GB2240663A - Metal base wiring board - Google Patents

Abstract

A metal base wiring board comprises a metallic plate (1) having adhered thereto a circuit-forming conductive metallic foil (3') via an insulating film (2) formed of a resin having a high glass transition point. An adhesive layer (not shown) having a thickness of not more than 10 mu m is interposed between the metallic plate (1) and the film (2) and between the film (2) and the metallic foil (3'). The gaps formed by etching the foil (3') to create the circuit pattern are filled with insulating resin (6). The foil (3') may comprise a layer (31) of aluminium or nickel and a layer (32) of copper. <IMAGE>

Description

METAL BASE WIRING BOARD This invention relates to a metal base wiring board comprising a metallic plate having provided thereon a circuit-forming conductive metallic foil. The board exhibits satisfactory properties in hot press wire bonding to the circuit pattern and in bending processability; and is therefore suitable for the production of a hybrid integrated circuit (IC).

To meet the latest demand for miniaturization of electronic equipment, there have been proposed hybrid ICs comprising circuit components, such as resistors, condensers and transistors, mounted on the same board.

For use in such hybrid ICs, a wiring board comprising a metallic plate as a base (referred to as a metal base wiring board) has been attracting attention because of its heat-dissipation properties. A metal base wiring board utilizing the characteristics of plasticity and processability possessed by metals, and which is capable of being bent, has attracted particular attention.

Known metal base wiring boards comprise a metallic plate made of, for example, aluminum, to which a circuitforming conductive metallic foil is adhered via an insulating layer formed of one of a variety of resins having a low glass transition point, such as a rubber-modified epoxy resin or a nylon-modified epoxy resin having a glass transition point of 60-8O0C.

However, when bare chips are mounted onto the circuit formed by processing the conductive metallic foil to produce a hybrid IC, difficulty has been encountered with the conventional board in accomplishing wire bonding, that is, the hot press bonding of a bonding wire by utilizing ultrasonic vibration while heating the metal base wiring board to connect the bare chips to the circuit.

The present inventors have conducted extensive investigations to overcome the above-described disadvantages of the conventional boards. As a result, it has now been found that the disadvantages can be overcome by using an insulating film comprising a resin having a high glass transition point as an insulating layer, and adhering the insulating film via an adhesive layer to a metallic plate base and to a conductive metallic foil by means of a special technique.

According to the present invention there is provided a metal base wiring board which comprises a metallic plate having adhered thereto a circuit-forming conductive metallic foil via an insulating film comprising a resin having a high glass transition point, with an adhesive layer having a thickness of not more than 10 pm being interposed between the metallic plate and the insulating film, and between the insulating film and the circuitforming conductive metallic foil.

In the present invention, the insulating film must contain a resin having a high glass transition point so that the insulating film retains a good rigidity even at a high temperature of at least 1000cm particularly at 2000C or more. Further, the thickness of the adhesive layer must be 10 pm or less. Since such a thin adhesive layer has low energy absorption, hot pressing can be stably conducted by ultrasonic vibration of a bonding wire while heating the metal base wiring board and, as a result, a bonding technique of great precision can be achieved. Furthermore, since the insulating film as described above has both excellent withstand voltage characteristics and excellent flexibility, the metal base wiring board of the present invention also has excellent bending processability.

embodiments of the invention will now be described, by way of example, with reference to the accompanying drawing in which Figure 1 is a schematic cross-section of a metal base wiring board according to the present invention; Figure 2 is a schematic cross-section of the metal base wiring board of Fig. 1 in which a circuit patterned layer has been formed; and Figures 3 and 4 are each a schematic cross section of a circuit board according to another embodiment of the present invention, in which the gaps in the circuit pattern are filled with an insulating resin.

The metal base wiring board shown in Fig. 1 comprises a metallic plate 1, an insulating film 2 formed of a resin having a high glass transition point, a circuit-forming conductive metallic foil 3, and adhesive layers 4 and 5.

As the metallic plate 1 functions inter alia as a heat sink, the type of metallic material is not particularly restricted. A material having satisfactory heat conductivity, e.g. aluminum, copper or iron, is preferably used. The thickness of the metallic plate is appropriately selected, usually in the range of from 100 pm to 5.0 mm, preferably from 0.5 mm to 4.0 mm, and more preferably from 1.0 mm to 3.0 mm.

The insulating film 2 functions to insulate the conductive metallic foil 3 substantially from the metallic plate 1 and is formed of a resin having a high glass transition point. The glass transition point as used herein can be measured by a generally known method using a DSC (Differential Scanning Calorimeter). Specifically, resins having a glass transition point measured by DSC of at least 800C, particularly at least 1000C,are preferably used in the insulating film 2 of the present invention, and further, those having a glass transition point measured by DSC of an environmental temperature employed in operating wire bonding treatment or higher (generally, 1200C or more) are most preferred.So-called high heatresistant resins which undergo heat decomposition are also included in the definition "resins having a high glass transition point" and are preferred for forming the insulating film 2.

Specific examples of the resins usable to form the insulating film 2 include heat-resistant resins such as polyimides, aromatic polyamides, polyphenylene sulfide and polyethylene naphthalate. Of these, polyimides and aromatic polyamides are preferred.

Examples of the polyimides include those obtainable from the reaction of an aromatic tetracarboxylic acid dianhydride (e.g. pyromellitic dianhydride, benzophenonetetracarboxylic dianhydride, naphthalenetetracarboxylic dianhydride and biphenyltetracarboxylic dianhydride) and an aromatic diamine (e.g. p,p'-diaminodiphenyl ether, p,p'-diaminodiphenylmethane and m-phenylenediamine).

As commercially available forms thereon, Kapton H/(manu

factured by DuPontT and Upilext(manufactured by Ube Industries, Ltd.) may be cited.

Examples of the aromatic polyamides include those obtainable from the reaction of an aromatic carboxylic acid (e.g. m-phenylenedicarboxylic acid and o-phenylenedicarboxylic acid) and an aromatic diamine (e.g. m-phenylenediamine and p-phenylenediamine). Nomex (manufactured by DuPont) may be cited as a commercially-available form thereof.

Further, Torelina t(manufactured by Toray Industries, Inc.) may be cited as a commercially-available form of the polyphenylene sulfide. Q-film (manufactured by Teijin Limited) may be cited as a polyethylene naphthalate obtained from the reaction of naphthalene dicarboxylic acid and ethylene glycol.

The thickness of the insulating film may be appropriately determined, and is generally from 10 to 200 pm, preferably from 12 to 100 pm, and more preferably from 20 to 50 pm. In view of the need for heat conductivity, the thinner the film is the better.

The circuit-forming conductive metallic foil 3 includes not only single-layer foil comprising a conductive metal, e.g. copper, but also multi-layer laminated foil comprising two or more different kinds of metal, e.g.

A1/Cu, Ni/Cu, Au/Cu and Au/Ni/Cu. Among these laminated foils, the combinations Au/Ni/Cu, Al /Cu and Ni/Cu are preferred. The multi-layer laminated metallic foil can be obtained by, for example, depositing a different kind of metal on the basic metal layer by plating or vacuum evaporation, or by press bonding a metallic foil of different kind onto the basic metal layer. The thickness of the conductive metallic foil is appropriately selected usually in the range of from 10 to 200 pm, preferably from 35 to 105 pm.

Where an adhesive is used for adhesion between the metallic plate 1 and the insulating film 2 and between the insulating film 2 and conductive metallic foil 3 as shown in Fig. 1, the thickness of each adhesive layer 4 and 5 should not exceed 10 pm, and preferably fall within the range of from 5 to 7 pm. When the surface of the metallic plate 1 or insulating film 2 exhibits fine unevenness, the surface is defined as a plane formed by connecting the apices of convex areas, and the thickness of the adhesive layer is defined in the present specification as the average distance between the surfaces defined above. The embodiment of Fig. 1 exhibits enhanced hot press wire bonding properties compared with conventional wiring boards.

Adhesives which can be used in this embodiment are not particularly limited and include, for example, epoxy adhesives (especially those containing novolak epoxy resins), polyimide adhesives, acrylic adhesives and silicone adhesives. Of these, epoxy adhesives and polyimide adhesives are parti#cularly preferred. Accordingly, adhesives comprising resins having a low glass transition point may also be employed. Adhesion between layers can be effected by an appropriate method according to the type of the adhesive used, for example by coating followed by drying or by heat-curing. Conditions for the hot press bonding are not particularly limited.

For example, hot press bonding may be carried out under the conditions of 20 kg/cm2 pressure at 1800C for 40 minutes.

Fig. 2 shows the structure in which a circuit patterned layer 3' is formed by etching a conductive laminated metallic foil composed of two or more different kinds of metal. Formation of a circuit pattern of the conductive metallic foil can be carried out by, for example, forming a resist layer on the metallic foil by using a pattern mask corresponding to the desired pattern, and then removing the unwanted area of the foil by an etching treatment.

According to the preferred embodiment of the present invention shown in Figs. 3 and 4, the gaps in circuit patterns of the circuit patterned layers are filled with an insulating resin, the wire bonding workability thereby being generally increased.

The insulating layer 2, which functions to insulate the circuit patterned layer 3' from the metallic plate 1 is formed by a resin having a high glass transition point as- described above. The layer 2 is usually formed by an insulating resin which is of a type which will also serve as an adhesive layer between the circuit patterned metallic foil layer 3' and the metallic plate 1.

The adhesive layers 4 and 5 shown in Fig. 1 are omitted from Fig. 2 and Figs. 3 and 4 for the sake of clarity.

The circuit patterned layer 3' is made of a multilayer laminated metallic foil comprising different kinds of metal 31 and 32. Such a multi-layer laminated metallic foil may be formed by an appropriately chosen method.

For example, the metallic layer 31 can be deposited on the basic metallic layer 32 by plating or by vacuum evaporation, or cladding metallic foil 31 may be pressbonded onto the basic metallic layer 32. The combination of different kinds of metals can be decided appropriately depending on the end use. For instance, a combination of Al and Cu or a combination of Ni and Cu may be employed. In these combinations, the Cu layer is generally formed as the lower layer (basic metallic foil).

The circuit patterned layer 3' can be formed by, for example, forming a resist layer on the multilayer laminated metallic foil by using a pattern mask, for example, corresponding to the desired pattern and then removing the unwanted area of the multi-layer laminated metallic foil by etching treatment. In using the above-described multi-layer laminated metallic foil composed of a Cu layer as a lower layer and an Al or Ni layer as an upper layer, the Cu layer is usually liable to undergo over-etching, resulting in the formation of the circuit patterned layer 3' as shown in Fig. 3.

That is, the formed circuit patterned layer comprises a Cu layer 32 whose cross section has a concave surface as shown in Fig. 3 on which an Al or Ni layer 31 is formed with projections 33 over the upper surface of the Cu layer. In general, the degree of over-etching becomes conspicuously higher as the thickness of the Cu layer increases.

The insulating portion 6 serves to prevent shortcircuits in the circuit pattern layer comprising a multilayer laminated metallic foil. It is formed by filling an insulating resin in the gaps in circuit patterns.

Since it is difficult to fill the insulating resin in the gaps so as to achieve an even surface on the circuit patterned layer, the gaps are in practice filled with the insulating resin so that a part of insulating resin bulges out as shown at 6a in Fig. 4. The insulating resin and the method of filling are appropriately selected. Examples of the insulating resin include thermoset solder resists and ultraviolet-curing solder resists.

As the filling method, screen printing is generally utilized. In order to ensure the prevention of short-circuits and to extend durability of the circuit board, it is preferable to use an insulating resin having a dielectric constant from 0.9 to 1.2 times that of the insulating material constituting the insulating layer 2. Further, for the purpose of preventing incorporation of air bubbles during filling which would accelerate deterioration of the circuit board, it is preferable to use a liquid insulating resin or a liquefied insulating resin, followed by solidification. It is particularly preferable that a resist resin is filled in the gaps by, for example, screen printing and then solidified by exposure to light or the like technique.

Since the circuit patterned layers are insulated from each other by the insulating resin filled therebetween, short-circuits between the circuit patterns or between the patterned layer and the metallic plate base are prevented. The circuit board according to this embodiment therefore has excellent durability with respect to voltage application.

The present invention will now be illustrated in greater detail with reference to the following Examples and Comparative Example.

EXAMPLE 1 A 25 pm thick polyimide film coated on both sides thereof with an epoxy adhesive layer was superposed on a 2 mm thick aluminum plate, and a 35 pm thick copper foil was then superposed thereon. The resulting laminate was then hot pressed at 1700C and 30 kg/cm2 for 60 minutes to obtain a metal base wiring board. The thickness of each of the adhesive layers was 7 pm.

EXAMPLE 2 A 25 pm thick polyimide film coated on both sides thereof with a polyimide adhesive layer was superposed on a 2 mm thick aluminum plate, and a 35 pm thick copper foil was then superposed thereon. The resulting laminate was next hot pressed at 1700C and 30 kg/cm2 for 60 minutes to obtain a metal base wiring board. The thickness of each of the adhesive layers was 5 pm.

COMPARATIVE EXAMPLE A metal base wiring board was prepared in the same manner as in Example 1, except for changing the thickness of each of the epoxy adhesive layers to 15 pm.

On the copper foil of each of the metal base wiring boards obtained in Examples 1 and 2 and the Comparative Example, an Ni layer having a thickness of about 2.5 pm was deposited by plating. An Au layer having a thickness of about 1.5 pm was further deposited thereon by plating.

An Au bonding wire was bonded to the Au deposit layer by ultrasonic vibration while heating the metal base wiring board to a temperature shown in the Table below. The ball shear strength of the joint and the percentage success of the bonding operation were measured to evaluate the hot press wire bonding properties of the metal base wiring board. The results obtained are shown in the Table.

Temp. of Example Example Comparative Board 1 ~ 2 ~Example ( C ) Ball Shear 25 45 43 46 Strength (g) 75 51 55 48 125 66 69 52 175 69 75 55 Percentage 25 100 100 100 Success of Bonding (%) 75 100 100 100 125 100 100 95 175 100 100 95 Ball Shear Strength Loading was applied to the bonding ball end of the Au bonding wire in the vertical direction, and the load at the moment of the peeling of the wire from the board was measured by using a tension gauge.

Percentage Success of Bonding The percentage success of bonding is defined as the percentage of bonding of the wire to the board without spring up of the wire (bonding miss) at the location of wire bonding.

It will be appreciated that, while the invention has been described and claimed in terms of a separate layer of adhesive on each face of the insulating film, the insulating film may be formed of an inherently ad hesive material; the film can thus be laminated directly to the metallic plate and to the metallic foil, enabling the separate layers to be dispensed with.

Claims (14)

1. A metal base wiring board which comprises a metallic plate, a circuit-forming conductive metallic foil and an interposed insulating film comprising a resin having a high glass transition point, an adhesive layer having a thickness of not more than 10 pm being positioned between the metallic plate and the insulating film, and between the insulating film and the circuit-forming conductive metallic foil.

2. A board as claimed in claim 1, in which the thickness of each adhesive layer is from 5 to 7 pm.

3. A board as claimed in claim 1 or 2, in which said high glass transition point is 1200C or more.

4. A board as claimed in any preceding claim, in which the thickness of the insulating film is from 20 to 50 pm.

5. A metal base wiring board as claimed in any preceding claim, in which said insulating film comprises polyimide or aromatic polyamide.

6. A board as claimed in any preceding claim, in which said circuit-forming conductive metallic foil is formed of a multi-layer laminated metallic foil comprising metals of different kinds.

7. A metal base wiring board as claimed in claim 1 and substantially as herein described.

8. A metal base wiring board substantially as herein described with reference to Fig. 1 of the accompanying drawings or in the foregoing Example 1 or 2.

9. A board as claimed in any preceding claim and which has been subjected to an etching treatment to form a circuit patterned layer, the gaps in the circuit patterns of the circuit patterned layer formed by the etching treatment being filled with an insulating resin.

10. A board as claimed in claim 9, in which the insulating resin has a dielectric constant of from 0.9 to 1.2 times that of the material forming the insulating film.

11. A metal base wiring board as claimed in claim 9 and substantially as herein described.

12. A metal base wiring board substantially as herein described with reference to Fig. 2 or to Figs. 3 and 4.

13. A hybrid integrated circuit including a board as claimed in any one of claims 9 to 13.

14. The features as herein disclosed, or their equivalents, in any novel patentable selection.