WO2002093989A1

WO2002093989A1 - Method for laser drilling in materials used for production of printed circuits

Info

Publication number: WO2002093989A1
Application number: PCT/SE2002/000900
Authority: WO
Inventors: Bernt Ekström
Original assignee: Metfoils Ab
Priority date: 2001-05-14
Filing date: 2002-05-10
Publication date: 2002-11-21
Also published as: SE521028C2; SE0101668D0; SE0101668L

Abstract

A method for laser drilling in a material for use in production of printed circuits is disclosed. The material is produced in a process comprising electroplating a layer of copper or copper alloy onto a foil of aluminium or aluminium alloy, said foil having one surface prepared for said electroplating, lamination by means of heat and pressure said copper plated aluminium foil onto at least one side of an insulating prepreg base of thermosetting, optionally fibre reinforced, plastics with copper layer facing said base, alkaline etching of said aluminium and acidic removal of residues not dissolvable in alkalines. The method for laser drilling is characterised in that said laser drilling is performed before said acidic removal, whereby said residues are used as absorbant for laser energy in said laser drilling.

Description

METHOD FOR LASER DRILLING IN MATERIALS USED FOR PRODUCTION OF PRINTED CIRCUITS

The present invention relates to a method for laser drilling in a material, primarily a copper clad thermosetting laminate, for use in the production of printed circuits.

Printed circuits are used to a large extent in the electronics industry. They are usually produced with a copper clad plastics laminate as starting material. A mask having a shape corresponding to desired wiring pattern is transferred to the copper layer by printing or by a photochemical method. The applied mask, formed of an etchant resistant material acts as a protection during a subsequent elimination by etching of superfluous copper. Thereafter electronic components are mounted on the laminate in electrical contact with the circuit thus obtained. The copper conductors of the circuit constitute the electrical connections and the laminate provides the required insulating base and mechanical support. The most common insulating bases used are paper reinforced phenolic laminates, which are used for comparatively simple circuits, and glass cloth or fibre reinforced epoxy resin laminates, which are used where the technical requirements are high. Use is also made of fibre reinforced plastics laminates of other types. Plastics films and plastics coated metal plates are also used to a certain extent as insulating bases.

For producing the copper coating or layer on the insulating base, it is common to use copper foils or sheets which are placed on a base-forming fibre material impregnated with partially cured or hardened plastics materials (so called prepregs) whereafter the composite material is moulded at high pressure and high temperature. The final curing of the plastics material is thereby effected and the fibre material is converted into a sheet bonded to the copper foil. A suitable and improved method for producing said laminates is disclosed in US 3,936,548 and in GB 1 403 976.

The creation of holes or vias (through holes) in a printed circuit board is a fundamental part of the manufacturing process. Holes and vias provide tooling locations for alignment during the fabrication, mounting points for components and other structures, as well as locations for mounting the board to a system cabinet. A very important use for holes, however, is to provide an electrical connection between circuit layers in the board. They have been serving in this capacity from the time the first double sided board was built. Increasing demand for higher interconnection density is putting pressure on the manufacturer of especially vias. As printed circuit board lines and spaces approach smaller and smaller dimensions, the area consumed by the vias becomes a limiting factor. The via and pad size reduction is a major effort in the printed circuit board industry today. In respond to that, alternatives to mechanical drilling of small holes are exploited, developed and improved. Mechanical drilling is the predominant method used today to make holes and vias in printed circuit boards and materials used in the production of printed circuits.

Alternative drilling methods include plasma drilling/etching, photolithographic drilling and laser drilling. Plasma drilling is an established and well understood process wherein panels or substrates are loaded into a plasma chamber. In this process the panel or substrate must have an etch mask to prevent unwanted areas from being etched during the plasma cycle. The hole or via pattern is first imaged into a dry film resist on the copper foil where every hole or via is to be formed. When the panels or substrates are loaded into the plasma chamber, the plasma discharge reacts with the dielectric film. Depth is controlled through a number of parameters. Another alternative to mechanical drilling is the use of photolithography to define holes.

Laser drilling of holes and vias can be described as a process using a laser beam, properly focused on the working surface, while controlling laser factors such as output power, pulse width, beam shaping, repetition rate, wavelength and minimum spot size for the of generating holes and/or through or blind vias. The laser energy evaporates the material being processed and remaining particles are either vacuumed away or in some instances high pressure gas jets are used followed by vacuum. The main lasers used for laser drilling of holes and vias in materials for printed circuit boards include excimer lasers, such as CO₂ lasers and YAG (yttrium/aluminium garnet) lasers, such as Nd:YAG lasers. Further lasers include liquid lasers, solid state lasers, glass lasers and semiconductor lasers. Laser drilling of holes and vias is typically done in one or two ways, point to point or mask imaging. In the point to point mode, the laser drilling process is a serial process with one hole or via drilled at a time. In the mask imaging mode, a mask is required to image several holes or vias in a parallel process.

The type of laser used is of decisive importance when selecting the base material for laser drilling. Neither copper nor glass can be stripped with excimer lasers without being adequately coated. CO₂ lasers can strip glass and organic materials. The hole pattern must, however, first be etched into the copper layer. Nd:YAG lasers can strip copper, as well as glass and resin.

Compared with plasma technology, lasers can only process one side of a circuit board at a time, but the quality of for instance micro-vias is extremely good. Copper typically reflects

80% at room temperature of the laser energy and laser drilling of holes and vias in copper clad materials used in the production of printed circuits accordingly requires that an absorbant, controlling the absorbance of laser energy, is coated on the copper surface. Commonly used absorbants include black oxide and cobalt clay. Applied absorbant must in a subsequent step be removed.

The present invention eliminates the need of applying an absorbant on the copper surface of materials, such as single sided, double sided or multilayer copper clad thermosetting laminates, produced in a process as substantially disclosed in US 3,936,548 and/or GB 1 403 976, for use in the production of printed circuits. The present invention accordingly refers to a method for laser drilling in a material for use in the production of printed circuits. Said material is produced in a process comprising electroplating a layer of copper or copper alloy onto a foil of aluminium or aluminium alloy, which foil has one surface prepared for said electroplating, laminating by means of heat and pressure said copper plated aluminium foil onto at least one side of an insulating prepreg base of thermosetting, optionally fibre reinforced, plastics with copper layer facing said base, and alkaline etching of said aluminium. After aluminium etching and rinsing, the copper exhibits a grey to black flat surface resulting from residues not dissolvable in alkalines. Said residues are normally acidically removed. The method of the present invention is characterised in that said laser drilling is performed before said acidic removal, whereby said residues are used as absorbant for laser energy in laser drilling of holes and vias having for instance a diameter of 0.010-1.50 mm, such as 0.025-1.25, 0.025 -0.5 or 0.1-0.25 mm.

Said laser drilling is in embodiments of the method of the present invention performed by means of an excimer laser, a liquid laser, a solid state laser, a glass laser or a semiconductor laser. Said lasers include:

- Excimer lasers, such as CO₂, He-Ne and Ar lasers, typically generating laser light in the ultraviolet to near-ultraviolet spectra, such as laser light at a wavelength of 9000 to 12000, such as 10000-11000, Nm;

- YAG lasers, such as Nd:YAG and Er.YAG lasers, typically generating laser light at a wavelength of 900-1200, such as 1000-1100, Nm;

- Semiconductor lasers wherein the active medium is selected from the third or the fifth group of the Periodic Table, such as Al, Ga, In, N, P, As or Sb; and

- Glass lasers, such as a Cr, Yb or Er:phosphate glass lasers.

The process, for production of said material for use in production of printed circuits, typically comprises applying a thin layer of zinc onto a foil of aluminium or aluminium alloy and plating a layer of copper or copper alloy onto said zinc layer. Said residues not dissolvable in alkalines comprise for instance iron and/or manganese.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilise the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative and not limitative of the remainder of the disclosure in any way whatsoever. In the following Examples 1 and 2 refer to preparation of copper clad laminates and laser drilling, in accordance with embodiments of the present invention, of holes in said laminates. EXAMPLE 1

A material (a copper clad thermosetting laminate) for use in the production of printed circuits were produced by applying a thin layer of zinc to a foil of aluminium and plating a 2 μm thick layer of copper to said zinc layer. The copper plated aluminium foil was laminated by means of heat and pressure onto one side of an insulating prepreg base consisting of glass fibre reinforced epoxy resin, said copper layer facing said base. The aluminium was subsequent the laminating removed by a 6 minutes alkaline etching in an etching solution comprising 50 g/1 of NaOH and at a temperature of 60°C. The laminate was subsequent said etching washed with water and dried in hot air (150°C) for 1 minute. The copper surface exhibited a grey to black flat surface substantially consisting of iron and manganese. The residues was used as absorbant surface for laser drilling of holes at the laser energies 11, 12, 13, 14 and 15 mJ. A number of five (5) holes were drilled at each energy. A Hitachi CO₂ Laser Drilling Machine, Model LC-1C21E was used for said drilling. Subsequent said drilling, the residues mainly consisting of iron and manganese were removed by a 1 minute acidic etching in a solution comprising 50 g/1 of sodium persulphate and 3 g/1 of sulphuric acid. The top diameter of obtained hole was measured in 2 directions (designated X and Y). The result is given in Table I below.

EXAMPLE 2

A material (a copper clad thermosetting laminate) for use in the production of printed circuits were produced by applying a thin layer of zinc to a foil of aluminium and plating a 5 μm thick layer of copper to said zinc layer. The copper plated aluminium foil was laminated by means of heat and pressure onto one side of an insulating prepreg base consisting of glass fibre reinforced epoxy resin, said copper layer facing said base. The aluminium was subsequent the laminating removed by a 6 minutes alkaline etching in an etching solution comprising 50 g/1 of NaOH and at a temperature of 60°C. The laminate was subsequent said etching washed with water and dried in hot air (150°C) for 1 minute. The copper surface exhibited a grey to black flat surface substantially consisting of iron and manganese. The residues was used as absorbant surface for laser drilling of holes at the laser energies 11, 12, 13, 14 and 15 mJ. A number of five (5) holes were drilled at each energy. A Hitachi CO₂ Laser Drilling Machine, Model LC-1C21E was used for said drilling. Subsequent said drilling, the residues mainly consisting of iron and manganese were removed by a 1 minute acidic etching in a solution comprising 50 g/1 of sodium persulphate and 3 g/1 of sulphuric acid. The top diameter of obtained hole was measured in 2 directions (designated X and Y). The result is given in Table II below. Table I

Table II

The relation between applied energy, thickness of copper layer and obtained hole diameter in the copper layer can be expressed as:

Pα = π(d/2)² - H - /? - E wherein P = laser energy, α = absorption ratio, d = hole diameter, H = copper thickness, p = density and E = specific energy per gram of copper.

Table I and II give at hand that laser drilling without especially applied absorbant, such as black oxide, is possible and give good relation between hole diameter, copper thickness and applied laser energy.

Claims

1. A method for laser drilling in a material for use in production of printed circuits, said material being produced in a process comprising electroplating a layer of copper or copper alloy onto a foil of aluminium or aluminium alloy, said foil having one surface prepared for said electroplating, laminating by means of heat and pressure said copper plated aluminium foil onto at least one side of an insulating prepreg base of thermosetting, optionally fibre reinforced, plastics with copper layer facing said base, alkaline etching of said aluminium and acidic removal of residues not dissolvable in alkalines characterised in, that said laser drilling is performed before said acidic removal, whereby said residues are used as absorbant for laser energy in said laser drilling.

2. A method according to Claim 1 characterised in, that said laser drilling results in a hole or via having a diameter of 0.010-1.50 mm, such as 0.025-1.25, 0.025 -0.5 or 0.1-0.25 mm.

3. A method according to Claim 1 or 2 characterised in, that said laser drilling is performed by means of an excimer laser, a liquid laser, a solid state laser, a glass laser or a semiconductor laser.

4. A method according to any of the Claims 1-3 characterised in, that said laser is a CO₂, He-Ne or Ar laser.

5. A method according to Claim 4 characterised in, that said excimer laser generates laser light in ultraviolet to near-ultraviolet spectra.

6. A method according to any of the Claims 4 or 5 characterised in, that said excimer laser generates laser light at a wavelength of 9000 to 12000, such as 10000-11000, Nm.

7. A method according to any of the Claims 1-3 characterised in, that said laser is performed a YAG laser.

8. A method according to Claim 7 characterised in, that said YAG laser is a Nd:YAG or Er:YAG laser.

9. A method according to Claim 7 or 8 characterised in, that said YAG laser generates laser light at a wavelength of 900-1200, such as 1000-1100, Nm.

10. A method according to any of the Claims 1-3 characterised in, that said laser is a semiconductor laser.

11. A method according to Claim 10 characterised in, that said semiconductor laser has an active medium selected from the third or the fifth group of the Periodic Table, such as Al, Ga, In, N, P, As or Sb.

12. A method according to any of the Claims 1-3 characterised in, that said laser is a glass laser, such as a Cr, Yb or Eπphosphate glass laser.

13. A method according to any of the Claims 1-12 characterised in, that said process, for production of said material for use in production of printed circuits, comprises applying a thin layer of zinc onto a foil of aluminium or aluminium alloy and plating a layer of copper or copper alloy onto said zinc layer.

14. A method according to any of the Claims 1-13 characterised in, that said residues not dissolvable in alkalines comprise iron and/or manganese.

15. A method according to any of the Claims 1-14 characterised in, that said material for use in production of printed circuits is a single sided, a double sided or multilayer copper clad thermosetting laminate.