Pekka Ruuskanen

Purpose: The aim of this study was to evaluate the feasibility of pad printing for producing electrical conductors and to define the factors affecting the print quality of polymeric silver ink conductors. Design/methodology/approach: Polyethylene terephthalate (PET) film and polyphenylene oxide (PPO) compound film were used as substrate materials. Three different polymeric silver inks, marked A, B and C, were used and tested. Findings: The results indicated that the important factors in the pad printing of silver ink conductors are the printing parameters and characteristics of the ink, pad, cliché and substrate. The interactions of these factors should be considered on a case-by-case basis. The sheet resistances of triple-pressed ink conductors varied between 20 and 110 mΩ/sq for 5.7- to 8.5-μm-thick conductors. Ink (B) had a higher sheet resistance than Ink (A) because of its lower silver particle content but also because of the shorter curing time and lower curing temperature. Ink (A) showed excellent adhesion on PET, and Ink (B) had moderate adhesion on PET without corona or plasma pre-treatments, but both inks adhered weakly on PPO compound. Both corona and plasma treatments raised adhesion of these two inks on all test substrates to the highest classification value, 5B. Originality/value: This paper contains a survey and preliminary testing of the pad printing of polymeric silver ink conductors on flexible thermoplastic foils. Finally, the paper introduces the advantages and drawbacks of the technique.

Purpose – The aim of this study was to test and survey a circuitry transfer technique where conductor patterns are electroformed on carrier substrates and thereafter the electroformed patterns are transferred from carrier substrates to their final devices. Design/methodology/approach – An electrically conductive pattern is built up by an electrodepositing metal or metal alloy on a carrier substrate, called a mandrel, using a resist image to define the outlines of the pattern. Thereafter, the electroformed structures are bonded on plastic substrates, for instance, by hot pressing or by embedding into a resin. In our experiments, the imaging of stainless steel carriers was done by the photolithographic process and the electroformed copper patterns were transferred by hot pressing onto thermoplastic substrates. Findings – The literature review revealed that the transfer of electroformed conductor patterns to plastic parts is not a very commonly used technology, although it could provide possibilities for even quite specific structures in electronics manufacturing at an affordable price. Our tests indicated that the acidic peroxide-sulfuric pre-treatment of electroformed copper patterns before hot press bonding clearly improved the adhesion of copper on both acrylonitrile butadiene styrene and polyphenylene oxide substrates and that a steel template around the substrate during hot pressing process can restrict dimensional changes in thermoplastic substrates significantly. Originality/value – This paper contains a survey and preliminary testing of the electroformed circuitry transfer technique. The analysis of the test boards focused on the adhesion between copper strips and thermoplastic substrates and on the factors affecting adhesion. Finally, the paper introduces the advantages and drawbacks of the technique.

Stable polarization of permanent magnets over the lifetime of the application is an important aspect in electrical machine design. Specification of the long-term stability of magnet material is difficult, since knowledge of the phenomenon is incomplete. To be able to optimize magnet material selection, the long-term magnetic behavior of the material must also be understood. This study shows that material with a very squareJHcurve is stable until a certain critical operating temperature is reached. Major losses are detected as the critical temperature is exceeded. Material with a rounderJHcurve does not show a well-defined critical temperature, but increasing losses over a large temperature range. The critical temperature of a material is also dependent on the field conditions. Results differ whether the tests are performed in an open or closed magnetic circuit. In open-circuit tests, the opposing field is not homogeneously distributed throughout the volume of the magnet and thus the long-term behavior is different than that in closed-circuit conditions. Open-circuit tests seem to give bigger losses than closed-circuit tests in cases where the permeance coefficient of the open-circuit sample is considered to be the average permeance coefficient, calculated according to the dimensions of the magnet.

Purpose: The purpose of this paper is to develop a wireless strain sensor for measuring large strains. The sensor is based on passive ultra high‐frequency radio frequency identification (RFID) technology and it can be embedded into a variety of structures.Design/methodology/approach: Silver ink conductors and RFID tags were printed by the screen printing method on stretchable polyvinyl chloride and fabric substrates. The development of the strain‐sensitive RFID tag was based on the behavior of the selected antenna and substrate materials. Performance of the tags and the effect of mechanical strain on tag functioning were examined.Findings: The results showed that large displacements can be successfully measured wirelessly using a stretchable RFID tag as a strain‐sensitive structure. The behavior of the tag can be modified by selection of the material.Research limitations/implications: New tag designs, which are more sensitive to small levels of strain and which have a linear response will be the subject for future work. Tag performance under cyclic loading and in a real environment will also be investigated. Future work relating the investigation of practical applications and the system designing for the strain sensor will also be required.Practical implications: Printing is fast and simple manufacturing process which does not produce much waste or material loss. The sensor is a new application of printed electronics. It also provides new opportunities for system designers.Originality/value: The paper provides a new kind of wireless strain sensor which can be integrated into many structures (i.e. clothes). The sensor is a new application of printed electronics and it is made from novel materials.

Strain measurements could be utilized in monitoring human bodily functions and movements. Cost effective and low maintenance wearable sensors are needed in this field. The goal of this paper was to produce wearable strain sensors based on UHF RFID technology and the behavior of the materials which were utilized in prototypes. Two tag geometries were compared. Polymer thick film silver ink was used as the conductive medium. The tags were fabricated by screen printing the ink on stretchable PVC and on fabric substrates. Performance of the tags and the effect of mechanical straining on tag functioning was examined. The prototypes tolerated large strain levels. The results showed that the behavior of the UHF RFID tag strain sensor is dependent on the material behavior under strain as well as the tag antenna geometry. These offer various possibilities in strain measuring.

In this study copper and silver ink microstrip lines (MSLs) are simulated, measured and compared. Implementation and performance of a coupled-line coupler using printed MSLs was investigated. At microwave frequencies it is important to consider conductor losses, especially in thin conducting structures. The effect of the conductivity of the conductor material, substrate losses and the effect of protective coating were also investigated. The results showed that despite the lower conductivity of the silver ink compared to copper, the screen-printed lines that authors studied are still applicable to microwave applications.

In this study, the effect of the conductive ink layer thickness on the performance of printed ultra-high-frequency (UHF) radio-frequency identification (RFID) tag antennas was investigated. A simple quarter wave dipole tag for European UHF RFID frequencies was designed to be tested in this study. All the tags were made by using screen-printing technique. Three different thicknesses for the ink layer were used. Performance of the tags was analyzed by the measurement of threshold and backscatter power. The results show that it is possible to produce RFID tag antennas by screen printing and it is possible to optimize the tag performance by adjusting the thickness of the electrically conductive layer. The results show how the performance characteristics deteriorated when the thickness of the printed ink layer was reduced. However, it was also shown that thin ink layers can be used in some applications and cost savings can be achieved in this way. It is therefore important to recognize these effects on the performance.

Purpose – The adhesion between electroless copper and a substrate is one of the most important factors in the reliability of thermoplastic printed circuit boards. The purpose of this paper is to investigate the effects of mechanical grinding and acid etching of thermoplastic substrate materials on the adhesion of copper deposited by an electroless copper plating process. The base material of the test substrates was a new high temperature thermoplastic polyphenylene oxide (PPO) compound. Design/methodology/approach – The effects of pre-treatment on plastic surfaces are analyzed by the following methods: Fourier transform infrared (FTIR), SEM, the Dyne surface energy test and the surface roughness test. The adhesion between electroless copper and thermoplastic substrate is measured with a peel strength test. Findings – The results showed that mechanical grinding of the substrates significantly increased adhesion but the highest adhesion is gained by using an acid etch treatment before electroless plating. These results indicated that adhesion between copper and the substrates was not directly proportional to the roughness and surface energy values. Originality/value – The conventional sweller/desmear treatment used in a printed circuit board factory for pre-treating epoxy based laminates prior to electroless plating is not suitable for these PPO compound boards. The copper adhesion is adequate when the substrates are etched with sulphuric acid/chromate solution. In that case the bonding between the metal layer and the plastic surface is stronger than the bondings between the polymer chains of the thermoplastic material. The adhesion mechanism of electroless copper in these mechanically abraded samples is mechanical interlocking of metal particles.