Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2002-01-17
2004-08-24
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S830000, C029S846000, C427S097100, C174S210000, C174S261000
Reexamination Certificate
active
06779262
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is entitled to the benefit of and incorporates by reference in their entireties essential subject matter disclosed in International Application No. PCT/EP00/02560 filed on Mar. 23, 2000, Luxembourg Patent Application No. 90 376 filed on Mar. 23, 1999 and Luxembourg Patent Application No. 90 475 filed on Nov. 19, 1999.
FIELD OF THE INVENTION
The present invention generally relates to the manufacturing of a multilayer printed circuit board and to a composite foil for use therein.
BACKGROUND OF THE INVENTION
The development of very compact and powerful electronic devices has been possible thanks to high-density printed circuit boards (PCB), obtained by sequential build-up (SBU) technology. Basically, a build-up multilayer circuit is a combination of several superimposed layers of different wiring densities, which are separated by dielectric layers and interconnected through micro blind vias with diameters of generally less than 100 &mgr;m.
Nowadays, essentially three different technologies are available for the manufacture of microvias: (1) the photochemical etching of photodielectrics; (2) the plasma etching process; and (3) the still relatively new process of laser drilling. Laser drilling seems to be the most promising technology for the production of microvias. Excimer, Nd-YAG and CO
2
laser sources are currently used for drilling of microvias, but each of these laser sources still has its specific draw-backs. Excimer lasers are not considered economically viable for industrial use. They have a low ablation rate per pulse and involve high investments in safety precautions, as excimer laser gases are extremely corrosive and highly toxic. Nd-YAG lasers are successfully used for smaller and medium sized volumes of high end products with microvias of diameters from 25 to about 75 &mgr;m. Larger holes must be produced by trepanning (i.e. by drilling multiple smaller holes), which of course reduces drilling speeds considerably. CO
2
lasers are increasingly gaining ground vis-à-vis the Nd-YAG laser for a large volume production of microvias. They are characterised by an ablation rate in non-reinforced polymer that is about twenty times as high as for Excimer or Nd-YAG lasers.
However, if CO
2
lasers are very much adapted for polymer ablation, they are not suitable for copper removal. Hence, an additional process step, the manufacturing of a conformal mask, is necessary before a hole can be produced in the dielectric layer with the CO
2
laser. During this additional step, openings are etched in the copper laminate at the positions where the dielectric should be removed later. This method allows to use the CO
2
laser for drilling blind microvias, but the manufacturing process is slowed by the conformal mask building step and there is a real risk of damaging the copper layer during the conformal mask building.
In order to avoid the above and other disadvantages of the conformal mask technology, it has been suggested to use a twin laser device for drilling the holes. Such a twin laser device is a combination of CO
2
laser source with an IR solid-state laser. First, the opening in the copper foil is carried out with the solid-state laser. The resin layer is then removed with the CO
2
laser. Such a twin laser allows microvia drilling in copper cladded build-ups, but the investment cost is higher than for a simple CO
2
laser, and the slow copper drilling step is responsible for a low process speed.
It has also been suggested to replace the manufacture of the conformal mask by a “half etching” step. A thin resin coated copper foil of about 18 &mgr;m is first laminated on the core board, with its copper foil upside. After lamination, the 18 &mgr;m copper foil is etched over its entire surface, in order to reduce its thickness down to about 5 &mgr;m. In the next step, the copper layer undergoes a black oxide treatment, to form a laser drilling adapted surface. Then, the CO
2
laser is used to drill the microvias directly through the 5 &mgr;m copper layer and the subjacent resin layer. The “half etching” step is of course less complex than conformal mask building, but the manufacturing process is nevertheless slowed down by the half etching step and the copper surface might still be damaged during the half etching step. Furthermore, CO
2
laser drilling on “half etched” copper foils does not yet produce satisfying results. The poor results are due to the fact, that etching the entire surface of e.g. a 600 mm×500 mm printed circuit board is neither a homogeneous, nor a precise operation. The most recent etching agents and etching machines claim a tolerance of ±2 &mgr;m. The thickness of a copper foil etched down to a nominal thickness of 5 &mgr;m may therefor vary from 3 &mgr;m to 7 &mgr;m. When drilling the microvias, the laser energy is set for a nominal copper thickness of 5 &mgr;m. If the copper layer at the incidence point is only 3 &mgr;m, the set laser energy is too high for the amount of copper to be vaporised. As a result, copper splashes are created on the border of the hole and the hole in the dielectric material is generally misshaped. If the copper layer at the incidence point is however 7 &mgr;m, the set laser energy is too low and the resulting hole in the dielectric material will have too small a diameter or will even not extend to the subjacent copper layer. Due to the disappointing results of the half etching method, CO
2
laser drilling is still exclusively used on non-copper cladded build-up materials or with conformal mask etching.
SUMMARY OF THE INVENTION
U.S. Pat. No. 3,998,601 discloses a composite foil and a method for manufacturing the latter. The composite foil comprises an electrodeposited copper support layer and a second electrodeposited copper layer of a thickness which is-not self supporting. Intermediate the copper support layer and the second copper layer is a thin layer of a release agent, preferably chromium. The second copper layer has a thickness no greater than 12 &mgr;m. A laminate may be formed by superimposing this composite foil on epoxy impregnated fiberglass with the ultrathin copper surface in contact with the epoxy-glass substrate, and subjecting this assembly to a conventional laminating process. After cooling of the laminate, the copper carrier coated with the release agent is peeled away to produce a thin copper clad laminate suitable for etching, etc. in the production of printed circuit elements.
A method for manufacturing a multi-layer interconnected board is described in JP 10 190236. According to a first step of this method, a circuit board with a desired circuit pattern formed thereon, a metal foil and an insulator layer are positioned, stacked up and laminated. In the next step, a point on a conductor layer desired to be laser processed is subjected to a process to increase the rate of absorption of the laser. In the following step a laser beam is impinged on the processed point so as to melt and sublime the metal foil and the insulator layer and thereby form a hole. In a final step, electroless plating is performed to electrically connect conductors through the hole.
The possibility of laser drilling into copper clad epoxy-glass, in particular by means of a CO
2
laser, is reported in “Laser drilling of microvias in epoxy-glass printed circuit boards” by A. Kestenbaum et al., IEEE Transactions on components, hybrids and manufacturing technology, vol.13, no. 4, Dec. 1990 (1990-12), pages 1055-1062, XP000176849 IEEE Inc. New York, US ISSN: 0148-6411. In one of the experiments, a CO
2
laser was used to drill a through hole in a 0.254 mm (10-mil) epoxy-glass layer clad with 4.4 &mgr;m (⅛-oz) copper on both sides. In another experiment a CO
2
laser was used to drill a blind hole in a 0.254 mm (10-mil) epoxy-glass layer clad with 4.4 &mgr;m (⅛-oz) copper.
DE-A-31 03 986 relates to a process for the production of drilled holes for the throughplating in printed circuit boards consisting of substrate materials on the basis of carbon. The throughholes ar
Gales Raymond
Michel Damien
Arbes Carl J.
Circuit Foil Luxembourg Trading Sarl
Phan Tim
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