Laminar polymeric sheet

Abstract: A sheet laminate of polyimide and amorphous polyamide especially suitable for making anisotropically electrically conductive articles by laser ablation drilling followed by metal plating. Preferred polyimides include those derived from polymerisation of 4,4′-biphenyldianhydride and 4,4′-diaminobiphenylether or p-phenylene diamine. Preferred amorphous polyamides include those derived from condensation of terephthalic acid with isomers of trimethylhexamethylene­ diamine.

Description: This invention relates to a laminar polymer sheet and to products made from such a sheet. Published EP-A-0213774 describes methods of making uniaxially electrically conductive articles by making through-holes in a laminar polymer sheet, plating metal in the holes, and removing surface laminae from the sheet to leave the plated metal in each through-hole projecting beyond the main surfaces of the sheet. These conductive articles are commercially valuable as compliant electrical connection interfaces, for example for connecting microcircuit chips to other elements in an electronic device. The through-holes can be made conveniently small (preferably not more than 200 micrometres diameter) and may be close-packed so that random positioning of the article between opposed connection sites produces electrical connection when the sites are brought into contact with the opposite main surfaces of the article. Other forms of such articles are described in copending British Patent Applications 8802567, 8802565, 8802568, 8815447.1, 8819895.7, and 8823053.7, the disclosures of all of which are incorporated herein by reference. In view of the wide use of polyimide materials in microelectronic devices, it is desirable to use polyimides for the uniaxially conductive articles, but this leads to problems in selecting suitable materials for the removable surface laminae. The present invention relates to a novel laminar structure which is useful for various purposes, and is especially advantageous for making the aforementioned uniaxially conductive articles. The invention accordingly provides a laminar sheet comprising a layer of polyimide material laminated in direct contact with a layer of amorphous, preferably amorphous aromatic, polyamide material. It will be understood that references to sheets include elongate tape-like sheets and other forms of substantially continuous laminar structures, in which individual laminae are preferably homogeneous. For the purpose of making the aforesaid uniaxially electrically conductive sheets, it is preferable to use a sheet with each of the two main surfaces of the layer of polyimide material laminated in direct contact with a layer of aromatic polyamide material. It may be convenient in many cases that the polyimide layer is substantially co-extensive with the polyamide layer(s). References to the respective layers being "laminated" do not imply any particular method of making the laminar structure. Solvent casting or melt coating of one material onto the other may be used, for example, as an alternative to lamination of two pre-existing solid films, provided that the desired laminar structure is achieved. For solvent casting, many solvents and solvent blends can be used including, for example: n-methyl pyrrolidone/10% methanol blend methanol/chloroform blend dimethyl formamide (DMF) dimethyl acetamide (DMAC). If desired, small quantities of additives (e.g. UV absorbing chromophores such as aromatic carbonyl compounds, e.g. benzophenone) can be included. Preferred amorphous polyamides include aliphatic/aromatic polyamides, (A) polyamides based on the condensation of terephthalic acid with trimethylhexamethylene diamine (preferably containing a mixture of 2,2,4- and 2,4,4-trimethylhexamethylene diamine isomers), (B) polyamides formed from the condensation of one or more bisaminomethylnorbornane isomers with one or more aliphatic, cycloaliphatic or aromatic dicarboxylic acids e.g. terephthalic acid and optionally including one or more amino acid or lactam e.g. epsilon-caprolactam comonomers, (C) polyamides based on units derived from laurinlactam, isophthalic acid and bis-(4-amino-3-methylcyclohexyl) methane, (D) polyamides based on the condensation of 2,2-bis-(p-aminocyclohexyl) propane with adipic and azelaic acids, and polyamides based on the condensation of trans cyclohexane-1,4-dicarboxylic acid with the trimethylhexamethylene diamine isomers mentioned above. (E) polyamides based on units derived from m-xylylenediamine and adipic acid. Other preferred polyamides include those based on polyether and polyamide blocks, especially the so called "polyether-ester amide block copolymers" of repeating unit: wherein A represents a polyamide sequence of average molecular weight in the range of from 300 to 15,000, preferably from 800 to 5000; and B represents a linear or branched polyoxyalkylene sequence of average molecular weight in the range of from 200 to 6000, preferably from 400 to 3000. Preferably the polyamide sequence is formed from alpha, omega-aminocarboxylic acids, lactams or diamine/dicarboxylic acid combinations having C₄ to C₁₄ carbon chains, and the polyoxyalkylene sequence is based on ethylene glycol, propylene glycol and/or tetramethylene glycol, and the polyoxyalkylene sequence constitutes from 5 to 85%, especially from 10 to 50% of the total block copolymer by weight. These polymers and their preparation are described in UK Patent Specifications Nos. 1,473,972, 1,532,930, 1,555,644, 2,005,283A and 2,011,450A, the disclosures of which are incorporated herein by reference. The polymer preferably has a C:H ratio of not more than 0.9, more preferably not more than 0.75, most preferably not more than 0.65 and especially not more than 0.55. It is notoriously difficult to cross-link polyamides, but it has been found unexpectedly that these amorphous polyamides in these sheets can be cross-linked by exposure to laser light at power levels which are insufficient to perforate or ablate the polymer. The sheet may be selectively cross linked to form resist patterns. At higher powers, exposure to laser light can be used to perforate or ablate the polymer to produce through-holes for making the uniaxially conductive article as aforesaid. Laser light at a wavelength greater than 248 nanometres is preferred, preferably from an excimer laser, especially a KrF excimer laser (249 nm) for perforation, or an XeCl excimer laser (308 nm) for crosslinking. The methods, material and dimensions described in the aforementioned EP-A-0213774 can be used to form through-holes and to put electrically conductive material in the holes, to produce sheets having through-holes containing electrically conductive material which provides an electrically conductive path between the main surfaces of the sheet. Preferably, the electrically conductive material providing the conductive path comprises metal plated on the interior surface of the through-holes, and preferably each such conductive path is substantially electrically separated from the other such paths. Preferably, portions of the polyamide layer(s) may be removed from the surface(s) of such a sheet so as to leave the electrically conductive material in the through-holes projecting beyond the main surface(s) of the sheet. Substantially complete removal of the polyamide layer(s) is preferred. The present invention is applicable to uniaxially electrically conductive articles disclosed in EP-A-0213774. It is an advantage of this invention that the polyamides adhere tenaciously to the polyimide, are readily and cleanly laser-drillable at the preferred wavelengths, and are readily removable by suitable solvents when desired. The unexpectedly good adhesion of the polyamides to the notoriously difficult-to-adhere polyimides enables the laminar sheet to be used as an electrically insulating covering of a wire, with advantages over known polyimide-insulated wires. Preferred polyimides include, for example, those commonly used in printed circuit boards or other components of electronic devices; those described in EP-A-0178185, the disclosure of which is incorporated herein by reference; and especially those resulting from polymerisation of 4,4′-biphenyldianhydride and (4,4′-diaminobiphenyl, 4,4′-diaminobiphenylether or phenylene diamine), preferably p-phenylenediamine. The laminate sheet may carry at least on further layer of polymeric material overlying at least one layer of polyamide material, and/or may carry at least one metallic layer overlying at least one polyamide layer. For example, acrylates, methacrylates, cellulose esters or other lacquers, mould release agents, and any other polymer coatings may be used as the further layer provided the material used has adequate adhesion to the polyamide. A sheet comprising the layer of polyimide material having a layer of the polyamide material on one or both of its main surfaces, and a metallic layer on the, or one, or both, polyamide layer(s) may be especially useful in making flexible circuitry. Specific examples of the invention will now be described by way of further illustration. EXAMPLE 1: Production of a Laminate Three laminates, respectively consisting of a sheet of the polyimide materials mentioned above derived from 4,4′-biphenyldianydride and (1a) 4,4′-diaminodiphenyl (1b) 4,4′-diaminobiphenylether, (c) p-phenylenediamine coated on both sides with amorphous aromatic polyamide (A) were prepared by solution coating. A solution, approximately 20% by weight, was obtained by dissolving the polyamide in DMF. A layer of the polyamide (A) hereinbefore mentioned in DMF was coated onto one side of the polyimide by doctor blading (other methods can also be used, e.g. spiral bar coating, spin coating). It was noted that there was no tendency of the single-sided laminate to curl on drying. Once the film was dry to the touch, the other side was coated in a similar manner. The completed three-layer laminate was subsequently dried in an oven at 150°C for one hour to remove all traces of residual solvent. The final polyamide thickness can be controlled by the polymer solution concentration and the height of the doctor blade. The finished laminate was then assessed for adhesion by peel testing. The adhesive bond was found to be so good that it was impossible to separate the polyamide layers from the polyimide without tearing the polyamide film. Similar results were achieved using the polyamide (C) hereinbefore mentioned instead of polyamide (A). In contrast, a film of polyetherimide (Ultem, a General Electric polymer) cast from dichloromethane onto the polyimide layer showed surprisingly poor adhesion, despite the apparent chemical similarity of the two polymers. The whole coated film peeled and completely delaminated a short time after drying, even without heating. EXAMPLE 2: Wire Coating Primary wires insulated with polyamide/polyimide composite can be produced by a variety of means: (a) A three-layer polyamide/polyimide/polyamide laminate, produced as described in Example 1, can be cut into strips about 1 cm wide, and these strips spirally wrapped round conventional conductors (with some overlap) using well-known means. The insulated wire is then heated beyond the softening temperature of the polyamide to consolidate the insulation, and bind the wraps together. (b) A copper conductor may be spirally wrapped with polyimide tape, dip coated with a solution of amorphous aromatic polyamide, and passed through a sizing die. The wire is then passed through a heated drying zone to remove excess solvent, producing a finished insulated primary wire. (c) A copper conductor may be spirally wrapped with polyimide tape, and then passed through the head of a conventional extruder to extrude a layer of aromatic polyamide onto the wrapped conductor, and cooled in a water bath to finish the wire. EXAMPLE 3: Adhesive Layer A solution of amorphous polyamide as described in Example 1 was used to form an adhesive layer between two pieces of polyimide film. The solution was applied to the film usirg a brush, though the coating techniques described in Example 1 could also be used. The second layer of polyimide was placed in contact with the wet adhesive layer and the 'sandwich' pressed together and heated to dry the solution. The sandwich was bake to remove traces of residual solvent and a bonded laminate of polyimide to polyimide was formed via the amorphous aromatic polyamide. This process can be repeated to form a multi-layer structure, which could subsequently be machined to shape. As an example of machining, a multi-layer sandwich was drilled with an ultraviolet excimer laser to produce 100 micrometer diameter through-holes. EXAMPLE 4: UV Excimer Laser Crosslinking A wire insulated with polyamide/polyimide composite was prepared as described in Example 2(a), and subsequently crosslinked by UV excimer laser light. This was achieved by irradiating the wire with laser light from a Lambda Physik XeCl excimer laser (wavelength = 308 nm). This laser produces pulses of UV light of about 30 ns duration, each pulse containing about 300 mJ. The laser beam was shaped using lenses and mirrors of types well known in the art so that lengths of wire were irradiated on all sides at an incident fluence of about 10 mJ/cm²/pulse for about 20 pulses. Because the laser can be pulsed repetitively at 100 Hz or more, it is possible to build a wire handling system that passes wire continuously through the crosslinking zone. The polyamide/polyimide-insulated wire so produced was compared with similar wire that had not been laser treated. Lengths of the insulated wires were placed in warm dimethylformamide (DMF). It was found that the aromatic polyamide part of the un-treated wire rapidly dissolved, leading to delamination of the insulation. In contrast, the laser-treated sample demonstrated superior solvent resistance. Microscopic examination showed that the aromatic polyamide had not been dissolved, indicating that crosslinking had indeed occurred. EXAMPLE 5: Drilling, Plating & Surface Removal (a) A laminate produced according to Example 1c, with benzophenone added as a chromophore at a level of 0.1% based on the weight of polyamide, was subjected to laser ablation using an XeCl excimer laser at a wavelength of 308 nm to produce a multitude of through-holes of about 50 micrometres diameter, which were electrolessly plated with copper, then nickel, then gold using known electroless plating baths, followed by removal of the polyamide surface layers from the sheet, all using the methods described in the aforementioned EP-A-0213774. (b) The laminate used for (a) above was subjected to laser ablation using an KrF excimer laser at a wavelength of 249 nm to produce a "1:1" pattern of through-holes (as described in the aforementioned copending British application) of about 50 micrometres diameter. The holes were electrolessly plated with copper, then nickel, then gold using known plating baths, followed by removal of the polyamide surface layers from the sheet to produce a "1:1" uniaxially electrically conductive article of the kind described in the aforementioned copending British application. (c) The procedures (a) and (b) were repeated to produce respectively a "multi-tube" article as in (a) and a "1:1" article as in (b) but with through-holes of only 10 micrometres diameter containing the projecting plated metal. A closely spaced regular "1:1" array of this kind can be especially useful for making connections to high-density regular arrays of electronic devices. Preferred polyimide materials for the present invention are those which are capable of retaining at least 50%, preferably at least 75%, more preferably at least 85%, of its original elongation after immersion in water of pH10 at 100°C for 4 days according to ASTM D882. It will be readily understood that a sufficiently fully cyclised polyimide having less than 15%, preferably less than 10%, more preferably less than 5%, or if possible substantially no open imide rings or uncyclised amic acid groupings may be better able to survive hot alkaline metal plating baths, which attack known polyimides such as Kapton (TM). The use of the aforesaid preferred polyimides derived from 4,4′-biphenyl-dianhydride has been found advantageous both in general and for the anisotropically electrically conductive articles described and claimed in the aforementioned EP-A-0213774. The currently more preferred commercially available polyimides are those available under the Trade mark "UPILEX" from Ube/ICI. One of these, "UPILEX R", is believed to be a relatively completely cyclised polymer having a repeat unit derived from biphenyl dianhydride and diaminodiphenylether, viz. Most preferred, however, is "UPILEX S", which is believed to have a repeat unit derived from the same anhydride and phenylene diamine, viz. The polyimide derived from the biphenyldianhydride and 4,4′-diaminobiphenyl may have thermal expansion characteristics which are particularly well suited to microcircuit requirements. The corresponding polymers derived from isomers of the diamines mentioned above, e.g. the 3,4′- or 3,3′-diamino isomers may also be useful, as may the corresponding polymers derived from pyromellitic dianhydride instead of the biphenyldianhydride (but excluding Kapton, of course). For the through-holes previously mentioned, laser drilling, preferably by ablative photodecomposition using a U.V. excimer laser, has the advantage of producing through-holes with less pronounced taper than alternative chemical etching methods, the lower degree of taper permitting closer pitch (hole-to-hole spacing). This is clearly advantageous, given that microcircuits, with which the resulting anisotropically conductive (after metal plating of the through-holes) sheets may be used, are becoming progressively smaller and more densly patterned. Through-holes with taper (measured on the substantially straight inner portions of the holes) less than 10°, preferably less than 8°, more preferably less than 6°, and especially less than 4° (relative to the axis of the through-hole) can advantageously be achieved by laser drilling of the laminates according to the present invention. This is especially useful for the generally preferred holes of less than 200 micrometres diameter, e.g. 5 to 150 micrometres or 10 to 100 micrometres, and especially less than 50 micrometres diameter.

Claims: A laminar sheet comprising a layer of polyimide material laminated in direct contact with a layer of amorphous polyamide material. A sheet according to claim 1 wherein the polyamide material comprises amorphous aromatic polyamide material. A sheet according to claim 1 or 2, wherein each of the two main surfaces of the layer of polyimide material is laminated in direct contact with a layer of the polyamide material. A sheet according to claim 1, 2 or 3, wherein the polyimide layer is substantially co-extensive with the polyamide layer(s). A sheet according to any preceding claim, which has been cross-linked by exposure to UV light. A sheet according to claim 5 which has been selectively cross-linked to form a resist pattern. A sheet according to any preceding claim, in which through-holes have been produced by exposure of selected areas of the sheet to U.V. laser light. A sheet according to any preceding claim, having through-holes containing electrically conductive material which provides an electrically conductive path between the main surfaces of the sheet. A sheet according to claim 8, wherein the electrically conductive material providing each conductive path comprises metal plated on the interior surface of the through-holes. A sheet according to claim 8 or 9, wherein each such conductive path is substantially electrically separated from the other such paths. A sheet according to claim 8, 9, or 10, characterised in that portions of the polyamide layer have been removed leaving the electrically conductive material in the through-holes projecting beyond the resulting main surface(s) of the sheet. A sheet according to any of claims 1 to 5 in the form of an electrically insulating covering of an electrical wire. A sheet according to any of the preceding claims, wherein the polyamide is derived from (A) the condensation of terephthalic acid with at least one isomer of trimethylhexamethylene diamine (preferably containing a mixture of 2,2,4- and 2,4,4-trimethylhexamethylene diamine isomers), or (B) the condensation of one or more bisaminomethyl-norbornane isomers with one or more aliphatic, cycloaliphatic or aromatic dicarboxylic acids and optionally includes one or more amino acids or lactam e.g. epsilon-caprolactam comonmers, or (C) laurinlactam, isophthalic acid and bis-(4-amino-3-methylcyclohexyl) methane, (D) the condensation of 2,2-bis-(p-aminocyclohexyl) propane with adipic and azelaic acids, or the condensation of trans cyclohexane-1,4-dicarboxylic acid with the trimethylhexamethylene diamine isomers mentioned in (A) above, or (E) polyamides based on units derived from m-xylylene-diamine and adipic acid. A sheet according to any of the preceding claims, wherein the polyimide is derived from 4,4′-biphenyldianhydride and (4,4′-diaminobiphenyl, or 4,4′-diamino biphenylether, or phenylenediamine). A sheet according to any preceding claim, comprising at least one further layer of polymeric material overlying at least one layer of the polyamide material. A sheet according to any preceding claim comprising at least one metallic layer overlying at least one layer of polyamide material. A sheet according to claim 16 comprising the layer of polyimide material having a layer of the polyamide material on one or both of its main surfaces, and a metallic layer on the, or one, or both of the said further layers. A sheet according to any preceding claim, wherein the polyimide material is capable of retaining at least 50%, preferably at least 75%, more preferably at least 85% of its original elongation after immersion in water of pH10 at 100°C for 4 days according to ASTM D882. A sheet according to any preceding claim, wherein the polyimide material is derived from polymerisation of 4,4″-biphenyl dianhydride and p-phenylenediamine.

Inventors: HORNER PATRICK JAMES, LUDDEN MICHAEL JOSEPH, NYHOLM PETER, PENNECK RICHARD JOHN, SMITH NICHOLAS J G, HORNER, PATRICK JAMES, LUDDEN, MICHAEL JOSEPH, NYHOLM, PETER, PENNECK, RICHARD JOHN, SMITH, NICHOLAS J.G.
Applicants: RAYCHEM LTD, RAYCHEM LIMITED
Application year: 1989
Main classification: H01L23
IPC codes: H01L21, G03F7, H05K1, H01L23, B32B15, B32B27, B23K31, B23K20, H01B3, B23K1, H05K3