Tuomas Happonen

All Silk-Screen Printed Polymer-Based Remotely Readable Temperature Sensor

In this paper, we present the fabrication and characterization results of a remotely readable flexible temperature sensor. The sensor has been fabricated with screen printing technology and it consists of a polymer-based capacitor (ASAHI CR18-KTI polymer paste) as a temperature sensitive component added to the antenna coil (ASAHI LS411-AW silver paste) for remote reading purposes. The resonance frequency of the sensor (i.e., the formed LC network) is measured as a function of temperature. The sensor has been characterized in the temperature range of 0 °C-70 °C and the total sensitivity over this range is ~9 kHz/°C. The sensors normalized temperature response is independent from the polymer capacitors thickness and the effective capacitor area, which enables reasonable performance despite fabrication process induced inaccuracies. Due to the inexact nature of the resonance peak measurement, a dataanalysis algorithm was developed to enhance the accuracy of the measurement method. In addition, the sensor response to the misaligned reader and the sensor self-heating effect, due to the inductive excitation voltage, are considered. Finally, the calibration of such a sensor is discussed.

Reliability Study on Adhesive Interconnections in Flex-to-Flex Printed Electronics Applications Under Environmental Stresses

The reliability of conductive adhesive interconnections in flex-to-flex printed electronics applications under thermal cycling and cyclic bending tests is investigated in this paper. The components under study consisted of a backplane, having screen-printed silver wiring on polyethylene terephthalate film, fabricated in a roll-to-roll process, and a flexible functional component mimic stacked on top of the backplane. Each test component contained four daisy-chained conductive adhesive interconnections, and their reliability was monitored in situ with a four-point dc resistance measurement during tests. In addition, the effect of supportive nonconductive adhesives on structure reliability was studied in various configurations. The accelerated life test results proved that the long-term reliability of the studied components can be enhanced by adding supportive adhesive between the flexible layers. In both thermal cycling and cyclic bending tests, the most reliable method was found to be the configuration with supportive adhesive around the top layer sides, attaching the two layers totally together, whereas the configuration with no supportive adhesive at all showed the worst reliability. The failure analysis emphasized that the critical factor, in terms of reliability, was silver conductor delamination from the backplane surface at the interconnection area.

Cyclic Bending Reliability of Silk Screen Printed Silver Traces on Plastic and Paper Substrates

The effect of the substrate on the cyclic bending reliability of screen printed silver traces is studied in this paper. The silver screen printed test samples in this study were three different plastic films and three varieties of paper sheets. The specimens with line widths of 250, 500, 1000, and 2000 μm were subjected to cyclic bending, and their electrical behavior was monitored with dc resistance measurements in situ. In this study, a 20% increase in resistance was defined as the failure criterion, and the reliability of each test population was assessed by a statistical Weibull analysis. The results showed that the reliability of the traces was highly dependent on the substrate thickness in all the studied materials. The traces on a 125-μm-thick polyethylene terephthalate substrate were found to have the lowest characteristic lifetimes of about 1000 bending cycles, whereas the traces with the same geometry on polyethylene terephthalate with a thickness of 50 μm endured hundreds of thousands of bending cycles. Similarly, decreasing the substrate thickness improved the reliability of the printed traces on paper substrates. This relationship was due to the reduced mechanical strain on the trace surface when using thinner substrates.

Modeling the Lifetime of Printed Silver Conductors in Cyclic Bending With the Coffin–Manson Relation

This paper discusses the modeling of the lifetime of printed conductors on a flexible substrate under cyclic bending. Roll-to-roll printed silver lines on a plastic film were exposed to cyclic strains accordant to the bending radii of 15, 20, and 30 mm, and their long-term electrical behavior was monitored in situ with four-point dc resistance measurements. By using 5%, 10%, and 20% increase in resistance as failure criteria, the lifetime of test populations was assessed utilizing the Weibull reliability analysis. Based on these results, the relationship between the applied strain and the lifetime was characterized and modeled by the Coffin-Manson equation. The Coffin-Manson relation accordant behavior for the lifetimes under various strains was also verified by confirming measurements with untapped bending radii with good results. As a conclusion, the lifetime of a printed conductor with known geometry and materials under cyclic bending was able to be modeled as a function of bending radius with the Coffin-Manson relation.

Screen-Printed Remotely Readable Environmental Sensor Pair

In this paper, we present the fabrication methods and the characterization results of a remotely readable flexible temperature/humidity sensor pair. The sensor pair has been fabricated with a screen printing technology, and it consists of a polymer-based capacitor as a temperature and humidity sensitive component added to the antenna coil for remote reading purposes. The sensor pair relays its operation to the double sensor structure in order to produce the environmental temperature and relative humidity readings. The resonance frequency of the sensor (i.e., the formed LC network designed to 13.56 MHz) is measured as a function of temperature and relative humidity. Temperature is sensed with the reference sensor with full lamination and the relative humidity with the active sensor with a lamination opening over the polymer capacitor. The sensor has been characterized in the temperature and humidity ranges of 0 °C-70 °C and 45%RH-95%RH, respectively. The simple data-analysis algorithm developed enables the calculation of both temperature and relative humidity from the corresponding measured resonance frequencies. The relative humidity sensitivity (ΔfRES/%RH) of the sensor was between 3.6-5.3 kHz/%RH in the temperature range of 30 °C-50 °C. The temperature sensor in the sensor pair had the sensitivity of 9 kHz/°C in the applied temperature range. In addition, the temperature compensation of the polymer humidity sensor is considered, based on the data-analysis algorithm. Finally, the calibration and the effect of the installation medium of such a sensor pair are discussed.

Processing and reliability of bare die LED chip bonding on flexible plastic substrate

Printed hybrid electronics combining traditional semiconductor devices and polymer substrates allows augmenting the performance of printed electronics. In this work, chip bonding on plastic substrate, i.e. chip on flex (COF), using anisotropically conductive adhesive and LED chips was investigated. Unpackaged semiconductor chips were bonded to PET foil with printed silver conductors. The yield and reliability of the bonding and the effect of several bonding parameters were studied. The experiments were done using automatic equipment to manufacture LED ribbons. Reliability of the ribbons was investigated by subjecting them to temperature and humidity of 85 °C and 85 % respectively and by bending tests. It was found that with correct choice of manufacturing parameters, it is possible to attain 100 % yield and the samples produced with optimal parameters can withstand the 85/85 environment for period over 100 days. Furthermore, samples produced this way are not damaged by bending them around cylinders with radiuses of 20 mm and 15 mm, for 10 000 and 27 000 cycles respectively. The good tolerance for bending can mostly be attributed to the small size of the chips.

Standardising remote testing: a universally applicable method

Testing has become a critical factor as high-tech products are increasingly complicated. Remote testing provides many benefits, including improved serviceability and cost efficiency. This paper discusses test communication for remote testing, which is lacking generic solutions. In this research, Universal Test Communication Specification (UTCS) is developed, with the involvement of research partners from telecommunication industry. The presented UTCS does not require special technical solutions and consequently has potential of developing into a standard. A practical demonstration is included to highlight a possible implementation. The presented specification and the Service List feature improve visibility over test solutions embedded in products.