The present invention is directed to a photo-mask used for imaging or photo-patterning three-dimensional objects such as a three-dimensional circuit board.
In recent years the concept of using three-dimensional injection molded plastic substrates for three-dimensional circuit boards has gained wide acceptance. The development of high temperature thermoplastic resins such as General Electric Company's Ultem.RTM. polyetherimide has further enhanced the importance of the three-dimensional circuit boards. Their advantages are obvious as such types of circuit boards provide immense design flexibility. Features such as standoffs, stiffeners, bosses, connectors, and internal snaps to hold items such as batteries, can be easily incorporated into these substrates. This design flexibility in the construction of the three-dimensional substrates can result in simplified assembly operations and installation of secondary component hardware. Besides the mechanical or functional aspects of the injection molded three-dimensional substrate, the majority of base resins utilized for molded packages feature electrical, mechanical or environmental properties superior to the conventional laminates used presently in the printed circuit board industry. In fact, as compared to more premium resins, such as polyimide, triazine or Teflon.RTM., thermoplastic injection molded grade resins are more cost effective. The applications based on the aformentioned manufacturing technology are not limited to just printed circuit boards. For instance, injection molded three-dimensional substrates have considerable applications in the designs of discrete components as well as electro-mechanical or electro-optical devices. In fact, the ability of the three-dimensional circuit boards to eliminate stamped or formed metal contacts, wires and cable has wide market utility in all sorts of electrical devices on non-electronic products. However, a major drawback of the three-dimensional substrates is that a key fabrication step involved in the manufacture of circuit boards namely photo-patterning or photo-imaging of the three-dimensional substrates, has not been solved satisfactorily. When a substrate is sufficiently flat, the desired photo-pattern exposure can be effected by using a contact photo-mask for selectively blocking light projected onto the coated surface of the substrate. The photo-mask can be fabricated from Mylar.RTM. polyester, which transmits UV (Ultraviolet) light and resists degradation by UV light. However, photo-masks made from glass plates are also used. Metal traces can be deposited in the desired occlusion pattern on the surface of the photo-mask. The Mylar.RTM. polyester photo-mask having the desired pattern can be aligned as desired, and the alignment may be maintained using a vacuum hold-down system while UV light is selectively directed through the photo-mask to the substrate.
Problems arise when the printed circuit board substrates are not flat. Those deviations from flatness due to, for example, features, such as "H" shaped stiffening ribs and pedestals for mounting integrated circuits, can interfere with the photo-mask placement. In addition, the smaller surface features and the imperfections such as minor surface undulations or roughness can disturb the contact between the photo-mask and the substrate. A lack of intimate contact between the photo-mask and the substrate results in light leakage under the occluded patterns on the photo-mask. As a result imaging failures such as narrower or wider than required conductive metal traces are subsequently formed on the printed circuit boards. Since in the printed circuit board industry it is not uncommon to have a photoresist layer protected by a transparent cover sheet, there is a further lack of intimate contact between the photo-mask and the substrate, thus aggravating the aforementioned light leakage defect even more.
Glass photo-masks fabricated to match the three-dimensional substrate surfaces have been tried, but with very little success due to the dimensional variability of the injection molded substrate surfaces and the difficulty of machining complex shapes on the glass photo-mask. Any discrepancy between the shape of the three-dimensional substrate and the photo-mask results in a gap where light leakage through the edges of the photo-mask occlusion pattern can occur. This light leakage in turn can cause unacceptable dispersion at the surface of the three-dimensional substrate coated with a photoresist. The problem of light leakage is further aggravated by the dimensional variations that are inherent in an injection molded three-dimensional substrate. Alternative materials which might be easier to fabricate into the desired photo-mask shapes are limited since the photo-masks must transmit, and resist degradation by the UV light.
A number of solutions such as shadow masking or projection imaging have been tried to resolve the photo-patterning of the three-dimensional substrates. The need to shape a photo-mask to avoid gross surface features such as surface roughness is obviated in shadow masking by spacing the photo-mask at a certain distance from the substrate during a collimated UV light exposure. However, it is harder to achieve accurate and high resolution exposures using the shadow masking techniques, since the position of the shadow generated by the photo-mask, depends upon the distance from the photo-mask to the substrate surface, and the angle of incidence of light relative to the substrate surface. Small errors in either of these parameters can lead to large errors in the position and the resolution of the shadow on the substrate surface. Further, the vertical surfaces on the three-dimensional substrates are difficult to image with the collimated UV light because of its inability to project the photo-mask image onto the vertical substrate surface. Moreover, the differential in incident photons on the vertical surfaces by the collimated UV light results in highly non-uniform exposures, thus further exacerbating the resolution problems. Thus, shadow masking with the collimated UV light is not considered a satisfactory alternative for manufacturing precision high circuit density, three-dimensional printed circuit boards. Methods such as pad-printing and two-step molding (insert molding of the plateable plastic traces on the non-plateable plastic substrates) are either very expensive or inflexible.
Three-dimensional photo-masks may be machined or etched out of a metal. However, these masks are expensive, difficult to make, limited as to pitch, i.e., line/space definition, very rigid and thus non-compliant, and are unable to overcome the inherent dimensional inconsistencies present in a molded three-dimensional substrate. The metal photo-masks are less effective when isolated circuits exist, and are often prone to physical distortion.
As indicated by the foregoing, so far, no truly satisfactory method for mass producing photo-patterned three-dimensional circuit board substrates has been disclosed.
Accordingly, the present invention provides a photo-mask and a method for improved photo-patterning of the three-dimensional circuit boards. More specifically, it is an object of the present invention to provide a photo-mask for photo-patterning a three-dimensional substrate having at least one side and more than one surface on which conductive metal trace patterns are deposited to form the printed circuits.
Another object of the present invention is to provide a three-dimensional photo-mask flexible enough to comply with surface irregularities on the underlying three-dimensional substrate being photo-patterned.
Another object of this invention is to provide a method for preparing the three dimensional photo-mask of the present invention.
Still another object of the present invention is to provide a method of preparing a three dimensional substrate having a conductive metal trace pattern thereon by utilizing the photo-mask of the present invention.
Other objects of the present invention, together with the features and advantages thereof will become apparent from the detailed description and when read with the accompanying drawings which illustrate exemplary embodiments of the present invention.