There are many different technologies used for printing electronic circuits such as screen printing, flexography, gravure, offset lithography and inkjet – a review of these can be found on the Wikipedia page for Printed Electronics. One of the printing technologies which is not currently used is electrophotography although there have been various publications including patent applications that have proposed this route.
A recent patent alert I received indicated that Eastman Kodak has just been granted a method patent for producing a printed circuit using electrophotography. The company is just emerging from chapter 11 bankruptcy and one of its stated aims is that it will focus on commercial printing. This patent therefore fits well with its new strategy. Although the patent title indicates a printed circuit board the description makes it clear that flexible devices are envisaged such as RFID, sensors and flexible displays.
The patent is US8497057 and was published on 30 July 2013.
Title: Method of Producing Electronic Circuit Boards using Electrophotography
Inventors: Thomas N Tombs; Donald S Rimal
Filed: 14 Sept 2012 and is a division of US 12/341099 filed on 22 Dec 2008
Abstract: The present invention provides a method producing printed electronic circuits using electrophotography.
The granted claims read as follows:
1. A method for producing a printed circuit, the method comprising performing the following steps in order:
charging a primary imaging member;
creating an electrostatic latent image by image-wise exposing the primary imaging member;
image-wise depositing thermoplastic particles onto the primary imaging member;
transferring the thermoplastic particles to an electrically insulating substrate;
a first fixing step of permanently fixing the transferred thermoplastic particles;
depositing conductive particles over the substrate;
a second fixing step of permanently fixing the conductive particles on the fixed thermoplastic particles so that conductive paths are formed between fixed conductive particles; and
removing conductive particles from portions of the substrate other than that coated with the fixed thermoplastic.
2. The method according to claim 1, wherein the conductive particles is permanently fixed by the application of heat.
3. The method according to claim 1, wherein the conductive particles is permanently fixed by the application of pressure.
4. The method according to claim 1, wherein the conductive particles is permanently fixed by the exposure to solvent vapor.
5. The method according to claim 1, wherein the conductive particles is metal.
This method patent will shortly be complemented with the original patent which has been granted and is due to be published later this month (info obtained from the USPTO PAIR database). I will post the details of this one when it is public. I was interested to see if this was one of the first granted patents for electrophotography and had a quick look through the citations and examiner’s search results. The USPTO PAIR database is a mine of information when you want to investigate the process by which a US patent is granted and see the comments made by the examiners.
Although a number of documents were found in the search none was considered relevant enough to impact the inventive step of the above claims as they are finally worded. It should be noted that considerable changes were required to the original 2008 application.
I did a brief search for the keywords electrophotography and printed electronics in titles and abstracts came up with the following:
US7638252: Electrophotographic printing of electronic devices
US20070234918 A1: System and method for making printed electronic circuits using electrophotography
This patent application was never granted and was abandoned.
From the references and citations it is clear that electrophotographic approaches for printed electronics have been documented well before 2008. However, the Eastman Kodak patent has described a stepwise process which is somewhat different to the prior art and reviewing the prosecution it is clear that considerable detail has been gone over in agreeing the allowed claims over the prior art.