The following chapter reviews potential applications, and the various demonstrator products which have been constructed.

The majority of the prototypes have used an epoxy based conductive adhesive (RS Part No. 496 265) for component attachment. This has proved well suited to the purpose. Trials with industry standard pick and place machines at Mitel Telecom have confirmed the suitability of this technology for high-speed component population of lithographic films.

Substrate choice is a major contributing factor to the cost and successful implementation of this technology. Coated, cellulose based (GlossArt) papers, and polyethylene based synthetic papers have been widely employed. About 30 different substrates have been trialed, including Industry Standard FR4 and materials such as polyimide (Kapton ®) and polyester (Melinex ®).

2.2.1 Microwave patch antennae
Following investigation of the characteristics of printed microwave striplines, a manufacturer of microwave frequency systems expressed interest in the process as mechanism for reducing manufacture costs. The company was particularly interested in patch antennae, devices which are currently fabricated from etched ‘FR4’. These capacitively coupled to the transmitting and receiving circuitry, avoiding electrical or soldered connections. The price of each antenna unit was approximately 15 pence and the primary driver was to reduce the cost.

A series of printing trials on ‘GlossArt’ and ‘PolyArt’ were used to determine the dielectric characteristics of the substrates at particular thicknesses using a parallel-plate capacitor model. These values were employed to generate substrate specific patch antenna artwork, designed to resonate at particular frequencies. In addition, the feed line to each patch was increased in thickness to compensate for the higher resistance of the printed films. The printed samples were evaluated using a microwave network analyser to determine actual resonant frequencies, and assembled into otherwise unmodified products to determine the functional performance against unmodified devices. The resonant frequencies of the two substrates varied from the ‘FR4’ sample by 4% and 13%, indicating the devices were not optimised for the resonant system they were to be included in. Measurements of the performance of the assembled devices resulted in reduction in performance, (compared to etched copper), of 40% and 75% respectively. These superficially disappointing results were attributed to mismatch loss and gain loss of the antennas not being resonant at the test frequency.

2.2.2 Flip Chip
‘Flip-Chip’ is a process of attaching integrated circuits directly to a circuit board substrate, instead of utilising standard DIL and SMT packaging. This is achieved by the following process: The bare silicon chip is bumped, a process of creating raised areas where connection to the board is required. The chip is then turned and placed face down, ‘flipped’ on the substrate. To ensure electrical contact between the silicon and the substrate a variety of adhesives are used.

For many applications, where the small size of the connections to be made prohibit the use of mechanical devices or standard conductive adhesives. Anisotropic adhesives offer the potential of good electrical connectivity and mechanical strength. Dr Samjit Mannan (Loughborough University) undertook an initial study of the suitability of conductive lithographically printed films for use as substrates for flip-chip assemblies using two anisotropic conductive adhesives.

Two test patterns were replicated from existing alumina tiles and printed, samples of ‘GlossArt’ and ‘PolyArt’ substrates were supplied to Loughborough for assembly. Whilst temperature stability of the polymer substrates was a problem, two of the three paper samples offered 100% connection of all joints, and the other 62.5%.

2.2.3 Printed Passive Filter Networks
Passive RC notch filter networks absorb a narrow band of frequencies whilst transmitting all others. The lithographic printing of conductive inks enables the fabrication of such devices from a single ink and plate. The ‘Twin T’ filter developed relies on critical matching of the component values to deliver a sharp notch at a frequency given by the rule:

The convoluted printed component structures had design resistances of 20kOhm and capacitances of 100pF. The interwoven resistor structures reduced relative variation in resistance to ±1% and ±10% absolute tolerances throughout the print run. Upon application of a swept frequency sinusoidal signal to the input of the device notch attenuations of ~25 dB were recorded at ~90 kHz, with notch bandwidths of ~30 kHz. The solid line represents results of a filter with trimmed resistors, whilst the dashed line represents an as printed specimen.

2.2.4 Display Structures
Several prototype reflective display devices have been fabricated from low cost thermochromic substrates and printed electrode structures. The possible applications for these types of display range from low-cost large area devices to inductively heated single shot tags, such as could be used to verify packaging integrity.

The devices function by heating a localised area of the thermochromic substrate, the electrode structure for each segment consists of a narrow track designed to dissipate a specific quantity of energy as heat. For the network illustrated each element has a dispersion of ~1.3 mW/mm when run from a 3V supply. If several elements are to be used in a single display care is needed to match the power dissipation/length of each, to avoid uneven heating.

Various thermochromic substrates are available, and a suitable colour change temperature can be selected for the desired environments. The lag time (the time taken for an electrical change to effect a visual change) is dependent upon the ambient temperature, thermal conductivity, specific heat capacity, and the power dissipation of the assembly, and can range from several seconds upward. Control of the power dissipated by the elements enables lag times to be reduced to a couple of seconds by delivering a high power ‘start’ followed by a lower power ‘maintenance’ phase, similar to chokes and starters on fluorescent tubes and large motors.

2.2.5 Polymer Light Emitting Devices
Light Emitting Polymer structures utilising optimised inks are under investigation in a joint Brunel University/Durham University EPSRC funded project (Project No. GRM01982). The Durham team is lead by Dr Ifor Samuel. There is currently great interest in the use of conjugated polymers to make light-emitting displays following the discovery of polymer electro-luminescence.

Polymer light-emitting diodes (LEDs) offer the prospect of colour light-emitting displays that are flat, operate at low voltage and are even compatible with flexible substrates. These devices typically consist of one or more polymer layers of total thickness approximately 100 nm in between two contacts. When a voltage is applied, the polymer emits light. To date the contacts have generally been deposited by thermal evaporation or sputtering. We have examined the feasibility of using offset-lithography to deposit contacts for polymer LEDs. Lithographic printing is faster than evaporation or sputtering, and could lead to the much-improved manufacturability of polymer displays.

CLF Light emitting diodes were fabricated by spin coating a solution of MEH-PPV (more fully known as poly(2-methoxy, 5-(2’-ethyl-hexoxy)-1,4-phenylene-vinylene) from a chloro-benzene solution onto a CLF printed track. The concentration of the solution determined the polymer layer thickness. The top electrode, a 30 nm aluminium layer, was thermal evaporated at a pressure of 0.000001 mbar The thin top contact layer thickness was necessary to allow light to pass through. Testing of the devices was performed under vacuum and for all results reported the CLF was wired as the anode.

The current-voltage characteristics are nonlinear and are typical characteristics for polymer LEDs, apart from a relatively high turn on field due to the large device thickness of typically 260 nm, and large barrier to charge injection from the contacts used. We have successfully demonstrated that polymer light-emitting diodes can be fabricated with silver-based conductive lithographic film printed contacts. These results represent a significant step in the low-cost manufacture of flexible light-emitting displays.

2.2.6 Printed Capacitors
A key area of research interest is the fabrication of capacitor devices using the CLF process. The aim of this work is to develop a strategy for manufacturing small capacitance value (<10nF) components as integral parts of the circuit’s substrate. Printed capacitors could be suitable for roles such as coupling, decoupling, timing networks and sensor applications.

A number of approaches to electrode capacitor manufacture have been explored including the printing of interdigitated structures, dielectric ink films and the use of plastic film dielectrics. Interdigitated capacitors are structures comprising two ‘comb-like’ electrodes separated by a fixed gap. The components are printed onto the surface of the substrate with standard conductive ink and can form an integral part of circuit interconnect. The capacitance of the structures is largely due to the fringing field passing through the substrate between the two electrodes. The components are typically low in Capacitance value per area (<15pF/cm2) but are useful in applications where low-cost non-critical devices may be specified. The picture above shows an unpopulated thermometer circuit that uses a printed capacitor in an RC network and printed pull-up resistors. Multilayer Capacitors have been constructed by sequentially printing conductive and dielectric ink films. This work required the formulation of suitable dielectric inks containing high proportions of titanium dioxide (rutile). The formulation of the inks was completed in collaboration with Gwent Electronic Materials (GEM). These inks were designed to offer high bulk dielectric properties, and exhibit shear-thinning characteristics necessary for offset lithographic printing. The relative bulk dielectric constant of the ink is ~14. Devices have been fabricated by printing a 3-5µm conductive film and overprinting with successive layers of dielectric ink to achieve a layer of ~20µm. A conductive layer was then printed onto the dielectric layer to form a multilayer capacitor structure. Capacitors manufactured in this fashion offer Capacitance Densities of ~650pF/cm2.

Recent efforts have focused on possible solutions for embedding capacitors within plastic substrates. Multilayer circuits containing capacitors can be built up with the use of thin polyester films. Capacitor structures and interconnect can be buried within a flexible substrate by printing conductive films and laminating over them with the plastic film. The various layers of the structure can be connected with drilled holes and printed vias. Devices manufactured in this fashion offer capacitance densities of ~110pF/cm2 for a single dielectric layer.

2.2.7 Surface-mount-technology component attachment

For CLFs to be fully integrated into a high-volume production process the adhesion characteristics of a range of surface-mount-technology (SMT) component packages and conductive adhesives were evaluated. A standard alumina tile employed in solder-bond shear strength measurements by Mitel Telecom was employed. Patterns were printed onto PolyArt and Teslin substrates which were then adhered to the alumina tile blanks to allow automatic handling, including adhesive dispensing, auto-placement of SMT components and adhesive curing.

The tile-mounted CLFs were populated in a similar manner to the standard alumina substrates. Each tile was screen printed with a specially chosen conductive adhesive, and populated with a range of solder-tinned SMT packages using a pick-and-place machine. The component bonds were thermally cured by a temperature profile matching the cure characteristics of the adhesive.

Analysis of the bond failure mechanisms have demonstrated that bond strengths of between 30% to 50% of the strength of soldered joints on sintered silver-palladium thick film conductors have been achieved. This is in agreement with adhesive bond strengths recorded by the manufacturer, involving similar SMT components and conventional circuit board substrate materials.

2.3.1 Statesman Telephone handset
To demonstrate the capacity of the printed films to form complex electronic interconnect, telephony circuitry was selected. Modern telephones contain a network of analogue and digital sub-systems operating at moderate currents and potentials approaching 50V.

The fabrication route adopted was to replicate a ‘Statesman’ (circa early 80’s British Telecom) telephone circuit. Circuit artwork was generated by desoldering and scanning an existing Statesman circuit board. The scanned artwork required substantial reworking to remove all the unwanted soldermask and component labelling. The image was transformed into 2-bit black and white, and the circuit tracks thickened wherever possible to accommodate the higher sheet resistivities of the printed film. The completed artwork was printed onto ‘GlossArt’ and ‘PolyArt’ substrates. The original circuit board from the telephone was stripped of the soldermask permitting the copper tracking to be etched off. The printed film circuitry was laminated to the bare board to provide the printed substrate with mechanical integrity. The 64 through-hole components, including two ICs, were attached with a conductive electrical touch-up paint (RS Part No. 186 3600) to the circuit board pads. The fully assembled and functional circuit was replaced in the original casing. The unit was tested for faults and load impedance on a British Telecom line simulator (Silicon Arrays Part No. Telecom Tester Type T-073), and used extensively on the internal university network.

The electronic success of this demonstrator proved unequivocally the ability of the process to form complex functional analogue circuits, and illustrated the possibilities of the process for switchpadapplications.

2.3.2 Microprocessor Thermometer
The thermometer consisted of a PIC microcontroller and associated circuitry, zinc/air cells and low power seven segment displays. The microcontroller used an external RC combination to set the speed of the processor to 4 MHz, and the temperature was calculated from the time taken to discharge a capacitor through a thermistor.

Several of these devices have been constructed and have proved very reliable. The substrate is used to form not only the electrical interconnect but the cell holder and switch for the device. Versions have been constructed on ‘GlossArt’, ‘Teslin’, polyester and ‘FR4’ substrates. The first of these demonstrators has now been operational for 2 years. The assembled circuits were not lacquered or otherwise protected, and though discolouration of the tracks has occurred the function of the devices has not been impaired.

2.3.3 Nortel Telephone handset
To demonstrate that lithographically printed films may be used successfully in complex mixed signal, double sided, surface mount circuits; a modern, fully featured telephone circuit was fabricated. Nortel provided three sample C8009 telephones, circuitry artwork and components.

The printed circuit board assembly comprised through-hole and surface-mount devices soldered to 1.6 mm ‘FR4’ laminated with 35µm copper. Plate artwork was prepared and printed with care to ensure accurate alignment of the two sides. Two versions, one with thickened tracks, were printed though ultimately this proved unnecessary as the unmodified circuitry was used and proved satisfactory. Assembly of the prototypes required manual drilling of the ‘PolyArt’ substrate to create via holes which were filled with conductive adhesive to achieve electrical continuity. The 133 components were attached in order of decreasing complexity, and checked for short or open connections, as no solder or adhesive mask was used.

The assembled board was tested in the same manner as the ‘Statesman’ before being connected to the university network. Successful operation was achieved, with crisp and clear audio.