Qi wireless charging has slightly lower efficiency compared to regular wired charging (around 72% compared to over 80%). Therefore, the phone may become warm during wireless charging. In general, the temperature will remain within the safe operating range of less than 113 Degrees Fahrenheit (45 Celsius) for high-quality wireless charging products, and it will not damage your phone or pose an injury risk. However, the efficiency and temperature may depend on many factors. as explained below:

First, make sure you use a wireless receiver made by a reputable company with a high-quality chipset (such as those made by Texas Instruments) and that features intelligent current control to avoid overheating. The MobilePal MP-QWIP6S for iPhone 6/6s and MobilePal MP-QWIP6SP for iPhone 6/6s Plus use Texas Instruments chipsets which prevent the temperature from exceeding 108 Degrees Fahrenheit (42 Celsius), which is well within the iPhone's safe operating temperature of up to 113 Degrees Fahrenheit (45 Celsius). In comparison, low-quality wireless receivers will have lower efficiency; Thus higher temperatures.

Second, make sure you use a high-quality wireless charging pad that has high-efficiency and built-in over-heating protection. One example is the MobilePal Gen-2 wireless charging power bank, which has an integrated high-quality wireless charging transmitter, a built-in 10000mAh lithium-ion polymer battery for on-the-go wireless charging, and integrated protection against overheating and over-charging.  When using a low-quality charging pad, the temperature may be higher.

Third, wireless charging is most efficient when you align the centers of the wireless transmitter and of the wireless receiver. If the centers are not aligned, the efficiency becomes lower and hence the temperature may be higher. Note that for some smartphones (such as Google Nexus 6) the wireless receiver is not centered, so it might take some trial-and-error for those phones to achieve best wireless charging efficiency.

In summary, as long as you use a high-quality wireless charging pad with a high-quality wireless charging receiver and use them properly, you do not need to worry about the slightly higher temperature damaging your phone.