Ask The Expert:
Permanent Power for RFID Tags
Steve Grady, V.P. of Cymbet's technical marketing team, discusses
the current status of thin-film batteries and what's in store in the future.
RFID tags are becoming increasingly integrated and miniaturized. One obstacle facing RFID tag designers is the lack of size and profile improvement in batteries and other energy storage devices that maintain sufficient capacity to deliver the necessary power and energy. Recent advances in thin-film battery technology are providing new means for active and hybrid RFID tag designers to power their tags. An example of these new thin-film batteries is the EnerChip ™ from Cymbet Corporation, www.cymbet.com. Its thin-film batteries can be reflow-soldered directly to the RFID electronics circuit card. These batteries can be recharged many thousands of times, making them ideal for reusable RFID tags.
Q: What does Cymbet mean by "Permanent Power"?
A: Permanent Power is the condition of always having power available on a device such as an RFID tag over the life of the devices. Until now, this has been accomplished through the use of storage devices such as coin cell batteries or super capacitors. However, these legacy storage technologies may not have the right mix of charging, storage, discharge, and physical size characteristics to provide Permanent Power. Recently introduced thin-film battery technology does have the right mix of capabilities to support Permanent Power.
How do these thin-film batteries operate?
The fundamental research on solid state Lithium thin-film batteries was conducted at Oak Ridge National Laboratories. There are several companies who have licensed the ORNL technology. The key for applications such as RFID tags is to make the batteries cost effective and easy to integrate into the technology. Over the last several years, Cymbet has been working to package these batteries in a semiconductor footprint.
How are these thin-film batteries packaged?
One of the key factors that limits traditional battery size is the packaging that houses battery chemicals. This packaging prevents cells from leaking, rupturing, or being degraded by external contaminants such as moisture. Because of the inherent differences in tolerances to heat and moisture, as well as safety considerations between ICs and electrochemical storage cells, electrochemical cells have their own unique packaging and handling requirements. Cymbet has created a thin-film battery construction and physical package that overcomes the limitations of traditional batteries. Surface mount chip scale packaging and bare die (the chip without its packaging encapsulated with other semiconductor chips) can be used in very thin packaging applications.
What are the electrical characteristics of these thin-film batteries?
Solid state thin-film batteries have a high energy density and come in capacities ranging from 12 uAh (micro Amp hour) to 85 uAh. The peak current output is typically 1mA (milliAmp), but often external boost capacitors are used to increase current output for burst applications. The charge voltage is 4.1V and the voltage output range is 3.0-3.8V.
Can these thin-film batteries be assembled easily onto an RFID tag board?
Since this is a Permanent Power solution, the battery is soldered onto the board using automated SMT pick and place equipment and the devices are available on tape and reel. Only thin-film batteries packaged like the EnerChip can withstand the 260 °C temperatures of the normal reflow-solder assembly. Additionally, the EnerChip can be used in a bare die format and attached to a flex circuit along with the RFID transponder chip.
How are these batteries charged?
Thin-film batteries are charged much like other on-board storage devices such as rechargeable coin cells and super capacitors. There are some design considerations that must be met to support the rapid charge times on a battery like the EnerChip. The EnerChip can be charged to 80% capacity in 30 minutes. It reaches 100% capacity in one hour.
When using thin-film batteries in RFID tag design, how many recharge cycles can be expected?
Thin-film battery technology can support many recharge cycles. The EnerChip for example, can support many thousands of recharge cycles.
What is "Energy Harvesting" and how can it be used in RFID tag design?
Energy Harvesting is the term for using ambient energy in various forms to charge storage devices. This ambient energy can be in many forms including: solar, motion/vibration, thermal, electromagnetic, and RF induction. An appropriate transducer such as solar cells or a piezoelectric device is coupled to a voltage converter. The resulting energy is stored in the battery. This Energy Harvesting combination of transducer and thin-film batteries creates a Permanent Power solution. Figure 2 shows an example of a Solar Energy Harvesting board coupled to a wireless transmitter.
Why are thin-film batteries such a good enabler of Energy Harvesting?
There are four key criteria for Energy Harvesting storage devices:
1. Low self-discharge
2. High energy density
3. High recharge cycle count
4. Rapid charge time
Thin-film batteries are the only storage devices that have all these key criteria.
For RFID tag design, how do thin-film batteries compare to super capacitors?
As can be seen in the Table 1, the key issues are: self-discharge, charge times, inability to automatically assemble in a SMT and reflow line, device size, and voltage output variation.
What about rechargeable coin cell batteries vs. thin-film batteries?
Table 2 highlights the key differences: recharge cycles, charge times, inability to automatically assemble in a SMT and reflow line, device size pulse capability, and environmental impact.
How are thin-film batteries more eco-friendly than other storage technologies for RFID tags?
Thin-film batteries, such as the Cymbet EnerChip, are solid state devices and do not have any liquids or harmful chemicals that might leak. Thin-film batteries meet all transportation safety criteria and will transport safely. Since thin-film batteries are like all the other semiconductor devices on the board, there are no special handling issues at time of disposal.
Final thoughts for RFID tag Permanent Power?
Thin-film solid-state batteries provide a cost-effective means of permanently powering RFID tags, given the advantages of small size, high energy density, quick charging, high recharge cycles, and eco-friendliness.
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