Coherent light generators – Particular beam control device – Mode locking
Reexamination Certificate
2000-06-08
2003-05-06
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Mode locking
C250S234000
Reexamination Certificate
active
06560248
ABSTRACT:
BACKGROUND
The present invention relates to laser direct image printing and more specifically to a method and apparatus including a mode locked laser utilizing scophony methods to increase image quality and therefore line sharpness and accuracy.
It is known today that printed circuit boards may be composed of several PCB panels, each panel having two sides, one or more of which is provided with a layer forming an electrical circuit. When there is only one panel having only two layers, the board is commonly called a double-sided board or PCB panel, and when there are more than two layers, the board is commonly called a multi-layer board. A common way of manufacturing a multi-layer board is by fixing several panels together, each panel having a single printed circuit on one side, or a circuit on each side. “Outer” panels are those that face the outside of a multi-layer PCB, and “inner panels” are the interior panels. Typically, the inner panels have a circuit on both sides, while the outer panels have a circuit only on one, the outer side. Each inner panel resembles a thin double-sided PCB in that the panel is comprised of an insulating substrate, which is clad on both sides with metallic foil, typically copper foil. A printed circuit is formed on any circuit side of an inner panel by that side's metal cladding having a light-sensitive layer laid on top of the metal. The light-sensitive layer is exposed to light (typically ultra-violet (UV) radiation) at selected locations, then processed by a photographic process that removes the layer at selected locations. A metal etching process is then applied to remove those parts of the layer of metal not necessary for forming the actual circuit. Once all the double-sided inner panels are produced, they are fused (pressed) together by placing an insulating binding material, typically a partially cured epoxy-resin material called prepreg, between the panels. Unexposed outer foils are placed on the outside of the double-sided inner panels, again with prepreg in between. All the layers are now laminated by applying heat and pressure that causes the prepreg to flow and bond to the surfaces of the inner panels and the outer foils. Holes are now drilled on the laminated multi-layer board, including holes for mounting electrical components inserted into the board (“mounting holes”), and holes for making contacts from one layer to one or more other layers (feed-throughs, also called vias or conductive vias). The holes typically are plated through. Each outer side of the multi-layer panel now is sensitized, then exposed and processed to form the two outer printed circuits in exactly the same manner as forming circuits on the inner panels.
Since a multi-layer panel is exposed in the same way as an inner PCB panel, “PCB panel” or simply “panel” means either a complete PCB board, an inner PCB panel, or a post-lamination multi-layer panel.
A common method for producing printed circuit boards is to first produce artwork, which is an accurately scaled configuration used to produce a master pattern of a printed circuit, and is generally prepared at an enlarged scale using various width tapes and special shapes to represent conductors. The items of artwork, once reduced, for example, by a camera onto film to the correct final size, are referred to as phototools and are used as masks for exposing the sensitized layers. Because the photographic reduction is never 100 percent accurate, more accurate phototools are produced nowadays using photoplotters rather than photographic reduction.
However produced, physical phototools are susceptible to damage. In addition, whenever any amendments need to be made to any circuit, new phototools need to be produced. Furthermore phototools, sometimes in the form of photographic negatives, are difficult to store. They also may not be stable; their characteristics might change with temperature and humidity changes and can suffer degraded quality over time.
Many of the disadvantages of using phototools can be overcome by using direct imaging technology, for example with a laser direct imaging (LDI) device. The working and benefits of such LDI devices are known. LDI may be performed by scanning a laser across the surface of a PCB panel from one edge of the PCB panel to the other edge, along one or more scan lines. For examples of LDI systems and their use, see U.S. Pat. No. 5,895,581 to Grunwald (issued Apr. 20, 1999) entitled LASER IMAGING OF PRINTED CIRCUIT PATTERNS WITHOUT USING PHOTOTOOLS, and U.S. Pat. No. 5,328,811 to Brestel (issued Jul. 12, 1994) entitled METHOD OF PRINTING AN IMAGE ON A SUBSTRATE PARTICULARLY USEFUL FOR PRODUCING PRINTED CIRCUIT BOARDS. See also co-pending U.S. patent application Ser. No. 09/435,983 to Vernackt, et al. (filed: Nov. 8, 1999), entitled: METHOD AND DEVICE FOR EXPOSING BOTH SIDES OF A SHEET, assigned to the assignee of the present invention and incorporated herein by reference for all purposes.
One difficulty in producing multi-layered printed circuit boards is the strict requirement for accuracy in positioning the different PCB panels together to ensure that the different circuits are positioned very accurately relative to each other. In particular, the mounting holes and vias need to be very accurately placed on each layer's circuits. For a particular tolerance for the placement of a circuit, it is clear that any deviations in the specified location of the circuits on each of the layers may be additive, so that at any one location, there could be large deviations. For the case of double-sided panels, including the multi-layer panel after lamination, it is even more difficult to position the circuits accurately enough relative to each other.
Registration is the process of positioning the PCB pattern on the panel at a particular physical location. Thus, in the case of direct laser imaging, it is where the panel is physically positioned relative to the laser beam.
The relationship between imaging process and the registration process becomes increasingly important when higher geometrical accuracy higher PCB layout density are desired.
The geometrical accuracy can be increased by the use of a laser direct imaging (LDI) device. However, to achieve such benefits, both geometrical accuracy and the quality of imaging are important. In particular, the repeatability, line edge quality and control of the line width of the tracks after etching (i.e., the widths of the conducting interconnects) are important. Further, more and more circuit components such as coils, high frequency (HF) circuits, and oscillator circuits are nowadays being implemented within the PCB layout itself. It is necessary to predict the characteristics of those components, and for this, a known and controlled fabricating process is needed to substantially eliminate later circuit trimming. LDI technology addresses some of these problems and increase the overall imaging quality.
New technology for making PCB panels like sequential build up (SBU) and direct ablation of the copper can be used with direct imaging technology. Accuracy is also important for such new technologies that include adding each new layer directly to the previous stack of layers as an additive process. In such a case, the relationship between the imaging process and the registration process becomes very critical.
FIG. 1
illustrates one method of producing the PCB
200
illustrated in
FIGS. 2A-2E
. In block
102
, a substrate
202
with a copper layer
203
is provided. Next, a layer of photoresist
204
is applied on top of the copper layer
203
, in block
104
. Then a mask layer
206
is placed on top oil the photoresist
204
in block
106
. The mask layer has at least one opening
208
substantially corresponding to the location, shape and size of the desired copper trace
220
. In block
108
the photoresist
204
that is not covered by the mask layer
206
is exposed with a high intensity light
210
such as an UV lamp. Next, in block
110
, the mask layer
206
and the unexposed photoresist
204
is etched away. Then, the exposed portion of
Inventek
Ip Paul
Mania Barco NV
Rosenfeld Dov
Vy Hung T
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