1. Field Of The Invention
The present invention relates generally to transfer metallization of various substrates and more specifically to the creation of electrically conductive and semi-conductive patterns through transfer metallization techniques.
2. Description Of The Prior Art
The manufacture of various electronic devices such as printed circuit boards or the like requires the placement of patterned layers of conductive material on a substrate. The method of placement must be suitable for high production rates and it must produce precise patterns. These requirements have become increasingly important with the recently developed surface mounting procedures used to mount components to a substrate.
One prior art method is the process of etching copper-clad laminates in accordance with standard negative photo-resist procedures. This process is slow, costly, and produces polluting and hazardous waste materials.
Another prior art method is the process of applying a pattern of conductive ink or paste directly onto the surface of a non-conductive substrate and then curing it. The curing operation of some conductive pastes comprises evaporating volatile solvents. During extended evaporation periods, these solvents may damage the surfaces of thermoplastic substrates with which they come into contact. In addition, once they have evaporated, these solvents become pollutants, requiring costly cleanup.
The use of conductive ink also presents a number of additional problems. One type of ink requires heating to an elevated temperature before it becomes conductive. As a result, its use is limited to substrates such as those made from ceramics which can withstand extreme temperatures. Another type of ink includes epoxy resin, which is unsuitable for high speed, mass production because of long cure times.
The methods used to apply the conductive ink to a substrate also present a variety of problems. In using screens to apply a pattern of ink, the screens become clogged, resulting in incomplete patterns. In addition, the need for regularly cleaning the screen makes the screening process unsuitable for high production rates.
Another known method uses a radiation curable ink containing particles of electrically conductive metal and radiation curable organic resins. This ink cures rapidly, resulting in higher production rates. However, the metal particles in it must have a certain predetermined shape for the ink to cure satisfactorily.
Finally, in placing multiple layers of patterned conductive material using the methods described above, production rates decrease substantially. This decrease occurs partly because of the added step of electrically interconnecting the conductive arrays comprising the various conductive layers. A conventional method of interconnecting the conductive layers of multilayer printed circuit boards and the like consists of forming a hole through the conductive layers and plating the walls of the hole with an electrically conductive material. The plating connects the exposed edges of the conductive layers. This operation consumes considerable time.
The present invention consists of a process which places patterned conductive layers of material on a substrate and which avoids the problems of the methods described above. It is simple, and suitable for high production rates. It does not require the use of special and expensive substrates or conductive inks. Thus, it provides a substantial reduction in the cost of manufacture of printed circuit boards and of various other electrical devices relying on single or multiple layers of conductive and non-conductive materials.