This handbook has been produced for those in the electronics industry as a guide to the specifications and performance of a novel manufacturing process for electronic interconnect which uses the offset lithographic printing process.

Over four years research work at Brunel University has demonstrated the feasibility of manufacturing electrical circuit interconnect via the established printing technology of offset lithography. It has been shown that offset-lithography can be used as a process for manufacture of low specification electrical interconnect, leading to reduced production time and raw material use when compared to conventional thick film printing approaches. Several industrial applications are at present being developed, and a case study is described in Chapter 3.

The specifications of a novel, lithographically printable ink with a cured electrical resistance sufficiently low to enable it to form interconnect for complex electrical systems are given.

The suitability of the offset lithographic process for fabricating a wide variety of products including switch pad membranes, antenna and interconnect circuitry is discussed.

Offset Lithography
Alois Senefelder (1771 - 1834) is accredited with the discovery of planographic printing, but it was not until 1904 that Ira Rubel began transferring the ink from the image plate to the paper using a rubber cylinder.

Lithography relies on the action of two wetting functions on the surface of a smooth and un-embossed printing plate. The planographic image plate is photochemically treated to repel water where the printed image is, and attract it in the non image areas. This allows an oil-based ink to adhere only to the image areas.

Contact with inking and damping rollers allows the printing plate to attract both water and ink as required, thus forming the image to be printed. However, the image is not deposited directly onto the substrate material (e.g. paper), but is instead “offset” on to an intermediate or “blanket cylinder” that has a yielding surface. The blanket cylinder then presses the ink film onto the surface of the substrate, which is now supported on a separate impression cylinder

Standard lithographic printing machines have the following characteristics:

• High Speed (Typically 3000-10000 Impressions/hour).

• Good Dimensional Control & Excellent Registration of Images.

• Low Cost per Sheet (Low Ink Volume/Substrate)

Types of Circuit which may be produced

• Single sided circuits,

• Double sided circuits,

• Experimental multilayer circuits have also been produced.

Substrate Materials
A wide range of substrates, such as paper, card, polyester, polyethylene, polysulphone, cellulose, and polyimide may be used. Best results are achieved from smooth but moderately absorbent materials such as synthetic and filled papers. Substrate thicknesses of 0.050 to 0.300 mm can be printed on most offset machines.

Sheet Resistance
Sheet resistivity of 100 mOhm per square attainable. Normalised sheet resistivity (25 microns) of 20 mOhm per square attainable. Sheet resistance is determined by choice of substrate and on press controls.

Electrical and physical characterisation of the printed films has enabled production tolerances to be determined and susceptibility to environmental attack and mechanical wear to be established. In brief the findings are as follows:

Achievable resistance tolerances:
Variation in resistance was found to be preliminary dependent upon the ink delivery mechanism of the press. For the 1982 model Heidelberg GTO46 used in this study the following tolerances were achieved:

± 2.5 % variation in resistance over print runs of several hundred units;
± 2.5 % variation in resistance from the leading to trailing edge of a sheet;
± 15 % variation in resistance across the width of a sheet on a manually set press.

Resolution and Alignment
Preferred minimum:
80 - 100 µm track, 60 µm gap

Current process limits:
60 µm track, 40 µm gap

60 µm track width structures have been successfully printed but exhibit higher sheet resistances (typically 500 mOhm per square).

Resolution limits are determined by press alignment characteristics, which for the press used in this study were found to be:

~40µm in side-lay alignment;
~20µm in front-lay alignment;

The alignment errors of larger presses may exceed the 40 µm stated above, resulting in a larger minimum feature size.

Component Attachment Process
The facilities and trials were provided and performed at Mitel Telecom, Portskewett.

• Screen print on conductive adhesive

• Auto pick and place components

• Low temperature bake to cure joints

• Bond strengths 30 - 50 % that of tin/lead solder

Surface Mount Adhesives
The following adhesives were trialed in this study:

• Ablebond 967 - 1

• Ablebond 8380

• Ablebond 8175 A

• RS 496 295

Ablebond 967 - 1 was considered most appropriate.

Through Hole Connection
Through hole connection is achieved by screen-printing adhesive through holes during component attach process. Alternative processes such as laser ablation may be appropriate for specific applications.

Environmental Test Specifications
The facilities and tests were provided and performed by Nortel Technology, Harlow, Essex. The test regimes used:

• IEC-68-2-60: pt.2: Ke: 1995. Flowing Mixed Gas Corrosion (Test Method 1)

• IEC 68-2-67:pt.2: Cy: 1995. Damp heat steady state accelerated test 85 °C - 85% Relative Humidity.

• IEC-68-2-20: Ca: 1969. - BS 2011:Pt.2.1: Ca: 1977. Damp Heat, Steady State, 40 °C - 93% Relative Humidity.

None of the samples exhibited a rise in sheet resistance greater than 10%. Most samples exhibited a reduced sheet resistivity resultant from film curing over the duration of the test.

Summary
All these figures are derived from a single model of press operated under typical print shop conditions. As a consequence aspects of these results, which are dependent upon press condition and characteristics, do not represent generic limits of the process. The printed films have also successfully withstood basic environmental test regimes, including humidity, corrosive atmospheres, elevated temperatures, thermal shock and mechanical wear.

The following is a checklist for assessing the suitability of conductive lithographically printed films as alternatives to other technologies.

The ‘Suitability of Process’ checklist facilitates an initial consideration of the process for new applications. This table illustrates clearly the suitability of the process to unpopulated, low current applications such as antenna, membrane switches, tags and sensors. The success of more complex demonstrator devices illustrates the capacity of the process, and highlights the additional developments and changes of perception necessary for the process to enter mainstream manufacture.

The appropriateness of the process to a product can be judged from the responses to the questions. An ‘ideal’ response to the first question in each section indicates high suitability. The likelihood for the CLF technology to offer significant savings in cost and time will be increased if the majority of the responses match the ideal model. If none of the answers for a section are ‘ideal’ there will be fundamental problems in implementing CLF technology.

Preliminary "suitability of process" checklist designed to assess circuit applications