Glass is Better for Wireless Charging: Here’s Why
The introduction of all glass phones has been one of the hottest trends in smartphone design since 2018. There’s no debating that phone designs keep evolving in beautiful ways. And glass delivers the beauty. It looks and feels great, and sells like hot cakes. Aesthetics, however, are only part of the value proposition of all-glass phones.
More than 100 smartphones from today’s top handset manufacturers support Qi, the dominant wireless charging standard, according to the Wireless Power Consortium. And wireless charging stations are showing up in cars, restaurants, hotels, airports, corporate offices and cars.
If you want to take advantage of wireless charging, glass beats metal backs. No need for replacement covers, cases and adapters: you already have the ultimate charging system and they are completely unnecessary.
How wireless charging works
Inside the typical charging pad there is a coil about an inch-and-a-half in diameter wound with 10 to 20 turns. There is a similar coil – its mate, if you will –in the back of the phone. The pad’s coil is driven with an alternating current of an Amp or so at a frequency of around 200 kHz. As is illustrated in the figure, this current generates a magnetic field that in turn induces a current in the phone’s coil. (For those in the technical know: Yes, just as in a transformer. In fact, this is a transformer.) The phone’s coil is connected to a rectifier that converts the alternating current to a direct current which recharges the phone’s battery.
Here, a metal phone back is shown, together with the eddy currents
that flow in it, lowering power transmission, and generating heat.
But why, exactly, does the case back material matter when you’re charging the phone?
The answer is simple: Metal hijacks charging power.
With a glass back, the magnetic field transfers power from charging pad coil to smartphone coil with negligible loss.
However, power loss occurs with metal backs. The metal itself consumes part of the power from the charging pad, turning it into heat. As a result, there are transmission losses, and charging time is increased. Moreover, there is a real risk that the heat could raise the temperature of the phone’s innards, not a good thing to happen, since electronics likes to run cool.
Zooming in to greater detail, this is what happens with metal backs. Part of the alternating magnetic field is consumed by generating circulating currents, known as “eddy currents” in the back. These eddy currents heat up the back in just the same way as electric current heats up the element in an electric heater. In a phone, the amount of heat generated depends on the metal of which it is made. The “conductivity” of the metal is a physical property that describes its propensity to turn electric current flowing through it into heat. The conductivity of aluminum is typically greater than that of stainless steel by a factor of 25. The conductivity of glass is almost exactly zero. (Remember that for conductivity in this context, low is good, so glass is perfect.)
Glass: a positive reception
This glass advantage is no problem for us at Corning because we love glass. Smartphone users love it, too. For now, and the foreseeable future, glass is the best material for wireless charging. It’s also shaping up to play a critical role in 5G wireless reception. Although metal backs work fine with today’s wireless signals, 5G will involve more channels and frequencies and will demand more performance of antennas. Metal, with its tendency to hijack signals, won’t help. Glass will.
So, whether you’re looking for form or function, glass wins.