NO140032B - METAL COATED DIELECTRIC SHEET FOR USE IN THE MANUFACTURE OF PRINTED CIRCUITS - Google Patents

Description

The present invention relates to a metal-clad dielectric sheet for use in the manufacture of printed circuits and comprising an electrically conductive metal layer, in particular copper, which adheres to a non-woven web comprising a mixture of discontinuous fibres.

Known metal-clad non-woven webs are not entirely suitable for use in the preparation of certain types, particularly large sheets, of flexible printed circuits. To be usable for such types of printed circuits, the non-woven fiber should hold

web does not shrink more than 100 micrometers per centimeter length during the preparation of the circuits. The preparation includes several operations that can cause changes in dimensions, including etching operations where metal is removed in flat areas, and soldering operations where the printed circuit briefly comes into contact with a molten solder bath heated to 260°C or more.

If the web during such operations shrinks more than 100 micrometers per centimeter length, parts of the printed circuit will not line up with electrical components or other circuits to be connected to the printed circuit.

Other required properties of nonwoven fiber holdigé webs include good toughness and tear strength (where a tear strength of at least 1.8 Kp according to the Graves tear test as specified in ASTM D-1004, or more is desired), ability to be cut cleanly without the formation of protruding fibers as well as low absorbency for moisture which tends to cause blistering of the metal layer away from the non-woven fiber holdigé web when the metal coating

tea sheets are heated in a soldering bath.

A known metal-coated non-woven fibrous web which generally has important properties for printed circuits, but does not always exhibit the extreme degree of dimensional stability noted

above, , is described in FR-PS 2.098.242 corresponding to US-PS 3.878.316.

The nonwoven fiber holdigé web in such a metal clad sheet includes undrawn heat softening fibers which are heat softened during the manufacturing process of the web to bind the fibers of the nonwoven web into a cohesive whole. (An example of a fiber mixture described in the above-mentioned patent document comprises 50% by weight non-drawn heat-softening polyester fibres, 25% by weight drawn polyester fibers and 25% by weight high-temperature-resistant polyamide fibres).

The applicant has found that the sheet material according to the present invention, which comprises a fiber mixture which does not include any fiber softening as is the case with the non-drawn polyester fibers according to the French patent document, really provides extreme dimensional stability. Instead of being held together with undrawn heat-softening polyester fibers, the sheet material according to the invention is held together with a polymeric binder that has a special combination of properties: It is firm and non-sticky at room temperature, but softens and flows easily under heat and pressure, iall case to begin with. Because of. these properties the binding material holds the mixture together during the compression operation. Formed as a film, the binding material will be able to be bent without cracking. In addition, the bonding material resists deformation when it is heated as a film for 10 seconds in a molten solder bath heated to 260°C. The binding material also absorbs less than 3% moisture when exposed to 65% relative humidity and 21°C for one day.

A metal-clad dielectric sheet according to the present invention is characterized in that essentially all the fibers have a softening point above 230°C, that the fibers are compressed and held together with a mass of film-forming polymeric binding material which resists deformation when exposed in film form for 10 seconds to a molten solder bath heated to 260°C, and which constitutes 15-75% by weight of the non-woven web, between 30 and 95% by weight of the fibers being drawn polyester fibres, between 5 and 60% by weight of the fibers being staple fibers of aromatic polyamide and up to 70% by weight of the fibers are acrylic fibres, that both the aromatic polyamide fibers and the acrylic fibers resist deformation when exposed for 10 seconds to a molten solder bath heated to 260°C, and that the metal-clad, dielectric sheet does not exhibit more than 20 micrometers of dimensional change per cm by etching away parts of or all of the electrically conductive layer, and does not show a dimensional change of more than 100 micrometres per about and substantially no blistering when exposed for 10 seconds to a molten solder bath heated to 230°C.

A favorable composition of the fiber mixture means that at least 70 percent by weight of the fiber mixture is drawn polyester fibers and 10-30 percent by weight of the fiber mixture is staple fibers of aromatic polyamide.

Preferably, the polymeric bonding material may be a cross-linked material which resists destruction when exposed in film form to a molten solder bath heated to 260°C.

The drawing shows a simplified printed circuit 10 as shown

from a dielectric sheet according to the invention. As shown, the non-woven fiber holdigé web or substrate 11 carries metal conductors 12 left after etching away the rest of the electrically conductive

metal layer on the track 11. An electrical component 13 is shown electrically connected to the conductors 12, with electrical wires 14 from the component being led through holes punched in the conductors, and then soldered in place.

In general, non-woven webs in metal-clad dielectric sheets according to the invention are produced by first mixing the desired discontinuous fibers or staple fibers, after which the mixture is preferably formed into an airy non-woven web

by conventional air laying processes, e.g. Rando track form

ning or tearing, and the web is then impregnated with a polymeric binding material, e.g. by being fed between two pressure rollers where-

of one is partially immersed in a dispersion or solution of the binding material, and finally compressed and dried, e.g. as the impregnated airy web is passed between heated pressure rollers and through an oven (which in a preferred embodiment also cross-links the binding material).

The staple fibers used in the fiber mixture should have lengths that provide good tear strength and easy web formation (by "staple fibres" is meant discontinuous fibres). Rando webbing, tearing or similar air laying processes are generally suitable for use with staple fibers longer than approx. 0.3 cm and preferably longer than 1.5 cm. Fibers longer than approx. 8 or 10 cm are generally not used even in the kardei hash machine. It is preferred that the discontinuous fibers or staple fibers according to the invention are monofilament fibers with filament diameters corresponding to 0.5 - 20 deriier> preferably less than 10 deriier. The fibers should be mixed into one

quantity of at least 10 kg per rice and preferably in a quantity of 20

kg. per rice to provide suitable tear strength of the substrate, although the weight will vary somewhat depending on the amount and type of high strength fibers used.

High-temperature stable fibers are important for the dimensional stability of the dielectric sheets according to the invention. Temperature resistance can be measured by observing fibers that have lain for 10 s on the surface of a molten solder bath heated to 260°C. Fibers showing significant deformation (e.g. polyethylene terephthalate fibers, which curl into a ball in this test),

do not have sufficient heat resistance, and dielectric sheets which are made entirely of such fibers will not have the necessary dimensional stability.

Even a small proportion of high-temperature fixed fibers which show little or no deformation in the described solder bath test will greatly improve the dimensional stability of the dielectric sheets. Obviously, the distribution of high-temperature-resistant fibers in the fiber mixture produces good dimensional stability, as high-temperature-resistant fibers come into contact with non-high-temperature-resistant fibers, together with the presence of a mass of high-temperature-resistant binding material. A proportion of high-temperature-resistant fibers as small as 5% by weight of the fiber mixture will provide good dimensional stability, although preferably at least 10% by weight of high-temperature-resistant fibers are used.

Preferred high temperature tenacious fibers used in nonwoven webs for sheets according to the invention are from the class of aromatic polyamides, e.g. as described in US-PS 3,094,511 and 3,300,450. Users of printed circuits according to the invention often wish to use temperatures of 300 or 350°C in their molten solder baths, and sheets based on aromatic polyamides have been found to provide the best dimensional stability at such temperatures. These aromatic polyamides have the following formula:

where R. is hydrogen or lower alkyl and A.sub.1 and Ar.sub.2 are divalent aromatic radicals. Among the preferred polymers are those where R 1 is hydrogen and Ar is a meta- or para-phenyl radical. These preferred polymers mainly retain their physical properties at temperatures above approx. 310°C and does not melt, but decomposes over approx. 370°C. The polymerization index ("n") should be

high enough to provide molecular weight for use in spun filaments.

Two particularly preferred commercial materials are those marketed by duPont under the trade names "Nomex" and "Kevlar", and which are generally believed to comprise poly (m-phenyl-isophthalamide) ("Nomex") or poly (p-phenyl- terephthalamide) ("Kevlar"). The latter fibers are particularly preferred because they seem to provide better dimensional stability and are less likely to absorb moisture.

Another useful class of high temperature tenacious fibers is the class of acrylic fibers, preferably those which are homopolymers of acrylonitrile, although they also include copolymers of acrylonitrile

(which generally contains at least 85% by weight of acrylonitrile)

and any additional monomer which does not deprive the fibers of their high temperature strength.

Preferred tough fibers with high tensile strength (which satisfy the requirements for elongation and toughness as stated above) that have been used in the sheet according to the invention are polyester fibers with the formula:

0-A-O-CO-Ar-CO,

n

where A is a divalent straight-chain or cyclic aliphatic radical, Ar is a divalent aromatic radical, e.g. meta- and/or para-phenyl and n is the polymerization index. These polyesters are produced in a known manner from di-functional alcohols, e.g. ethylene glycol and 1,4 - cyclohexane-dimethanol - and difunctional carboxylic acids (or esters thereof) - e.g. terephthalic acid, isophthalic acid and mixtures thereof. In order to obtain the best strength properties, the fibers are pulled, i.e. stretched and oriented, so that they have a crystalline structure.

The above-mentioned fibers of aromatic polyamide are therefore often tough fibers that have great strength and satisfy the requirements for elongation and toughness stated above.

In the case of nonwoven webs, the general ranges for the amounts of the fibers summarized above, which are used in the present invention, are as follows: For drawn polyester fibers 30-95 percent by weight, preferably at least 50 percent by weight and most preferably at least 70 percent by weight. For fibers of aromatic polyamide 5-60% by weight, preferably 10-30% by weight, and for acrylic fibers up to 70% by weight, preferably up to 40% by weight.

The useful binding materials such as non-woven fiber holdigé webs or substrates according to the invention are compressed and

compared to, are high-temperature-resistant materials that are preferred

vis reacts to a cross-linked or .heat-set state. The binder material is impregnated or incorporated into the fiber mixture to form a substantially continuous mass in which the fibers are contained. Particularly useful resins are acrylic-based reactive resins, generally comprising copolymers with a major proportion of lower alkyl (generally 1-8 carbon atoms) esters of acrylic or methacrylic acid, e.g. ethyl acrylate, butyl acrylate and 2-ethylhexyl acrylate

lat, and a smaller proportion of acrylic or methacrylic acid. Other useful bonding materials are thermoplastic polymers, e.g. vinyl chloride-based polymers (eg the resin marketed by Union Carbide under the name "Vinyon"). The binding materials should not absorb moisture, i.e. they should absorb less than approx.

3% humidity when in film form they are exposed to 65% relative humidity at 21°C for one day (after cross-linking if they are reactive). Furthermore, when heated to 230°C, they should resist rapid degradation

of their polymeric structure (e.g. when in film form they are heated in a molten solder bath. Conveniently, the polymeric binding material may be a cross-linked material which resists destruction when exposed in film form to a molten solder bath heated to 260°C.

In non-woven fiber holdigé webs for sheets according to the invention, the binding material is generally added in an amount of between 15 and 75 percent by weight of the substrate, preferably less than 35 percent by weight. Nonwoven fiber holdigé webs useful in the invention,

are generally quite flexible, e.g. as pliable as a 250-micron-thick biaxially oriented polyethylene terephthalate film. The webs are usually less than 0.5 mm thick.

The non-woven fiber holdigé webs or substrates are supplied

with a continuous electrically conductive metal layer at least on

one side and usually on both sides, preferably by sticking a previously formed metal sheet or foil on the web with an adhesive or binding material. It helps if the web is somewhat porous before the application of the adhesive material to provide better anchoring of the adhesive material to the web. A particularly suitable adhesive material is an epoxy-based composition as described in US-PS 3,027,279. The composition described in this patent generally comprises a mixture of branched acid-terminated polyester and an epoxy composition. The polyester is a product of a) dicarboxylic acid, b) dihydroxy alcohol and c)

a polyfunctional compound selected from the class polyvalent

alcohols down to at least three non-tertiary hydroxyl groups and polybasic acids with at least three carboxyl groups. No more than half of the total content of the acids and alcohols have aromatic rings, and the polyester contains an average of 2.1 - 3.0 carboxyl groups per molecule, has an acid-

number of 15 - 125 and a hydroxyl number of less than 10 and is free of unsubstituted ethylene groups in its skeletal chain. The epoxy compound contains on average at least 1.3 groups that readily react with the carboxyl group, at least one of the groups being the oxalrane group, the reactive groups are separated by a chain of at least two carbon atoms and the chain is free for unsaturated ethylene groups. For example, an epoxy-polyester composition of this type may comprise a mixture of (1) a polyester derived from adipic acid, isophthalic acid, propylene glycol and trimethylol propane, and (2) a liquid epoxy resin, e.g. the polyglycidyl ether derived from bisphenol A or resorcinol, the condensation product of 1,1,2,2-ttStrakis-(4-hydroxyphenyl)-ethane and epichlorohydrin, or cyclopentadiene dioxide. Halogenated versions of the described polyesters can also be used to provide flame retardancy, and in some cases the halogenated version is used as a base coat and a non-halogenated version is applied over this. The adhesive has no solvent in general

a thickness of 50-259 micrometers after removal of the solvent, and the metal foil and coated web are brought together and passed through pressure rollers and an oven.

Suitable conductive layers include foils of copper, aluminium, nickel, silver, gold or suitable transition metals. The thickness of the metal foil is usually of the order of 0.02 - 0.05 mm. Conductive layers can also be obtained in a laminate according to the invention by non-electrolytic plating processes. Typically, the dielectric sheets of the invention include an electrically conductive metal layer on each side of the nonwoven web. Other products include an electrically conductive metal layer on only one side of the web, and then often have a pre-formed polymer film on the other side of the web.

The invention will be further illustrated by the following examples:

Example 1

The following fiber mixture was opened and mixed

together on a fiber mixer:

The well-blended mixture was then shaped into

a path on a "Pando-Webber" machine at a speed of approx. 3.6 m/sec. The course, which was very airy and had a height of approx. 6 mm, was then pressed between two press rollers placed directly above each other, where the polymeric binding material was added. The binder was a copolymer believed to contain a large component of acrylic or methacrylic ester, e.g. ethyl acrylate and acrylic acid, together with a cross-linking reagent e.g. hexa-methoxymethyl-melamine (Rhoplex AC 172 from Rohm and Haas Co.). This binder was dispersed in water as an emulsion. The lower of the two press rollers was placed in a trough containing the emulsion, and transferred the emulsion to the web, and part of the emulsion was pumped up to the top of the press rollers. The impregnated web was passed through an oven, where it was dried at 120°C, and then between rubber and steel rollers heated to 200°C, where it was compressed to a thickness of approx. 150 micrometers. The weight of the resulting porous, dense and tough web was 20 kg per rice.

The untreated web thus produced was then coated by dipping with the epoxy-polyester resin specified under example 2 in US-PS 3,027,279, i.e. the reaction product of a) an adipic acid/isophthalic acid/propylene glycol-trimethylolpropane polyester, b) an epichlorohydrin-bisphenol-A epoxy resin and c) tri (2,4,6-dimethylaminomethyl)-phenol. The resulting coating in the dry state had a thickness of approx. 0.076 mm (total on both sides). This coating was dried and brought to the B stage for 20 minutes at 149°C. Then, "Treatment A" quality 0.03 g/cm o (Circuit Foil Corporation) copper sheets were laminated to each side of the web between pressure rollers heated to 138°C, one of which was steel and the other of rubber. After

lamination, the adhesive was cured for 15 minutes at 205'c.

The resulting flat laminate was flexible and had a total thickness of 0.25 mm. It was found that the copper was securely attached to the substrate.

The resulting metal clad dielectric sheet was then tested for deformation using 7.62 x 7.62 cm sheet test pieces. A test piece was immersed in a solution of ammonium persulfate etchant to remove the copper layer and then dried for 30 minutes at room temperature. The etched specimen was then first heated in an oven for 30 minutes at 121°C to simulate typical steps in the printed circuit manufacturing process, and then immersed for 10 minutes in a solder-tin/lead bath maintained at a temperature of 260°C. After each of the etching and heating steps, the dimensions of the test piece were measured. After etching away, it was found that the test piece had expanded 0.1% in length, after heating to 121°C it had shrunk 0.3% in length and after heating to 260°C it had shrunk<*>1 %. ;Another sample of the metal-clad sheet according to this example was tested for solder blistering by first conditioning it under controlled humidity conditions (24 hours in a chamber heated to 21°C - 1°C and with a relative humidity of 50% - 5%) and was then immersed for 10 s in a bath of molten solder tin/lead maintained at a temperature of 230°C or 260°C. Any blistering that occurred would have been caused by escaping steam blowing the copper up or otherwise loosening it, but no blistering was observed. With regard to other properties that have been mentioned, the measurement of these was carried out as follows: Tensile elongation and toughness of the fibers were measured by standard methods* The moisture absorption of the fibers was measured by weighing a sample, placing it in a test chamber with 65% relative humidity and temperature 21°C for one day and then weigh it again.

Examples 2 and 3

Additional samples of metal clad dielectric sheets were prepared by forming a non-woven fibrous web or substrate of fibers as indicated in the following table,

used the binding material and the method described

written in Example 1, and a copper foil was laminated to the substrate, likewise in the manner described in Example 1. The fiber mixtures were as follows:

Example 2

Example 3

When the samples were tested as indicated in Example 1, the results were as follows:

Dimensional changes

Claims (4)

1. Metal-clad dielectric sheet for use in the manufacture of printed circuits and comprising an electrically conductive metal layer, in particular copper, adhering to a non-woven web comprising a mixture of discontinuous fibers, characterized in that substantially all of the fibers have a softening point above 230° C, that the fibers are compressed and held together with a mass of film-forming polymeric binder material which resists deformation when exposed in film form for 10 seconds to a molten solder bath heated to 260°C, and which constitutes 15-75% by weight of the non-woven web, in that between 30 and 95% by weight of the fibers are drawn polyester fibers, between 5 and 60% by weight of the fibers are staple fibers of aromatic polyamide and up to 70% by weight of the fibers are acrylic fibers, that both the fibers of aromatic polyamide and the acrylic fibers resist deformation when exposed for 10 seconds to a molten solder bath heated to 260°C, and that the metal-clad dielectric sheet exhibits no more than 20 micrometers in dimension change per cm by etching away parts of or all of the electrically conductive layer, and does not show a dimensional change of more than 100 micrometres per cm and essentially no blistering when exposed for 10 seconds to a molten solder bath heated to 230°C. 2. Dielectric sheet as specified in claim 1, characterized in that at least 70 percent by weight of the fiber mixture are drawn polyester fibers and 10-30 percent by weight of the fiber mixture are staple fibers of aromatic polyamide. 3. Dielectric sheet as stated in claim 1 or 2, characterized in that the polymeric binding material is a cross-linked material which resists destruction when exposed in film form to a molten solder bath heated to 260°C. 4. Dielectric sheet as stated in claim 3, characterized in that the binding material comprises a cross-linked material on an acrylic basis.