Tag to Fit: Custom / Application Specific RFID Tags

Written By Peter Kuzma

As Radio Frequency Identification (RFID) technology has evolved it has become apparent that the number of applications in which the technology can be utilized is much wider than anticipated even one or two years ago. The popular press has focused on wide scale deployments and mandates particularly in logistics and supply chain applications, typified by US DoD and Wal-Mart mandates.

In contrast, there are myriad applications in which there is a substantial Return on Investment (ROI) for RFID; however the performance requirements of these RFID systems, particularly those for the tags, do not mimic those of the supply chain applications. These applications generally involve closed loop asset tracking, security, and/or contactless payment where the tag is used within a specific and unique manner. As these applications have developed so has the need for implementation of Application Specific Identification Devices (ASID's).

RFID systems take many forms; in common is a tag, usually placed on or internal to an object to be tracked and/or authenticated, a reader, which broadcasts an RF signal by use of a fixed (interrogator) antenna, to which the tag responds, and a communication link that delivers the tag's data to a host system. In general tags are small RF transponders made up of a suitable configured antenna, coupled to a single integrated circuit which receives, interprets and responds to queries from the reader. The antenna and IC combination is referred to as an inlay; the inlay is packaged in a suitable format tag such as a label, plastic tag, paper card etc. The tags are classified as passive; in that it receives all its power from the RF field of the reader or active, in that upon activation the tag draws its power from a battery that is part of the tag itself. The readers are generally fixed and linked to a host computer system. The sophistication, power and RF communication mode of readers also varies by manufacturer and application. Communication between the tag and reader occurs in well defined frequency bands set by local governing authorities. Generically these are defined as Low Frequency (LF) at approx. 130 kHz, High Frequency (HF) at 13.56 MHz and Ultra High Frequency (UHF) in 2 modes at approx. 900 MHz and 2.45 GHz respectively. However the mode of communication, both data protocol and methodology, varies from system to system. Moreover the algorithms used to decode the RF signal differ between systems and are often highly proprietary. A partial table, by no means comprehensive, of RFID communication protocols and data encoding methods is given below:

From the RF standpoint the method of electromagnetic coupling between the reader and the tag must be considered. For LF and HF implementations the coupling is through the magnetic component of the EM field, the interaction is similar to that of an air coupled transformer. For UHF tags the coupling may take place through either the magnetic (near field) or electric (far field) component of the EM field. This transition is dependant on the distance from reader to RFID tag and occurs, as a rule of thumb, at a distance of ½ to 1 times the wavelength of RF radiation from the reader antenna, e.g. 16 to 33 cm for 900 MHz radiation.

While the value of the technology lies in the value of the information contained in the tag, for which there is by no means a single standard of format or content. For example in a library application the tag may be required to carry only a ID number that is linked with a particular object by a local database, the so-called "license plate" tag. A supply chain application may require data in a common format, shared by multiple parties in distinct and unrelated organizations. In a pharmaceutical supply chain application the tag may be required to carry information about the pedigree of the individual dose, further this information may require encryption as it may be linked with a specific patient's name and medical history. For RFID passports the tag may be required to carry a great deal of encrypted biometric information that must be reliably and quickly accessed by legitimate authorities but must be secured from electronic eavesdroppers.

The tag as a means to an end, enabling the accurate and reliable dissemination of tracking information, is valuable only so long as it is able to effectively work within the RFID system as a whole. Given the myriad of applications and objects to be tagged it really should come as no surprise that the conformation and construction of the tags should vary widely. The challenge to the user is to establish the fitness for use of a particular tag and system. Fitness for use is measured by the ability of the tag to perform the appropriate data interchange in its operational environment. There is by no means a single parameter that determines a tag's suitability. In some cases maximum read distance between the tag and the interrogator is desired, in other cases shorter read distance and faster data interchange are desirable.

The underlying reason for the variation of a particular tag's performance lies in the variability of the use environment, e.g. variability of objects to which the tag is attached, the relative positions of other tags in the area, the use of materials that interact with the electro-magnetic field by which the tag and reader communicate and objects in the vicinity of the tag/reader combination, all may greatly affect the performance of the system. This can be seen in applications and associated environments such as the tracking of timber harvested for lumber, to authentication of fine art, to the use of a contactless payment card/fob/wristband to conduct a financial transaction.

System Considerations in ASID Implementation

The initial challenge is to understand the overall requirements of the solution and the environment in which the implementation is to occur. There is by no means a single correct answer to the question, "What tags and readers do I want to use?" In almost all cases a series of engineering compromises must be made to achieve the goal. The parameters that characterize the system are not independent. Thus a successful implementation requires a holistic view of the ASID implementation and use case.

Factors that influence the design of the system and tag selection are:

• How much data must be carried on the tag
• Is the data to be open or secured
• Does the data need to be static or dynamic during the life of the tag
• Is the tag to be multi or single use
• At what distance must the tag be read, or not read
• What are the environmental factors that influence the electromagnetic field in use
• What are the physical limitations on the format of the tag
• How will the reader interact with the tag (far field/near field)
• How is the tag to be handled
• How is the tag to be applied to the object
• What is the life expectancy of the tag

The first five factors tend to influence the choice if IC and reader architecture that is used while the last six tend to influence the tag conformation.

In most cases the user will select the system and reader hardware prior to making the determination of the tag. Physically reader hardware is standard and less configurable than the tag itself. Further since the reader is almost always more limited in its ability to adapt to its surroundings, it must be a static variable in the system. Unfortunately, it is at this point in tag selection where designers become bound by the limited selection of commercially available tags and begin to make trade offs in performance, quality, and cost of the overall desired solution in order to accommodate the "off the shelf" non-optimized tags that are typically considered. A more comprehensive approach would have one answer the above questions and then work in collaboration with a tag supplier to provide a tag that has been specifically optimized for their given application.

Tag Design and Function in an ASID Implementation

From a design perspective the tag generally allows the most freedom for variation and thus when done properly can have the most beneficial impact on the overall solution. The principle factors that can be manipulated are IC (chip), antenna design, IC attachment modality, selection of construction materials (laminates, protective coatings, adhesives, etc.) and overall package design itself. The integration of these for elements and their interplay are what determine the overall performance and characteristics of the tag.

IC Selection: There is rarely a single IC choice that must be made for a given tag, as standards for communication protocols and design specifications have evolved the capabilities of IC's available from different manufacturers have tended to cluster together in terms of chip memory size and frequency. Different IC's will conform to the required portion of various ISO or EPC standards and specific functions as needed, but will vary widely in the additional features and their implementation such as security, multi-tag reads and individual tag delineation, capacitance for longer read ranges and unique identifiers to name a few. The choice from a designer perspective is driven by the required functionality desired and limitations that accompany the user environment and electrical parameters of the RF interface and format in which the chip is provided. The first influences the design of the ASID antenna and the latter influences how the chip is assembled on to the antenna to produce the inlay.

Antenna Design: The real art of ASID design occurs with the antenna design. The tag antenna must harvest the RF energy from the reader and transfer it efficiently to the chip. Since the chip has a specific input impedance, it is vital to match the impedance of the antenna and the chip. While the chip characteristics are for most purposes static the antenna characteristics are anything but. The RF field interacts with the antenna and the local environment and it is this interaction which will ultimately determine the functionality of a particular tag in the application. For instance, the presence of metal or moisture either as part of the item being tracked or the surrounding environment has a profound effect on the read performance and accuracy In short the environment of the tag will greatly influence the antenna performance, it is vital that the antenna used in the tag is optimized for the working environment of the entire system.

IC Attachment Modality: There are several methods within the RFID industry for attaching the IC to the antenna to complete the inlay construction. The most mature is bonding what is known as a chip package or module, which contains the actual die, and attaching this module to the antenna using applied conductive epoxy. This process is proven, but has two major draw backs; it is costly, less pliable and constrains the minimum size that can be achieved. Another popular approach is to attach the die directly from the IC wafer via "Flip Chip" bonding approach. This method has become widely deployed over the past few years and has throughput and size advantages over module packaging. But because the IC is attached directly onto the antenna and its substrate, the bond is susceptible to stress in flexible applications. Moreover as IC sizes decrease, the process complexity of attachment of a single chip directly to an antenna increases dramatically. The approach that most chip developers are favoring is "Strap Attach", whereby the IC is bonded to a small flexible substrate (strap) and then attached to the antenna to form an inlay. This approach has several key advantages over "Flip Chip" and modules. First, the reliable attachment of ICs to straps can be done at very high throughput and therefore very low cost. The straps can be applied to inlays a similarly rapid approach, in a common format, thus dramatically improving throughput and thus achieving lowest assembly cost. Additionally, the strap itself creates a stress relief mechanism via its flexible construct and thus achieving higher levels of durability in applications where the inlay will be exposed to bending, physical pressure and thermal changes.

Construction Materials & Package Design: There are two main criteria in selecting appropriate tag construction materials; first, how do these materials affect the functionality of the RFID system and second how will they affect the durability and longevity of the device. In many cases packaging material selection is driven by non-RFID factors such as the need to incorporate human readable variable or fixed data, in other cases they are driven by compatibility requirements with existing infrastructure.

The physical demands of downstream process as well as in-use durability requirements such as the need to embed a RFID device as part of a mold injected process present unique and significant challenges. Additionally, constructing a RFID enabled card for limited use as a payment and/or entry device as found in limited use mass transit tickets also requires strict adherence to specific performance criteria to overcome the harsh conditions that the device will be susceptible too such as water, bending creasing, and crushing.

Building the ASID Tag

What are often ignored in the design process are the economic realities of the tag requirements. Implicit in the design for manufacturing approach is that the materials and processes designed in can be implemented and controlled in a cost effective stable manufacturing process.

From the discussion above it should be apparent that the successful implementation will involve two issues as paramount;

1. Clear and complete understanding of the application, from both the technology and use case perspectives.
2. Understanding of the design and manufacturing process, implementing a design that balances the manufacturing approach

This requires close integration of the design and manufacturing team with the customer. In general a careful and thorough up front effort to qualify and quantify the application the will bring a greater the chance of success. Further a vertically integrated tag manufacturer will have a better chance of controlling the disparate parts of the process as the customer volumes increase. Not only tag manufacture but also data personalization.

Fully as important as design for manufacturing is change control in the manufacturing process. The responsible product group must understand the implications to the customer of manufacturing, material or process changes that may be implemented after the initial deployment. For ASID tags this of particular relevance since by definition the material set is ‘designed in' and matched to the application. Implementation of an ISO certified manufacturing process is de facto de-rigueur in order to maintain a stable supply.

Recomended Approach to the ASID Tag

Commonality must exist in prototyping, pilot production, and full manufacturing in order to keep control of a stable and seamless product delivery through these various stages and volumes of commercialization. Look for vendors with a scalable and flexible manufacturing process that is common to all volumes of tag manufacturing. They should have a flexibile manufacturing process that allows economies of scale to be built into the production process.

Of the antenna technologies available in the market (subtractive copper, subtractive aluminum, printed silver conductive ink, physical vapor deposited copper or aluminum or stamped metal) none offer a better combination of flexibility, performance and economics inherent in an additive copper. The ability to dial in the line spacing and width along with the thickness of the deposit to offer the correct combination of conductivity and physical durability suitable to the high conductivity requirement of an HF antenna and the lower conductivity requirements of a UHF antenna can be achieved on the same production line with the same process.

The adoption of a strap-attach method for inlay assembly has the obvious advantage of opening up antenna design process window. The strap-attach modality feeds into the ability to maintain a common manufacturing process across products. Strap-attach also gives the less obvious advantage of a scalable process that will survive through future generations of IC's that will get progressively smaller with each succeeding generation. Presently all the major IC manufacturers active in the RFID space have smaller chips on their roadmaps as a key to enabling the cost reductions that will be required for mass adoption, particularly as item level tagging looms nearer.

Peter Kuzma is a subject matter expert and trusted adviser in the areas of RFID passive smart lables, security of RFID labels, HF and UHF chips, antennas, and inlays.

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