ScreenWeb | screenweb.com

(June 2006) posted on Tue Jun 27, 2006

Learn about the function and construction of RFID tags and how screen printing and other imaging methods are used to produce these devices.

By Wim Zoomer

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Table 2 Comparison of Antenna-Printing Methods
Characteristic	Screen printing	Flexo/gravure
Investment	low to medium	medium to high
Print speed	low to high	medium to very high
Ink-deposit thickness	variable	low
Run length	very short to medium	large to very large

Direct printing Direct printing allows antennas to be produced in one pass, whereas the other methods require multiple steps. In direct printing, a solvent-based, conductive, silver polymer-thick-film (PTF) ink is used to print the antenna on a plastic or paper substrate (Figure 2). UV inks for this application are currently in development.

HF antennas require a relatively thick ink deposit to provide the proper level of conductivity, so screen printing is a leading technique for producing these antennas. Screen printing is an ideal technique for delivering a measured deposit of ink. And the deposit thickness can be easily modified by switching to a mesh with a different thread count. After printing, hot-air drying causes the silver particles in the ink to become connected, which creates an electric circuit—the final antenna.

In addition to PTF inks, thermal silver conductive inks also are available. These inks don't contain a polymer binder; instead, they contain electrically conductive organo-metallic particles. These particles are melted together under high temperature. Antennas printed with thermal silver conductive inks provide greater conductivity than those produced with PTF inks.

UHF antennas are basically thinner than HF ones. Consequently, other printing methods may be more suitable than screen printing for producing UHF antennas. Flexography and gravure printing both deposit significantly less ink than screen printing, but enough for UHF antennas. These print methods are typically used with jobs involving longer run lengths.

Flexography and rotogravure print with a speed of 100-500 ft/min (30-150 m/min). The speed of flatbed screen- printing equipment is far slower, so flatbed screen presses generally just are used for producing prototypes and very small jobs. Rotary screen printing, on the other hand, delivers speeds of approximately 80-200 ft/min (25-60 m/min), which puts it prominently in the middle between flatbed screen printing and flexo/gravure. The performance attributes of both flatbed and rotary screen printing are compared in Table 3.