. Thereafter a bonding agent is coated on the exposed surface of the nickel and the nickel coating is laminated to a glass polyimide resin base, the pressing of the nickel coating to the base being accomplished in a manner similar to that set forth in Example I above. When the pressing is completed, the caul plate is removed from the metal-clad laminate. Other laminates are prepared according to the above process possessing a slightly thicker nickel coating, these laminates then being etched and the nickel coated on the laiminate is subjected to a peel value test. The peel value of the nickel coating on the laminate which was prepared according to the above process is greater than the peel value which is possessed by a nickel coating pressed on a laminate which has not undergone the treatment comprising the steps of high current density, oxidation, and the placing of a bonding agent thereon.

EXAMPLE IV In this example a tin coating is plated on a steel caul plate in a manner similar to that set forth in Example I above. The caul plate comprises a stainless steel plate which has been prctreated with a release agent such as a vinyl silane. The release agent acts as a point of fracturc. By utilizing the vinyl silane, the formation of the release agent on both surfaces is prevented and the fracture which occurs later in the process between the conductive metal and the stainless steel occurs between said metal and the silane release agent. Upon completion of the plating operation in which the tin coating reaches a thickness of from I to 20 microns the plating step is terminated. agitation is discontinued. and the voltage is increased so that the exposed surface of the tin is treated with a high current density. Thereafter the tin coated steel caul plate is removed and oxidized in an oven or by treatment with a hydrogen peroxide solution in a manner similar to that hereinbefore set forth in Example I. Thereafter a bonding agent comprising a gamma-aminopropyltriethoxysilane is placed on the exposed surface and the tin coating is bonded to a paper phenolic resin which is in a prepreg state. The bonding is accomplished by pressing at a pressure of about 1000 pounds per square inch and an elevated temperature of about 350"F. for a period of time sufficient to realize the desired stage of cure. When this stage of cure is reached. pressure and heat are removed and the caul plate is separated from the tin-clad paper phenolic resin laminate. Other tin-clad laminates possessing a thicker coating of from 15 to 20 microns are prepared according to the invention, etched and the strips are subjected to a peel value test. said peel value test disclosing the fact that these strips possess a peel value greatly in excess of the values for those strips which have not undergone the aforemcntioned three stages of treatment prior to bonding to the laminatc.

WHAT WE CLAIM lS:- l . A method of preparing a metal-clad laminate which comprises depositing a layer of an electrically conductive metal having a thickness of from l to 20 microns on a surface of a carrier bearing a release agent. treating the upper side of the metal layer to improve its adhesive properties with respect to the bonding of a base member. bonding the metal layer to a base member and separating the resulting metal-clad laminate by removing the carrier bearing release agent.
2. A method as claimed in claim I wherein the electrically conductive metal is copper.

nickel. tin or gold.
3. A method as claimed in claim I or 7 wherein the base member comprises glassreinforced epoxy resin. glass-rcinforccd polyimide resin. paper-impreganted phenolic resin.
4. A method as claimed in any of claims l to 3 wherein the carrier is metallic and the electrically conductive metal layer is deposited electrolytically.
A A method as claimed in any of claims l to 3 wherein the carrier is non-metallic and the electrically conductive metal layer is deposited electrolessly.
6. A method as claimed in any of claims I to 5 wherein the upper side of the metal layer is subjected to a high current density to improve its adhesive properties.
7. A method as claimed in any of claims I to 5 wherein the upper side of the metal layer is subjected to oxidation conditions to improve its adhesive properties.
8. A method as claimed in any of claims l to 5 wherein after deposition of the electrically conductive metal layer on the carrier. the upper side of this layer is subjected to a high current density and then oxidized. a bonding agent is applied to it and it is then bonded to the base

member.
9. A method as claimed in any of claims l to 8 wherein the metal layer is bonded to the base member by application of heat and pressure.
10. A method as claimed in any of claims l to 9 wherein a metal layer is bonded to one face of the base member.
I I. A method as claimed in any of claims l to 9 wherein metal layers are bonded to two opposed faces of the base member by use of two of the metal-coated carriers.
12. A method of preparing a metal-clad laminate carried out substantially as hereinbefore described or exemplified.
1 3. A metal-clad laminate when prepared by a method as claimed in any of claims I to 12.
14. A copper-clad laminate prepared by depositing a layer of copper having a thickness of from 1 to 20 microns on a stainless steel carrier coated with a silane release agent, treating the copper surface by subjecting it to a high current density and oxidizing it by the application of heat, coating the oxidized surface with a bonding agent, bonding the coated surface to a base member and removing the stainless steel carrier and adhering silane release agent.
15. A laminate as claimed in claim 14 wherein the bonding agent is a silane bonding agent and the base member is a glass-reinforced epoxy resin.
16. A circuit board comprising a metal-clad laminate as claimed in claim 13, 14 or 15.