GB2169240A

GB2169240A - Sandwich structure laminate for printed circuit board substrate

Info

Publication number: GB2169240A
Application number: GB08521898A
Authority: GB
Inventors: David Dobson
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-09-07
Filing date: 1985-09-03
Publication date: 1986-07-09
Also published as: GB8422578D0; GB8521898D0

Abstract

Substrates for printed circuit boards having overall system dielectric efficiency and mechanical strength characteristics comprise a sheet form core material of low mechanical strength, stiffness and dielectric constant, and affixed to a planar surface thereof to confer mechanical strength and stiffness thereon a thinner sheet or skin of a second material of higher mechanical strength and higher dielectric constant.

Description

SPECIFICATION Sandwich structure laminate for printed circuit board substrate This invention relates to printed circuit board (PCB) substrates and methods of producing same.

FOB substrates are normally laminar materials which are used as a means for carrying conductors and devices which go to make up electrical circuits.

PCBs are used throughout industry as building blocks for electronic hardware.

The term 'substrate' as used herein relates to the discrete structure used by the industry as the base to which conductive layers are applied for the production of PCBs per se. To form the PCB, such substrates may be coated in a step subsequent to substrate production, with conductive materials, either directly or by use of bonding agents which attach the substrate to the conductive materials.

Many materials are known for use as substrates for printed circuits. The type of application or environment in which the circuit will be operating will often dictate the type of FOB substrate used. Specific mechanical properties e.g. strength or stiffness may be required; resistance to hostile environments such as corrosive gases or ultra violet radiation may be needed; and furthermore the ambient working temperature is important as are the temperature peaks induced by the active circuit elements. In addition, the actual electrical performance of the material, its insulation value (or breakdown voltage), dielectric constant and loss tangent, have an effect on the circuit designer's choice of FOB substrate. Cost always has to be assessed as this affects the market price of the hardware.Finally, the FOB substrate needs to be capable of accepting state of the art processing techniques.

Such techniques conventionally involve the bonding onto the substrate surface of conductive layers, the selective removal of conductor to produce a circuit and the soldering of electrical devices to various parts of the circuit.

As FOB substrates need to be electrical insulators, the most common materials are ceramics and plastics. The latter are acknowledged to be relatively weak, soft and flexible by comparison to other engineering materials. Consequently, the plastics are often reinforced to enhance the mechanical strength, hardness and stiffness. This reinforcement is generally of a fibrous nature where stiff fibres are impregnated throughout the plastic.

The diameter, length and mechanical characteristics of these fibres can vary enormously. They may be randomly orientated in their impregnated form or be pre-woven into a fabric. Another type of reinforcement is bulk fillers. These are of particulate form and again may have a wide variety of shapes and sizes. They also enhance strength, hardness and rigidity, but not as efficiently as fibrous reinforcement.

There are certain FOB applications where the dielectric properties of the substrate (dielectric constant and loss tangent) have a very important effect on the efficiency characteristics of the FOB as a whole. For example, in a microstrip antenna application, the attenuation due to dielectric losses in the FOB substrate are known significantly to reduce the system efficiency. As an electric current travels down the FOB conductor which is carried on the substrate, magnetic field lines are generated which interact with the substrate. The result of this interaction is dependent upon the dielectric proper- ties of the substrate, that is, dielectric constant and loss tangent.The dielectric constant is a measure of the material capacitance under a given set of conditions (such as voltage frequency and ambient temperature). It is the ratio of the permittivity of the material to the permittivity of a vacuum. Permittivity determines the size of the force acting between a pair of electric charges separated by the dielectric material. The loss tangent is a measure of the phase lay between the electric field and the polarisation induced by the field in the dielectric.

Both dielectric constant and loss tangent are very important factors in the overall loss characteristics of the system; the higher their values the more power that would be attenuated in a microstrip antenna as previously mentioned. This power loss is a function of current frequency, as are values of dielectric constant and loss tangent.

The foregoing is particularly true of microstrip and stripline circuits where the conductors and dielectric FOB substrate are in contact over a relatively large area and the operating circuit voltage is at high frequency, often in the microwave region. In applications where the system attenuation must be minimised, dielectric losses must also be minimised.

There are many commercially available materials, mainly plastics, which are produced with controlled and minimised dielectric constants and loss tangents. However most have poor mechanical properties. As described above, the properties of such materials are conventionally enhanced by bulk reinforcement (fibrous and particulate). It is a disadvantage of such systems though that this reinforcement generally deteriorates the good dielectric properties of the base polymer.

Thus, there is a desideratum in the industry for a means of giving mechanical and/or environmental enhancement to the properties of dielectrically good materials, without detracting from those good dielectric properties. According to the present invention there is provided a laminar printed circuit board (PCB) substrate which comprises a first sheet material (also termed "core" herein) of low mechanical strength and low dielectric constant and, affixed to a planar surface thereof to confer mechanical strength and stiffness thereon, a thinner sheet of a second material (also termed "skin" herein) of higher mechanical strength and stiffness and higher dielectric constant than said first material.In.a preferred embodiment the FOB substrate comprises two of said thinner sheets or skins of second material, these being affixed to respective planar surfaces of the first sheet material or core.

In practice, the skin(sYmay be attached to the core surface(s) for example by means of an adhesive or by a hot bonding technique, although the invention is not meant to be limited in this respect. Generally, therefore, the substrates of the invention are those in which a dielectrically good but mechanically weak core material is provided with one or more outer skins which are stiff and strong, although generally of poorer dielectric quality than the core. These outer skins may also protect the core material from, say, impact abuse or a harsh environment.

The outer skins are thin by comparison to the core of the FOB substrate, and as mentioned above, the dielectric properties of the skins will be less desirable than those of the core. However, by having the ratio of skin to core thickness relatively small, the reduction in dielectric performance of the whole substrate compared to the core alone will also be small, whilst the good mechanical properties of the skin are such as to confer on the substrate as a whole handling characteristics which are not a feature of the core sheet per se. In substrates according to the invention which are suitable for a range of FOB applications, the core sheet preferably has a thickness of from 0.10 to 20 mm, more preferably 0.15 mm to-15 mm. The thickness of the or each skin is preferably from 0.01 mm to 10 mm, more preferably from 0.05 mm to 5 mm.

Depending on the nature of the core and skin materials, and consistent with the specified differences in their mechanical strength characteristics, the skin : core thickness ratio in substrates of the invention is preferably from 2 - 30 %, more preferably 5 - 15 % and most preferably 8 - 12 %. Substrates with skins about 10 % of the thickness of the core have been found to be particularly useful in the production of PCB's, for example those having a 1.3 mm core and a 0.13 mm skin affixed thereto.

The outer skins, which may be termed flexural stress carrying members, are thin, but provided they are stiff and strong, massive increases in the flexural properties of the FOB substrate can be achieved compared with the core sheet per se. On the thicknesses quoted for the typical example above, stiffness and strength increases of between 100 % and 10,000 % can easily be achieved with the correct material combinations. It has been shown that laminating a low density polyethylene (PE) core with flexural stiffness of nominally 2.0 MPa and thickness nominally 2.0 mm, with outer skins of a nominally 0.13 mm thick glass reinforced plastic material having a tensile stiffness which is typical, i.e. not exceptional, for its class of material, yields a resultant composite having a flexural stiff ness of about 150 MPa.

The core material dielectric constant is preferably from 1.01 to 15 (at say, 10 GHz), although a more preferred range is from 1.02 to 8 for low loss systems and most preferably from 1.02 to 3. Core loss tangents are preferably from 0.00001 to 0.002 (again at 10 GHz) with a more preferred range of 0.00001 to 0.0015. The dielectric constant value (at 10 GHz) of the skin material is preferably from 2.5 to 15 with a preferred range of 3.5 to 5. The preferred loss tangent value of the skin material is from 0.002 to 0.04 (again at 10 GHz) with a more preferred range of from 0.004 to 0.02.

Preferably the dielectric constant of the skin material is not more than 1000 % that of the core, more preferably not more than 200 % thereof, and most preferably not more than 150 % of same. The dielectric constant and loss tangent of the laminated composite are a function of their relative values in the core and skin materials prior to laminating, together with the relative skin thicknesses. Provided the core to skin thickness ratio is high, the dielectric properties of the laminated composite will not be significantly impaired by the presence of thin outer skins. Typically, with core to skin thickness ratios in the region of 10 1, the re- sultant dielectric constant and loss tangent values of the composite substrate may be increased over and above the values of the core material by between 5 % and 15 %.

By use of the invention, therefore, it is possible to impart gross increases in mechanical properties with minor overall detrimental effects on dielectric properties by laminating dielectrically desirable core materials with one or more stiff, thin outer skins.

These outer skins are preferably designed to impart additional impact resistance, and more over are preferably designed to give good envirnomental tolerance; to this end, the edges of the laminate may be sealed to prevent, say, water ingress which would have a detremental effect on the substrate and ultimately on the FOB produced therefrom.

The core materials may be any low dielectric loss material such as polytetrafluoroethylene (PTFE), polystyrene (PS), polyethylene (PE), polysulphone, polyetherulphone, polyimide, polyetherimide, or polyphenylene oxide. Foamed versons of the core materials are also a preferred option, since foaming generally reduces the dielectric losses of the bulk material.

The skin materials may be any electrically insulating material that has a relatively high strength and stiffness, for example a plastic such as polyester or epoxy resin. Preferably the skin materials are reinforced,.for example with high strength and high stiffness reinforcing fibres such as glass or aramid fibres or with a particulate filler. The fibres may be randomly orientated or woven into a fabric, so giving strength increase characterstic of such formations.

A second aspect of the invention provides a meThod of producing the defined substrate which comprises providing a sheet of first material and thinner sheet of second material, and affixing together the planar sheet surfaces by means of adhesion or hot melt bonding. It is to be understood that such method differs from the technique of producing PCB's by conventional processes wherein the dielectric core may be treated with an adhesive layer which is not self supporting, immediately prior to the application of the conductive layer which is then converted into patterned format by conventional FOB production techniques, or immediately prior to a masking layer which is employed to determine and facilitate the applicationof patterned conductor member.The skin of the present method is applied to confer mechanical strength on the core material and the combination of core and skin constitutes a distinct element of FOB production, rather than a transitory configuration such as would be the case with a mere adhesive layer or masking member. The method of the present invention may therefore be considered as being absent any simultaneous or immediately subsequent application of conductive material to the substrate.

As mentioned above the skins may be attached to the cores by various techniques, and to one or more planar surfaces of the core. An adhesive may be applied to the interface between the two. This adhesive may vary in type and thickness but a typical example is an epoxy adhesive with nominal thickness of the bond line being 0.04 mm. This adhesive may be applied in liquid, paste or sheet form. Pressure may be applied during the cure cycle of the adhesive, so may heat. Thermal bonding may be used by locally melting either the skin or core material in the region of the interface, whilst the skins and core are kept under pressure.

This method of reinforcing dielectrically superior core materials with stiff outer skins to produce a composite FOB substrate is a cost efficient means of achieving good mechanical and dielectric properties, using base materials which may be significantly cheaper than bulk reinforced FOB substrate materials. The substrates of the invention are particularly suitable for use in the production of, for example, microstrip or stripline PCB's which have a relatively large area of the substrate coated with conductive material. It is therefore important in such PCB's to minimise the dielectric value of the substrate, whilst maintaining good mechanical strength properties of same in order to facilitate production techniques and minimise raw material costs.

According to yet another aspect of the invention, therefore,there is provided a method of making a printed circuit board which comprises in a first step producing a substrate by applying to a first sheet comprising material of low mechanical strength and dielectric constant, a second sheet being a flexural stress carrying member which is thinner than the first, of a material having higher mechanical strength and dielectric constant than said first material, to form a substrate having overall system dielectric efficiency and mechanical strength characteristics acceptable for microstrip applications, and in a second step applying to the surface of said second sheet, a conductive member in patterned format or suitable for conversion to said patterned format, or a mask adapted for receiving such a conductive member. The first step may therefore be carried out at a position remote from that where the second step is performed; for example the second step may be carried out after storage of the substrate product of the first step.

Claims

1. A laminar printed circuit board (PCB) substrate which comprises a first sheet material of low mechanical strength and low dielectric constant, and affixed to a planar surface thereof, to confer mechanical strength and stiffness thereon, a thinner sheet of a second material of higher mechanical strength and stiffness and higher dielectric constant than said first material.

2. A substrate according to claim 1 which comprises two of said thinner sheets of second material affixed to respective planar surfaces of said first sheet material.

3. A substrate according to claim 1 or 2 wherein the or each thinner sheet is affixed to the or the respective planar surface of the first material by means of an adhesive or by means of a hot bonding technique.

4. A substrate according to claim 1, 2 or 3 wherein the thickness of the or each thinner sheet is from 2 - 30 % that of the first sheet material.

5. A substrate according to claim 4 wherein the thickness of the or each thinner sheet is from 5 15 % that of the first sheet material.

6. A substrate according to claim 5 wherein the thickness of the or each thinner sheet is from 8 12 % that of the first sheet material.

7. A substrate according to any one of the preceding claims wherein the dielectric constant of the second material is not more than 1000 % that of the first.

8. A substrate according to claim 7 wherein the dielectric constant of the second material is not more than 200 % that of the first.

9. A substrate according to claim 8 wherein the dielectric constant of the second material is not more than 150 % that of the first.

10. A substrate according to any one of the preceding claims wherein the first material is of foamed construction.

11. A substrate according to any one of the preceding claims wherein the first material comprises polytetrafluoroethylene, polystyrene or polyethylene.

12. A substrate according to any one of the preceding claims wherein the second material comprises a plastic substance.

13. A substrate according to claim 12 wherein the plastics substance comprises polyester or epoxy resin.

14. A substrate according to any one of the preceding claims wherein the second material contains high strength and high stiffness reinforcing fibres.

15. A substrate according to any one of the preceding claims wherein the second material contains glass reinforcement.

16. A substrate according to any one of the preceding claims wherein the second material contains particulate reinforcement.

17. A substrate according to any one of the preceding claims wherein the edge surface of the laminar substrate is sealed.

18. A method of producing a substrate according to any one of the preceding claims which comprises providing a sheet of first material and thinner sheet or second material, and affixing together the planar sheet surfaces by means of adhesion or hot melt bonding, absent any simultaneous or immediately subsequent application of conductive material to the substrate.

19. A method of producing a substrate according to claim 18 where the sheet of second material is applied to both surfaces of the first material.

20. A printed circuit board comprising a substrate according to any one of claims 1 to 17 or produced by the methods of claims 18 and 19.

21. Electrical or electronic equipment which comprises a printed circuit board according to claim 20.

22. Electrical or electronic equipment according to claim 21 when in the form of microstrip or strip- line equipment.

23. A method of making a printed circuit board which comprises in a first step producing a substrate by applying to a first sheet comprising material of low mechanical strength and dielectric constant, a second sheet being a flexural stress carrying member which is thinner than the first, of a material having higher mechanical strength and dielectric constant than said first material, to form a substrate having overall system dielectric efficiency and mechanical strength characteristics acceptable for microstrip applications, and in a second step applying to the surface of said second sheet, a conductive member in patterned format or suitable for conversion to said patterned format, or a mask adapted for receiving such a conductive member.

24. A microstrip antenna comprising a printed circuit board which has been made by the method of claim 23, or which comprises a substrate according to anyone of claims 1 - 17 or produced by the method of claim 18 or 19.