Bright sunlight disables sticker tags not screened for IR, causes rare false reads
By Peter Samuel
2012-06-26: Some windshield-mounted sticker tags used for vehicle identification and electronic toll collection cease functioning in bright sunlight - those without screening against IR. Some even malfunction to the extent they'll generate a false tag identification (TID) number, raising the possibility of the wrong toll account being debited via the sun. The disabling effect producing non-reads is a widespread problem in some models, while the false TID occurs only in those models rarely, at a narrow band of light intensity and energy level.
The problem arises where no provision is made for screening sunlight's Infra Red (IR) energy. Unscreened the IR portion of solar energy penetrates deep enough into the silicon circuitry to overwhelm the normal operations of the chip.
There is no permanent damage. The chip works fine again out of direct sunlight.
But in sunlight an unscreened chip regularly malfunctions. Screening against IR can be provided by the materials in the tag packaging or by the windshield inside which the tag is mounted. But some windshields do not have IR screening and some tags are not engineered with IR screening.
One engineer who has done extensive testing and researched the scientific literature says the IR problem is readily understood by reference to the properties of silicon and its handling of different wavelengths of energy. And the disabling effects of IR can be readily overcome with a protective coating built in when the tag is manufactured.
He says such a screen coating has been built into tags to protect them against IR disruption in the case of tag brands he has been associated with since 2009, but not in other brands.
ISO standards ignore the problem so conforming tag can malfunction
None of the present international standards, notably ISO 18000 6B or 6C, governing design of vulnerable sticker tags address the problem or provide for any IR screening - though the OmniAir certification group has been alerted to the issue and is likely to take it up in its forthcoming certification program for 6C.
An engineer says that with some makes of 6C tags all those tested "failed to operate" in unscreened sunlight. All!
The tags were also tested by exposing the backside of the tag to sunlight. Once again, the tags failed to function.
That backside vulnerability shows the IR is the problem. Visible and UV light have wave lengths less than 1.1 microns and won't penetrate right through the 150 microns substrate to get to the printed silicon circuitry. The 1.1 microns at which IR begins correlates to the so-called bandgap of single crystal silicon.
The silicon electronic circuitry of a sticker tag is located in the outside 2 microns of the 150 micron thick substrate on which it is 'printed.'
150 microns is close to perfect in allowing IR light to penetrate deeply and the engineer says IR unscreened can pass through as many as 10 silicon wafers. The substrate at 150 microns is almost 'tuned' for sunlight disruption.
"The data certainly supports the matter of sunlight affecting an unprotected silicon integrated circuit. This is not an issue of ISO 18000-6C or even RFID, but the proper screening of sunlight when operating any integrated circuit. The reason it has become an issue for certain RFID (sticker) tags is that most of the new RFID tags are fabricated by placing a bare die on a flexible substrate which is transparent to sunlight.
"Another interesting aspect identified when testing the (brand withheld) tags was that under just the right conditions of light intensity, the tag would respond with an altered TID (transaction identification number.) These events were very rare and seemed to be associated more with certain tags. Sometimes it was only one or two bits but in some cases it was many bits that were altered.
"In general it would be expected that RFID chips will be more sensitive to light than most integrated circuits. RFID chips are designed to operate at very low power levels. Consequently, the internal nodes are much higher impedance, or have much less current flow. So a small amount of charge created by light creation of carriers in the silicon will have more of an effect on these high impedance nodes than high speed circuits involving much higher current operation.
"Fortunately, most windshields block certain wavelengths of sunlight. Usually this is the UV spectrum and thus the reason that a person inside a vehicle will not suffer sunburns. Most windshields block IR but this is associated more with newer vehicles and also higher value vehicles.
"The cure for making tags light resistant is quite simple. The light that is causing the failures must be blocked from reaching the RFID chip. (One company) has accomplished this by placing a barrier film (or screen) over the die."
EDITOR'S NOTE: We got more information than this but were asked to withhold some specifics for the time being because of late corporate sensitivity.