Electric heating – Metal heating – By arc
Reexamination Certificate
2002-06-06
2003-12-02
Dunn, Tom (Department: 1725)
Electric heating
Metal heating
By arc
Reexamination Certificate
active
06657159
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed towards a method and system for laser drilling in structures, such as creating vias.
BACKGROUND OF THE INVENTION
Drilling precise holes or vias in structures, especially in multilayered structures such as printed wiring boards (PWB), can be difficult to do in a controlled and rapid manner. While a CO2 laser may quickly make incisions in a structure, it cannot do so precisely. A solid state laser, on the other hand, can precisely cut a hole but it does so at the expense of speed.
The dilemma of speed versus precision is especially prominent when drilling vias in PWBs. A PWB typically has a top copper layer, a second dielectric layer, an underlying copper layer and more layers of similar construction. Vias can be through vias which go right through the board; blind vias which go through one copper and one dielectric layer, stopping at the underlying copper layer; or step vias which go through more than one pair of copper/dielectric layers, ultimately stopping at a copper layer. Blind vias in PWBs must be cut only through the top copper and dielectric layers and must not affect the underlying layer. The sides of the via must be perfectly straight and smooth, without interference from the residue of dielectric fillers.
One prior art technique used to drill vias in multilayered materials, particularly PWBs, used a solid state laser to drill through the first two layers (copper and dielectric), stopping at the underlying layer (copper), forming a blind via. This technique is discussed in U.S. Pat. Nos. 5,593,606 (Owen et al. 1) titled “Ultraviolet laser system and method for forming vias in multi-layered targets” issued on Jan. 14, 1997 and 5,841,066 (Owen et al. 2) titled “Method employing UV laser pulses of varied energy density to form depthwise self-limiting blind vias in multi-layered targets” issued on Nov. 24, 1998. A small beam from the laser was trepanned to create a via of a larger size whose shape could be manipulated by changing the trepanning motion parameters. Owen et al. 1 and 2 specifies the use of a solid state laser to precisely and completely remove all the material from at least two layers of the PWB. However, by using a solid state laser with a trepanning motion, the laser will damage the underlying copper layer unless the machining power is reduced before it arrives there. This is not possible for some dielectric materials, such as FR4, that contain non-uniform glass fillers requiring high machining power to remove. Further, the precision of this process slows down the drilling process.
Another technique uses a solid state laser to drill through the copper using trepanning and a CO2 laser to drill through the dielectric using percussion. The wavelength of the CO2 laser is reflected by copper so the underlying copper layer is not damaged, creating a self-limiting process. This technique is discussed in German patent application DE 19719700-A1 (Roelants) titled “Blind hole production in circuit board” issued on Nov. 12, 1998. When this process is used on RCC (resin coated composition) dielectric, the walls of the via can be undercut (i.e. a meniscus shape) rather than the positive taper preferred in the chemical plating process. Also, when the process is used on FR4 dielectric the glass filler material can remain along the walls. The problems in arise from the percussion method used for the entire dielectric material which lacks the precision of the trepanning motion. Roelants specifies the removal of the residual dielectric material by the use of a chemical solution.
U.S. Pat. No. 5,910,255 (Noddin), titled “Method of sequential laser processing to efficiently manufacture modules requiring large volumetric density material removal for micro-via formation” issued on Jun. 8, 1999, describes drilling vias with a first laser and a first trepanning motion, followed by a second laser and a second trepanning motion. This involves drilling the via twice with two different lasers and trepanning motions. Cutting the via a second time smoothes the edges of the via formed by the first laser and trepanning motion. However, re-drilling the via is time costly.
SUMMARY OF THE INVENTION
It is an object of the present invention to improve the process of laser drilling in a structure to form vias, slots and the like.
It is an object of the present invention to provide a method of laser drilling for creating vias in a multilayered structure that forms precisely shaped sides of the vias.
It is an object of the present invention to provide a method of laser drilling for creating vias in a PWB that produces smooth and precise sides of the vias for a variety of dielectric materials in the PWB.
In accordance with one aspect of the present invention there is provided a method of drilling vias in a multilayered structure having at least three layers, the method using a first laser and a second laser, said method comprising the steps of: (a) removing a first layer of the multilayered structure with the first laser; (b) removing a portion of a second layer of the multilayered structure with the first laser; and (c) removing the remaining portion of the second layer using the second laser.
In accordance with another aspect of the present invention there is provided a system for drilling vias in a multilayered structure having at least three layers, said system comprising: a first laser configurable for removing a first layer of the multilayered structure and a portion of a second layer of the multilayered structure; and a second laser configurable for removing the remaining portion of the second layer.
REFERENCES:
patent: 4839497 (1989-06-01), Sankar et al.
patent: 5222617 (1993-06-01), Gregory et al.
patent: 5536579 (1996-07-01), Davis et al.
patent: 5593606 (1997-01-01), Owen et al.
patent: 5744780 (1998-04-01), Chang et al.
patent: 5837964 (1998-11-01), Emer et al.
patent: 5841066 (1998-11-01), Bocherens
patent: 5841099 (1998-11-01), Owen et al.
patent: 5910255 (1999-06-01), Noddin
patent: 197 19 700 (1998-11-01), None
patent: 0 299 702 (1989-01-01), None
M. Owen, E. Roelants and J. Van Puymbroeck, “Laser drilling of blind holes in FR4/glass”, Circuit World 24/1 (1997), pp. 45-49.
Lobin Joseph
McKee Terry
Dunn Tom
GSI Lumonics Inc.
Hoffman, Warnick & D'Alessandro
Johnson Jonathan
Merecki John A.
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