Ultra-Fine Thick Film Printing with Foil Based µ-Screens
R. D. Shipton, C. J. Robertson, D. R. Gray, J. Coit, W. White, ERA Technology Ltd. Leatherhead, Surrey. UK. KT22 7SA, Tel: +44 (0) 1372 367366, Fax: +44 (0) 1372 367134, e-mail: microscreen@era.co.uk
Abstract
Thick film printing of features such as interconnection tracking, microwave components and sensors devices, has taken a quantum leap in feature size and definition. If mesh based screens will print down to 250-200µm (10 - 8 mil) features easily and 125 - 100µm (5 - 4 mil) features in limited quantity with great care, then the foil based µ-Screen will print 100µm (4 mil) features - tracks and gaps - easily and 50µm (2 mil) features with care. Edge definition is also greatly improved.
The µ-Screen is fabricated by etching a pseudo-mesh into a stainless steel foil. The pseudo-mesh is computer generated to exactly match the customer pattern which is formed into an organic emulsion or gasketting layer on the underside of the screen. There are no randomly placed mesh wires, so no staggered and stepped line edges and no half blocked mesh openings. The µ-Screen is a screen, not a stencil, because there is a pseudo-mesh to hold the features together and it can print totally enclosed features such as rings and spirals.
This paper explains what the screens are like, how they are made, how easy they are to use, and the benefits of this new technology developed by the Communications and Sensors Solutions (CSS) business of ERA Technology. µ-Screen printed parts - interconnects, sensors and microwave/millimetre wave components - developed by some of its customers in association with ERA are illustrated.
Key words: Screen printing, µ-Screen, Fine line, Thick film, MCM.
What is the µ-Screen?
The µ-Screen is a precision, high definition printing screen capable of printing features, both lines and spaces, down to 50 µm (2 mil). The working region of the screen is fabricated from a stainless steel foil (as opposed to the wire mesh of a normal printing screen) which is micro-etched with a custom, computer-generated pattern of ink feeder holes, to form a pseudo-mesh which precisely matches the print pattern.
An organic gasketting layer is applied to the underside of the foil, which allows the ink to flow beneath the metal ‘bridges’ between the feeder holes, and defines the sides of the printed lines and areas as shown in Figure 2.
The custom pseudo-mesh avoids the interference produced by the random alignment of a pattern to a standard wire mesh, which causes stepped and “saw tooth” edged features and wires blocking very narrow tracks. The µ-Screen is a screen, not a stencil, because the pseudo-mesh holds the features together so that it can print totally enclosed features such as rings or spirals as shown in Fig. 3.
How a µ-Screen is made
The manufacture of a µ-Screen starts with the customer’s own CAD design file. This is sent to ERA, for example, as an e-mail attachment. From this file, a version is CAD generated which turns every line and area into an appropriately shaped array of ink feeder-holes. The fabrication process requires two mirror copies of the ink feeder-hole artwork and one of the original print pattern. Standard photo-lithographic exposure and development techniques are used to form the ink feeder hole pattern into photo-resist on both sides of the foil. This pattern is wet etched into and through the foil from both sides. After etching, the resist is stripped and the gasketting layer screen printed onto the underside of the foil, dried and photo-processed. The alignment of both the double sided etching, and the gasket layer exposure must be within a few microns of each other.
After an inspection, the foil is bonded onto tensioned mesh on a standard screen frame, and then the mesh cut away in the working area. This mesh provides tension to the foil and flexibility to permit it to be displaced across the snap-off gap.
In principle the foils can be mounted into any screen frame and a variety of frames from 8 x 6 inch (e.g. Dek 65/1200) up to 12 x 12 inch have been used. Printing areas up to 4.5 x 4.0 inch have been worked on to date, although this is expected to be increased in the coming months.
Printing with a µ-Screen
Printing with a µ-Screen is very little different from printing with a normal mesh screen. It is used in the normal off-contact mode with a snap off of typically 0.75 - 1.0mm (30 - 40 mils). Print speed and pressure are within the normal range, although slightly slower / lighter may be advantageous.
High viscosity inks are preferred, at least equivalent to a good fine line printing paste, however, feature quality with lower viscosity inks can be significantly controlled by selection of a harder squeegee: up to 95 Shore A, or even a light stainless steel blade being beneficial. The µ-Screen is perfectly robust in normal usage, and wear out, coining, and slackening with age have not been experienced. Accidental damage should be avoided.
The benefits of µ-Screen technology
Screen printing through a traditional mesh screen has a fundamental limitation: At feature sizes less than approximately 2 mesh counts, the quality of the printed features start to significantly degenerate. Most people will print readily and at production volumes down to 250 - 200µm (10 - 8 mil) features, while the more entrepreneurial will succeed in reaching down to 125 - 100µm (5 - 4) mil features in limited quantity and with great care.
With the foil based µ-Screen it is possible to print down to 100µm (4 mil) features - tracks and gaps - easily and 50µm (2 mil) features with care. Edge definition of the printed features is also greatly improved.
At a time when the drive to size reduction is as inexorable as ever, thick film technology has been losing out to other solutions, such as thin film and other subtractive techniques: This new screen brings thick film back into the running.
ERA sees the µ-Screen as offering significant advantage to clients in the areas of interconnect, communications, sensor, wireless sensors, automotive radar, broadband network communications, microwave and millimeterwave circuits. It is applicable to both low cost high volume applications, and also more modest volume custom applications.
µ-Screen printed components
This Lange coupler, printed in ultra-fine line gold with a µ-Screen, featured 50µm (2 mil) lines and spaces. The print dimensions displayed outstanding conformance to the design dimensions. It operated in the 10 GHz range.
A series of Tchebycheff filters were printed in ultra-fine line gold with a µ-Screen. Different designs featured line separations down to the finest example shown here with 33µm (1.3 mil) separations. Without any correction for spread, the fired components had an actual gap of 37µm (1.5 mil).
They all operated in the 7-16 GHz range.
Fig. 8 shows a small, high sensitivity, quick response humidity sensor printed at 70µm (2.8 mil) lines and spaces.
“That’s not a pencil: It’s a pin” was the caption to the above picture used by General Hybrid in their brochure. It shows part of a thick film MCM circuit featuring 50µm (2 mil) lines and spaces printed in ultra fine line gold paste. It allowed the number of conductor layers in the design to be reduced enhancing yield and reducing cost.
Wire bonding
Wire bonding, both gold and aluminium, has been carried out on µ-Screen printed components. Pictures below show some test examples of gold bonding.
Conclusion
The µ-Screen is a major step forward in the ability to print thick film high density tracking and ultra-small features.
The technology has been developed and proven and is now available for use by manufacturing industry. Many have already seen the benefits of this new technology in action, and it will surely not be long before this process is adopted as one of the standard manufacturing techniques, as more and more communications and sensor applications demand fine-line printing at low cost.
Acknowledgments
The µ-Screen was developed by ERA with the assistance of the Commission for the European Community “Framework” Programme, and in association with the following companies:
Alcatel ETCA (Belgium)
Custom Interconnect (United Kingdom)
EKRA (Germany)
Extec Hybrids (United Kingdom)
General Hybrid (United Kingdom)
Heraeus (Germany)
Hymec (The Netherlands)
MCI (Cambridge) (United Kingdom)
Micro-Hybrid Electronik (Germany)
Microtel Tecnologie Elettroniche
(Italy)
Thalis (formerly Dassault Electronique and Sorep) (France)
Photographs are reproduced by permission of Extec Hybrids, Microtel, Hymec, and General Hybrid.
About the Author and the Company
Russell Shipton, the main author, gained an honours degree in Electrical Engineering in 1969, and has worked in R&D, production and QA in the thick film and hybrid circuits industry ever since. He is a Chartered Engineer, a member of IMAPS, and also a member if the Institution of Electrical Engineers and The Institute of Quality Assurance.
ERA Technology, where he currently works has been involved in thick film research and development since the inception of the technology in the late “60s. It offers design and consultancy services for a wide range of electronic systems, communications and sensor solutions developments, where it aims to deliver cost effective solutions for its clients world-wide. It also has well known expertise in reliability and failure analysis.
ERA is actively seeking to offer the technology to industry world-wide either by way of a custom screen supply service, or by licensing the fabrication technology to other service suppliers. For companies with existing high quality thick film facilities, the implementation of the technology requires no costly investment and time to market is rapid: This is in marked contrast to considering a switch to thin-film or other subtractive processes.
| 
