Mittuniversitetet

The development of printed electronics on flexible substrates is increasing rapidly, where the main benefit is large area and low cost. However, the functionality and performance of printed circuits cannot compete with standard silicon based microprocessors or integrated circuits, though the functionality and performance of printed circuits are increasing. Therefore, in this work we investigate the possibilities of using Sn42/Bi57.6/Ag0.4 low-temperature solder paste together with a reflow solder oven and hot air solder iron to mount regular SMD components on screen-printed silver tracks. It was found that it is possible to solder standard Si SMD pack-ages onto screen-printed Ag ink tracks on paper substrate, however, the component bonding strength to Polyimide were not satisfactory. The resistance of the solder joints was found to be no more than 240 mΩ. The bond strength was found to be higher using the manual hot air sol-der iron than the reflow solder oven. Bending tests show that the bonding strength is higher for the hot air soldered components. Reference samples on FR-4 based copper PCB show a bond strength ~10 times higher but we conclude that it is possible to solder regular Si SMD components onto Ag-printed conductors on paper substrate with good results. The process could be used to fabricate hybrid printed electronics on a standard solder process line.

This paper describes mounting of standard surface mount component packages on a paper substrate using an industrial solder process with a low-temperature solder. The use of paper as a substrate for printed flexible electronics is becoming more and more widespread as an alternative to the more commonly used plastic substrates, such as polyethylene and polyimide. Paper has the benefits of being environmentally friendly, recyclable, and renewable, as well as inexpensive. It is shown that it is possible to mount standard surface mount device components on paper substrates using low-temperature solder in an industrial soldering process. The contact resistances obtained are mostly low, although the yield of functioning contacts is low. The reason is cracking of the substrate coating layer that goes through the printed silver tracks. It was observed that the cracks appear mostly close to the contact pads, the most likely cause is thermal mismatch between the coating layer and solder and also thermal expansion of the photo paper resin coating. The smallest component package size, 0201, resulted in the highest yield of >80% with decreasing yield for larger package sizes.

Implementing electronics systems on paper is an important area of flexible circuit technologies. One of the approaches is to print conductive inks onto paper substrates, on which silicon-based surface mount device components are mounted. However, one of the problems is that the printed conductors have unneglectable resistivity. In this paper, we present paper-based flexible circuits, using copper and aluminium conductors that are laminated onto paper substrates using a high-speed roll-to-roll method. Edge roughness inspections and repeated two-point bending tests are carried out to evaluate the manufactured flexible circuits. Three surface mount techniques are used to assemble standard surface mount device components onto the flexible circuits, including an isotropic conductive adhesive, an anisotropic conductive adhesive, and a low-temperature solder paste. Several characterizations are performed to the surface mount techniques, including contact resistance measurements, component bonding strength tests, assembled circuit bending tests, and scanning electron microscopy. The results of the characterizations suggest that flexible circuits made from Cu with paper substrate achieve satisfactory results for mechanical reliability, all surface mount techniques, and have the potential to be used on automatic component assembly lines. In order to test whether such flexible circuits and surface mount techniques can be used in implementing electronics systems, passive NFC tags with relative humidity sensing functionality are made, which are interrogated by an NFC equipped mobile phone.

Electronic textiles, integrating functional electronics circuits into fabric materials, are emerging as an important branch of flexible circuits. In this paper, we introduce a novel material combination for electronic textiles that can be used in implementing hybrid electronics. This type of circuits is fabricated by laminating patterned aluminum foils onto a nonwoven substrate in a high-speed roll-to-roll method. An isotropic conductive adhesive and an anisotropic conductive adhesive are used to assemble standard surface mount device components onto the fabricated circuits. The surface mount techniques are characterized by means of contact resistance measurements, component bonding strength tests, circuit bending tests, and scanning electron microscopy. An NFC tag with relative humidity sensing functionality is fabricated to evaluate the fabricated circuits to an electronic system level.

RF electronics commonly incorporate PCB-materials with low loss tangents, which limits its use for large-area applications due to its high cost. This work verifies one alternative solution how non-conventional flexible circuit materials can be used to manufacture large functional surfaces for RF-based applications. Laminated Al foils are used for conducting layers and a flexible foam material is used for substrate. An RFID reader system has been developed to demonstrate the functionality, comprising of eight microstrip antenna elements arranged in a SP4T switching structure covering an area of 1.2 m × 0.6 m. Each antenna element is individually addressable with aid of distributed digital and analogue multiplexer circuitry and it is shown how these components can be soldered directly onto the Al conductors. The constructed system shows good RF performance, both with regards to the materials and to the interconnections with the distributed multiplexer circuitry. It can perform far-field RFID tag reading above its surface without dead zones and the system characterization implies that the concept can be further expanded to cover geometrical areas up to 1000 square meters.

RFID readers serve the obvious role of extracting information from RFID tagged objects. Objects without RFID tags or objects with tags that for some reason are unreadable will not be noted at all when positioned within an RFID reader antenna's interrogation zone. In this paper, we investigate how UHF RFID reader systems for smart shelf applications could also be used for classification and distance estimation of non RFID tagged objects, if the reader modules provide access to antenna S-parameters. The investigation is performed with an inset fed microstrip antenna where objects of different materials are positioned at different heights above the antenna. It is shown how objects are detected and classified in terms of their materialistic properties through S-parameter analysis and how the distance from the antenna to the object could be estimated.

Screen printing is a stencil process where conductive inks are patterned onto substrates through a fine mesh of threads. Nowadays, screen printing can be used to print RFID antenna structures onto flexible and ultra-low-cost substrates such as pa-per. In this manuscript, we present an HF RFID reader antenna system, operating at 13.56 MHz, using screen printed Ag particle ink as conductor and using HP photo paper as substrate. The proposed antenna system comprises four loop antenna elements, matched to 50 Ω, and one I2C addressed SP4T multiplexer circuitry, controlled through an exterior embedded system. The geometries, designs and characterizations of the antenna system are described in the manuscript in details. Measurement results show that the antenna system has low power reflections and a suitable Q factor. It has a maximum 11.1 cm RFID tag read range at an antenna system input power of 33 dBm. 2D RFID tag positioning can be enabled by utilizing the RFID tag interrogation zones formed by the four loop antenna elements. In addition, a parametric study is carried out to investigate the effect of loop antenna element DC resistance on the antenna element performance. It can be concluded that the proposed method can be used to create low-cost and large-area HF RFID reader antenna systems.