A recently granted patent caught my eye because it made reference to a method of ink jet printing conductive inks onto non-uniform surfaces. The patent was granted to the Cabot Corportion who currently provide a number of inkjet formulations for printed electronics applications. This patent indicates that they have also been working on a system of direct printing which can compensate for the uneven surface of a substrate.
US8167393: Printable Electronic Features On Non Uniform Substrate And Processes For Making Same
Inventor(s): Karel Vanheusden, Chuck Edwards
Assignee(s): Cabot Corporation
Filing date 13/01/2006
Issue date 01/05/2012
Prior Publication Data: US20060158470 – 20/07/2006
Abstract: A system and process for compensating for non-uniform surfaces of a substrate when direct printing traces is provided. The system and process provided herein measures the surface of a substrate and can determine whether the surface is substantially flat, rises or falls, or whether a mesa or valley is encountered. Depending on the surface feature (i.e., mesa, valley, falling or rising surface), the direct printing system can change the frequency of the printing timing signal, advance or retard the print timing signal, advance or retard the print data, or make repeated passes over certain areas. In addition, the process disclosed herein can determine whether two, three or all of the aforementioned steps for compensating for non-uniform substrates should be combined to most effectively and efficiently print on the non-uniform surface of the substrate as intended.
The invention seems to be primarily for printing on surfaces which have already received some printed circuit tracks or components and need to have additional conductive tracks to complete the device. However, the text describes the situation where you might want to print directly onto an uneven or flexible substrate. With a conventional printhead the drops will become more spaced out if the distance between the head and the substrate increases due to a downward slope. If there was an upward slope then the drops will land closer together.
Various ways are described to overcome this problem and enable a uniform print density and line thickness to be achieved. The distance between the printhead and the surface can be continually monitored with a laser interferometer so that the surface topography is mapped and used to control the drop delivery.
The patent has a huge number of document references and also contains a useful review of the background art associated with more traditional printing processes such as photolithography and screen printing.
This does seem to be a serious attempt at reducing the impact of an uneven surface on the reliability and quality of printed conductive tracks and I would be interested to hear from anyone who has looked at other ways of addressing this problem.