Process for making substrates for printed circuit boards.

This invention relates to a process for manufacturing substrates for printed circuit boards and the products made by that process. More particularly, it relates to a process which can be used to manufacture, from the same basic materials, a wide variety of substrates especially suited for printed circuit boards and having a low dielectric constant and electrical characteristics meeting various end-use requirements.

The various types of substrates made by the process are especially suited for printed circuit boards which meet diversified needs ranging from general electronic devices to computers requiring high-speed signal transmission.

Base materials for printed circuit boards used in conventional electronic equipment have included: <ul id="ul0001" list-style="none"><li>(1) Glass/epoxy or paper/epoxy composite base materials prepared by impregnating glass cloth or paper with an epoxy resin, following by curing; or paper/phenoplast base materials prepared by impregnating paper with a phenoplast resin, following by curing;</li><li>(2) Flexible insulating materials such as polyimide film and polyester film;</li><li>(3) Heat resistant base materials made of a glass fiber/polyimide composite;</li><li>(4) Low-dielectric-constant base materials, such as epoxy/Kevlar (Kevlar is a registered trade mark), polyimide/Kevlar and epoxy/quartz composites; and</li><li>(5) Low-dielectric-constant base materials composed of glass cloth and polytetrafluoroethylene.</li></ul>

Each of the above-mentioned materials provides final products having well defined characteristics; that is, it is impossible or impractical to manufacture products having varying characteristics from the same base material.

Materials of type (1), which are the most widely used, have relatively high dielectric constant and can usually not be applied to computers and other equipment for which processing at increasingly higher transmission speeds has been required in recent years. The same is true of the materials of types (2) and (3), which normally do not provide printed circuit boards of sufficiently low dielectric constant. The level of dielectric constant of these materials at a frequency of 1 MHz is as follows: <ul id="ul0002" list-style="none"><li>4.5-5.5 for (1), 3.5-5.0 for (2), 4.3.-4.7for (3)</li></ul>

Materials of type (4), though excellent in thermal resistance, also have a dielectric constant ranging from about 3.5 to 4.0 and normally cannot meet the need for higher speed of signal transmission.

Materials of type (5) have dielectric constant of about 2.5 to 3 and are regarded as low-dielectric-constant base materials. But their disadvantages include low adhesion to metal foil and poor workability.

All of these base materials are not only unable to meet the transmission speed requirements for computers or the like in terms of dielectric constant but also exhibit different physical property characteristics, such as thermal resistance, heat conductivity and mechanical properties and flexibility. Hence, the type of base material to be used must be selected to meet the specific requirements for each end use. Thus, manufacturers have to store many types of materials in inventory in order to satisfy a variety of needs, or they can accept orders ony for a few, selected types of products.

Even when various base materials are stored in inventory, it is not always possible to meet every specific need.

According to the present invention there is provided a process for the manufacture of a composite sheet of a dielectric material comprising impregnating a sheet of porous, expanded, sintered polytetrafluoroethylene (PTFE) with a solution of at least one other resin dissolved in a solvent, heating the impregnated sheet to evaporate and remove said solvent, thereby providing a composite sheet of expanded, porous PTFE containing at least one other resin and air within the pores of said sheet, and thereafter compressing said sheet to expel at least a portion of said air from said sheet.

The sheet may be compressed following removal of the solvent only slightly or to an extent that substantially all of the air is expelled from the sheet or to any level in between, to provide for a variety of physical and electrical characteristics from the same base material. Use of a sheet of porous, expanded PTFE containing at least one other resin within its pores as a dielectric material, particularly as a dielectric material in a printed circuit board, is disclosed. The other resin may be contained within the sheet in an amount of at least 15 weight percent, based on the total weight of the sheet and the other resin, and may exceed at least 24 weight percent, based on the total weight of the sheet and the other resin. The other resin may be a thermoplastic resin, a thermosetting resin or a fluoropolymer. The other resin may be partially cured or fully cured. An inorganic particulate filler may be incorporated in the expanded, porous PTFE.

The composite may be laminated to a layer of rigid insulating material such as a layer of a ceramic. The composite may be laminated to other substrates such as a layer of fiberglass/epoxy composite sheet, a layer of gallium arsenide, a cloth of glass fibers, a layer of quartz fibers, a layer of aramide fibers, a layer of resin such as a layer of a polyimide resin, a layer of a polyether-ketone resin, a layer of a polyether-sulfone resin or a layer of a polyester resin. The composite may also be laminated to a sheet of electrically conductive material such as a sheet of metal, including a sheet of copper or a sheet of aluminum. The composite sheet may be laminated to at least one other composite sheet.

The other resin may be an epoxy resin, a polyimide resin, a polyester resin, an acrylic resin, a triazine resin, a bismaleinimide/triazine resin, a polyolefin resin, a polysulfone resin, a polyether-sulfone resin, a fluorinated ethylene propylene resin, a polyether ketone resin, a polyamide resin, a phenolic resin or a perfluoropropylvinylether resin.

A polymeric coating may be applied over the composite to seal it from the atmosphere.

The invention will now be particularly described, by way of example, with reference to the accompanying drawings in which:- <ul id="ul0003" list-style="none"><li>Figure 1 is a photomicrograph showing a typical node-fibril structure of the expanded, porous PTFE used in the invention;</li><li>Figure 2 is a photomicrograph showing another node-fibril structure of biaxially expanded, porous PTFE used in the invention;</li><li>figures 3-6 are enlarged cross-sectional views of the products obtainable according to the invention, and</li><li>Figure 7 shows a product according to the invention having an airtight, polymeric coating thereover.</li></ul>

A process is provided for manufacturing a composite of a sheet of expanded, porous poiytetrafluoroethylene (PTFE) impregnated with another resin, the other resin being impregnated within the pores of the expanded PTFE. The other resin imparts desired characteristics to the composite such as high thermal resistance and low dielectric constant which make it useful as substrates for printed circuit boards. The composite is useful as a dielectric generally and as an adhesive.

In particular, a process for manufacturing substrates for printed circuit boards is provided which comprises using porous, expanded PTFE as a base material, impregnating the porous base material with a resin solution that will give characteristics (thermal resistance, low dielectric constant, etc.) required for intended circuit boards, and curing said resin under pressure conditions that will give the intended characteristics.

Expanded, porous PTFE has a dielectric constant as low as about ₁.₁ to 1.8, and can be readily impregnated with suitable resin solutions because of its porous structure, although PTFE itself is chemically very stable and is difficult to bond to other types of resins or adhesives.

The electrical, physical and chemical properties of the resin solution used for impregnation according to the invention are the primary factors that determine the characteristics of the resulting composites.

A second factor that determines the characteristics of the composites is the pressure conditions adopted for curing of the resin-impregnated, porous base material.

Substrates for printed circuit boards having widely different electrical, physical and chemical characteristics can be manufactured from the same base material by properly combining these two factors.

Characteristics required for printed circuit boards used in electrical equipment are very diverse. For example, ultra-high-speed processing elements based on gallium arsenide and other relatively new materials have been developed to meet the need for high-speed signal transmission in computers, and there is a great demand also for printed circuit boards having characteristics that match this need. On the other hand, substrates to which IC chips are to be directly attached are frequently required to have, in combination, adequate thermal expansion coefficient, heat conductivity and mechanical strength, as well as low dielectric constant.

The signal transmission delay time,τ , as in high-speed computers, is proportional to the square root of the dielectric constant of the material used, and may be represented by the following formula in a strip line:<maths id="math0001" num=""><img file="EP0248617A2_D0001.tif" /></maths>wherein ε is the dielectric constant of the material and C is the velocity of light (3x10⁸ m/sec). Therefore, materials having low dielectric constant usually must be used in order to provide for higher speeds of signal transmission.

With substrates to which IC chips are to be directly attached, on the contrary, priority normally should be given to, in combination, adequate thermal expansion coefficient, heat conductivity and mechanical strength, even at the sacrifice of dielectric constant to a certain extent.

This invention meets the various needs for substrates for printed circuit boards as described above. Polytetrafluoroethylene itself has a very low dielectric constant, and hence its porous products also possess a value as low as 1.2, even at a porosity of 80%. This resin, however, is rather poor in resistance to tensile and compressive forces. These disadvantages of polytetrafluoroethylene can be avoided by the use of porous, expanded products thereof and by impregnation with a resin of another type, followed by curing of the impregnated resin.

Previously known porous, expanded PTFE impregnated with a resin of low dielectric constant or high thermal resistance, thus obtained, still normally contains a considerable volume of pores. As a result, the etching solution can work into these pores, thereby etching unintended areas. Part of the components of the etching solution is unavoidably left attached to the base material and moisture tends to work into these pores during service of the resulting printed circuit boards, thus adversely affecting their insulating and dielectric characteristics. In the process of this invention, these difficulties are avoided by effecting pressure curing. However, the dielectric constant tends to increase as the porosity is decreased by this press forming process.

This means that final products haying different dielectric constant, insulating characteristics, flexibility, mechanical strength and other properties can be obtained, even when the same type of resin is used for impregnation, by performing the compression operation under different pressures, followed by curing.

As shown in Figs. 1 and 2, the porous, expanded polytetrafluoroethylene ₁ used in the process of this invention can be obtained by rolling and stretching polytetrafluoroethylene film as disclosed in U.S. Patent 3,953,566 in the form of minute fibers 12 intertwined with one another between nodes ₁₁ like a spider web. Several sheets of such porous, expanded resin may be laminated one upon another, as required. to give base materials with a porosity ranging from 30 to 90%. The dielectric constant decreases with increasing porosity, which ranges from about 1.1 to 1.8 for the range of porosities mentioned above.

The resins to be impregnated into the porous polytetrafluoroethylene include heat-resistant resins and low-dielectric-constant resins. Illustrative examples are as follows: <ul id="ul0004" list-style="none"><li>Heat-resistant resins: epoxy, polyimide, polyester, acrylic, triazine and bismaleinimide/triazine resins.</li></ul>

Low-dielectric-constant resins: polyolefin, polysulfone, polyether-sulfone and FEP resins.

The porous polytetrafluoroethylene thus impregnated with a resin solution may then be subjected to a pressure curing operation under different conditions appropriate to the intended characteristics of printed circuit boards. For example, press forming is conducted under mild conditions when low dielectric constant is intended, whereas higher pressure conditions are adopted when priority is given to insulating charactertistics. If a press or rollers are used to effect the pressure curing, the porous structure will be moderately altered and the porosity of the resulting base material will be lower. Electrical and mechanical characteristics, as well as dimensional stability, of final products will be determined by the pressure conditions adopted.

In a base material obtained as described above, incorporating an inorganic substance, such as SiC, SiO₂ and quartz powder, into the polytetrafluoroethylene further enhances its heat conductivity, dimensional stability and mechanical strength. Dimensional stability and mechanical strength can also be improved by lamination of cloths of fibrous reinforcing materials, such as glass fibers, quartz fibers and aramide fibers or by lamination or incorporation of short-fiber reinforcing materials.

Furthermore, lamination of a resin film reinforcement, such as polyimide, polyether-ketone, polysulfone and polyester, to the base material or on at least one side thereof can further enhance mechanical properties (tear resistance, tensile strength, etc.) and prevent permeation of air and moisture into the structure.

Providing a rigid insulating material, such as glass/epoxy and ceramic sheets onto both or at least one side of the base material can also be effective in improving bending strength and dimensional stability.

In addition, providing a heat-conductive material, such as a soft or rigid metal foil, can allow effective dissipation of heat generated during the service life of printed circuit boards. Use of a soft metal material, in particular, gives flexible boards that can be deformed as desired.

When forming a conductive circuit on the base material obtained above by using metal foil (e.g., copper), an adhesive may be used for its bonding if the impregnated resin is poor in adhesion to that metal foil or if that metal foil is to be laminated onto a fibrous reinforcing material, a resin film reinforcement or a hard insulating material. Conductive circuits may also be formed by ion plating, vacuum deposition or spattering methods, in addition to a plating process using a plating catalyst layer.

The base materials for printed circuit boards manufactured according to the process of this invention may have a certain degree of porosity. When polymeric coatings 7 are provided on both sides as shown in Fig. 7. application of a resin solution around the edges makes the structure airtight, thereby preventing entry of air, moisture and etching solution to avoid lowering of insulating and dielectric characteristics and enabling etching of fine circuit patterns.

PREFERRED METHODS

Method 1

A 100 micron thick, expanded PTFE membrane of 70% porosity is fixed on a pin frame. The membrane is impregnated with MEK as a primary solvent and placed on absorbent paper. Epoxy resin is impregnated into the membrane under the driving force of the absorbent paper. The membrane with pin frame is placed into an oven to remove the solvent by drying. This results in a prepreg in which the resin is B-stage. The prepreg is laminated with copper foil on both sides under appropriate pressure.

Method 2

PTFE dispersion is mixed with 4 to 8 microns particle size quartz powder to 50 weight percent and then coagulated. Epoxy resin is added to 30 weight percent, based upon the total mixture, and uniformly mixed. This is preformed, extruded and calendered to a sheet material. The sheet is finally cured by hot pressing.

Method 3

Fabric woven from quartz fiber is impregnated with epoxy resin and dried to B-stage. Prepreg made in this way and prepreg made by Method 1 are clad with copper as in Fig. 3. This is laminated by pressure to 20%

Method 4

Polyether-ether-ketone film is laminated to the prepreg of Method and clad with copper under pressure to 33% thickness reduction.

Method 5

Glass epoxy board or film is used instead of quartz fabric in Method 3, under pressure resulting in 20% thickness reduction.

The following Manufacturing Examples will further illustrate the invention.

Manufacturing Example 1

A sheet of porous, expanded polytetrafluoroethylene film 1 having a thickness of 0.1 mm and a porosity of 70% was impregnated with 40 weight percent of an epoxy resin as described above. After drying and curing the impregnated resin by press forming to a thickness of 0.08 mm, copper foil was bonded to both sides of the film. The final product obtained had a dielectric constant as low as 2.2, and was found suitable for use as a printed circuit board in high-speed computers.

Manufacturing Example 2

The same type of 1 mm thick, porous, expanded polytetrafluoroethylene film as used in Example 1, also containing 50 weight percent of quartz powder 4 to 8 µm in particle size, was impregnated with 30 weight percent of an epoxy resin. After drying and curing the impregnated resin by press forming to a thickness of 0.09 mm, copper foil was laminated onto both sides of the film. The final product thus obtained had a dielectric constant of 2.2 and high heat conductivity and was found suitable for use in high-speed computers.

Manufacturing Example 3

Sheets of polytetrafluoroethylene film having a porosity of 70% were impregnated with an epoxy resin in the same manner as in Example ₁, a cloth layer of quartz fiber 3 was provided between the two sheets as shown in Fig. 3, and copper foil 4 was laminated onto both sides thereof and the resulting assembly was press-formed to a thickness reduction of 20%. The final product thus obtained had a dielectric constant of 2.5 and high dimensional stability and was found suitable for use in high-speed computers.

Manufacturing Example 4

A substrate for printed circuit boards was manufactured in a similar way to Example 3 except that a sheet of polyester-ether-ketone 5 was provided as shown in Fig. 4 in place of the quartz fiber 3 and a thickness reduction of 33% was adopted. It had dielectric constant of 2.8, showed high mechanical strength, and was also found suitable for use in high-speed computers.

Manufacturing Example 5

A sheet of porous polytetrafluoroethylene film having a porosity of 70% was impregnated with 40 weight percent of an epoxy resin, a glass/epoxy sheet 6 and copper foil 4 (both 0.1 mm thick) were laminated on both sides as shown in Figs. 5 and 6, and the assembly was press-formed to a thickness reduction of 20% and heated to cure the impregnated resin. The final product thus obtained had a dielectric constant of 3.1 and high mechanical strength (e.g., bending strength), and was found suitable for use in high-speed computers.

As is apparent from the foregoing, the process of this invention can be used to manufacture substrates for printed circuit boards having varying characteristics from the same material, i.e. porous expanded, polytetrafluoroethylene. This process assures manufacturers easier management of raw materials and allows them to flexibly respond to diversified needs.