Printed wiring boards produced for the electronics industry have previously been manufactured by a variety of methods. The choice of the method is dependent upon anticipated production volumes, required delivery times, performance requirements, component density, conductor spacing, etc. Presently available methods include printing processes, engraving processes, and forming processes.
In the printing process, negative or positive ink images are printed upon a substrate using one of a variety of prior art printing techniques. The process may be an additive process where the substrate is originally electrically non-conductive. The ink which is printed may dry or cure into a conductive material, or may require additional processing such as firing at elevated temperatures and possibly in highly controlled environments to result in a suitable conductive pattern. If the process is a subtractive process, an essentially continuous conductive material is laminated to a substrate material. The conductive coating is patterned by a resist ink, where the resist ink resists a following removal process of the exposed (non-inked) conductive.
In the additive process with kiln firing, particulate inks are commonly used, usually composed of palladium-silver or one of the newer base metal materials (usually copper). This firing necessitates expensive kilns with controlled atmospere. The printing of the inks has been proven to be best accomplished with a screen printing technique, due in part to the particulate characteristics of the inks and in part to the requisite rheology of the ink necessitated by the printing and then firing sequence.
The screen printing and firing method has proven out in high volume high quality production, but clearly the cost of a screen printer and kiln would not be justified for low-volume or reduced cost application. Additionally, since the requisite firing process involves elevated temperatures, the choice of substrate materials which will survive the high temperatures is severely limited. With this process a screen must be fabricated prior to any printing process, the kiln must be pre-heated, and finally the substrate must be printed and fired for a particular time period. The turn-around time from wiring pattern to finished circuit is clearly extensive, and not justifiable for prototyping of wiring patterns.
A relatively recent process utilizing a polyethylene coated substrate dusted with copper powder, heated in a pattern by a moderately powered laser so as to melt the copper into the polyethylene in the pattern, blowing off the remaining loose copper, and firing the substrate to produce a pattern offers a solution to some of the drawbacks associated with a screen printing process in the way of prototyping and turn around time, but the initial capital investment is still very large.
Where the process is additive without firing, a conductive paint consisting of a filled polymeric material may be used. The paint typically will be a silver filled epoxy or a carbon filled polymer. With this method many different substrate materials may be utilized. However, the material used will not survive elevated temperatures and often will not be solderable. Electrical connections have been made by either mechanical pressure, painting the connection, or plating through an additional additive step to provide a solderable surface.
The extra expense and lengthened turn-around time caused by poor solderability as well as the potential for reduced quality is apparent and is the subject of extensive research. This method has found limited application in the prototyping of wiring boards, where a simple circuit without significant dimensional limitations may be hand painted upon a substrate. Devices may then be mounted with additional painting. Very little capital investment is required with this method. However, the turn-around time for this method is still very great, as the patterning process has been performed by hand or with the aid of a screen for screen printing. Once the pattern is formed, the wiring board must be allowed to dry and fully cure. This will typically be 24 hours in the case of a filled epoxy. The board is not readily repairable, and any alterations to the design, as is common in the case of prototyping, require repeating the turn around time. Additionally, the finished product may not accurately represent a production quality board, due to the process variations inherent in this prior art method.
Additive plating processes have been devised but have not gained significant a supplies and the environmental hazards associated therewith. In order to selectively plate, some type of printing process must occur prior to the plating, and the plating process itself has not yet been competitive with other available processes.
In the subtractive processing of wiring boards, several mechanical and chemical variations exist. With this method the patterning of some type of resist over the conductive material is followed by a chemical etch of exposed conductive and then removal of the resist. The resist may be applied by lithographic, photolithographic, screen, or other known printing process. Alternatively, adhesive backed appliques may be applied to form the resist, or commercially available drawing pencils may be used.
The prior art subtractive processing techniques are faced with reliability and labor intensity problems in the case of the pen or applique methods, and expense and turn-around problems in the case of the known printing methods available for resist patterning.
Engraving processes are automatically accomplished with either a high power laser or a numerically controlled router. These methods, while highly effective, require an enormous capital investment and are not readily suited for high volume production. In summary, they have found application in those shops where computer aided design and prototyping is both frequent and necessary for the work. Hand engraving is also possible, though rarely used due to the time required to form a wiring pattern.
Forming processes include the process of wire wrapping. This method probably has been used for the greatest time period and continues to be in use today. It requires little initial investment, can be applied to volume production with the automated machinery available (although with correspondingly larger cost), and is suited for rapid modification in the case of prototyping. However, as the demand for higher density circuitry with greater speeds is felt throughout the electronics industry, wire wrapping is most unable to answer these particular demands.
The primary objective of the present invention is to overcome the disadvantages of the prior art while adding or retaining the desirable features of low initial investment, high quality production, rapid production, and ease of design modification.