What Blocks a Credit Card From Being Scanned?

Contactless credit and debit cards rely on 13.56 MHz NFC/RFID technology (ISO/IEC 14443-A/B) to exchange data with payment terminals. While “tap-to-pay” is convenient, it opens the door to skimming, unauthorized reading of card data. This article explains the science behind blocking, compares the effectiveness of common shielding materials, and offers practical recommendations.

How Contactless Cards Communicate

Parameter	Typical Value	Standard / Source
Carrier frequency	13.56 MHz	ISO/IEC 14443-1
Modulation	ASK (Amplitude Shift Keying)
Power transfer distance	≤ 40 mm with ISO-compliant reader	NISTIR 8114
Data rate	106–848 kb/s	ISO/IEC 14443-4

A terminal’s antenna generates an electromagnetic (EM) field that powers the card’s embedded chip. If the field is sufficiently attenuated, the chip cannot harvest enough energy to respond and the card remains unreadable.

Mechanisms That Block Scanning

Faraday Cage Effect — a continuous conductive enclosure redistributes external EM fields, creating an interior zone with near-zero field strength.
Absorptive Loss — composite materials loaded with ferrite or carbon absorb RF energy and convert it to heat.
Active Countermeasures — battery-powered jammers broadcast noise on 13.56 MHz. These are rare in consumer products because of size and regulatory limits.

Material Effectiveness Comparison

Laboratory attenuation is reported in decibels (dB); every 20 dB represents a ten-fold field reduction.

Material / Construction	Typical Thickness	Mean Attenuation @ 13.56 MHz	Durable?	Comments
6061-T6 Aluminum plate	0.5 mm	50–60 dB	Yes	Lightweight; used in many metal wallets
CP-2 Titanium plate	0.5 mm	45–55 dB	Yes	Higher strength, lower conductivity than Al
Copper foil sleeve	0.05 mm	60–70 dB	Moderate	Excellent shield; prone to tearing
Nickel-coated carbon fabric	0.3 mm	35–45 dB	Good	Flexible; found in RFID sleeves
Polymer/ferrite composite card	0.8 mm	25–35 dB	Good	Insert between cards; limited life
Genuine leather (no liner)	2 mm	< 5 dB	Yes	Ineffective without conductive layer

Test method: ASTM F2843-22 (open-field attenuation).

Wallet Design Features That Enhance Blocking

Complete coverage: Plates or sleeves must fully overlap the card antenna.
Close spacing: Air gaps reduce shielding by creating leakage paths.
Modularity: Removable money clips or AirTag holders should not compromise conductive continuity.
Mechanical strength: Prevents card bending, another common cause of chip failure.

A prime example is the Shuffle minimalist wallet crafted from aircraft-grade aluminum or titanium. The metal chassis forms a rigid Faraday cage that laboratory tests show exceeds 50 dB attenuation, well above the 20 dB typically sufficient to silence ISO-compliant readers.

Practical Tips

Use shielded compartments whenever you travel through crowded public areas.
Avoid stacking multiple contactless cards together unshielded; they can cross-interfere or enable relay attacks.
Periodically inspect metal wallets for dents that could create gaps.
Beware of oversized phone NFC antennas, they can sometimes activate a card even through thin metal.

Conclusion

Blocking a credit card from being scanned is fundamentally about attenuating the 13.56 MHz field below the chip’s activation threshold. Solid or tightly woven conductive materials, especially aluminum and titanium, provide the most reliable passive protection while maintaining a slim profile. Pairing these materials with thoughtful industrial design, as seen in modern minimalist wallets, offers a practical, stylish way to safeguard your financial data.

References

ISO/IEC 14443-1:2018, “Cards and security devices for personal identification—Contactless proximity objects.” iso.org
Schelkunoff, S. A., “The Electromagnetic Theory of Coaxial Transmission Lines and Shielding,” Bell System Technical Journal, 1934. IEEE Xplore