You changed case and now calls drop in the kitchen, or wireless charging has stopped working, or your card reader will not tap. Most phone cases are electrically irrelevant. A specific few are not, and the ones that cause problems share obvious characteristics once you know what to look for.
The short answer
Plastic, silicone and leather cases have essentially no effect on mobile or Wi-Fi signal. What does affect things: metal plates fitted for magnetic car mounts, cases with substantial metal in the body, wallet cases holding cards, and any case thick enough to block wireless charging. If something stopped working when you changed case, take the case off and test rather than assuming.
What is affected and what is not
- Mobile signal. Barely affected by ordinary cases. A metal plate over the antenna region can reduce it measurably, particularly in areas where reception was already marginal.
- Wi-Fi and Bluetooth. Same picture. Ordinary cases are fine; metal is not.
- Wireless charging. Very sensitive. Anything over about three millimetres, and absolutely anything metallic, will reduce or block it.
- Contactless payment. The most sensitive of all. Cards in a wallet case are the classic cause of a phone that will not tap.
- GPS. Rarely affected by cases, though a metal plate can reduce the satellite signal.
- The compass. Strongly affected by magnets. A magnetic mount plate is a common cause of a map that points the wrong way.
Take the case off and test the same thing
Same room, same task, same time. This is the whole test and it takes a minute. If the problem disappears, you have your answer.
Check for a metal plate
Magnetic car mounts work by sticking a thin steel disc to the phone or inside the case. That disc sits directly over the wireless charging coil and often near the antennas. It is the most common culprit.
Empty a wallet case
Cards between the phone and a charger or a card reader block both. Bank cards can also be damaged by a wireless charging field, so this is worth fixing for more than one reason.
Check the case thickness for wireless charging
Over about three millimetres and charging becomes unreliable. Measure at the centre of the back, which is where the coils align.
Check the signal bars in the same spot with and without
Better still, use the field test or network settings screen that shows actual signal strength in dBm, which is far more informative than bars.
Test the compass
Open a maps app and see whether the direction indicator is correct and stable. A magnetic plate or a magnetic closure will send it wandering.
Check the case is not covering a microphone or speaker
A misaligned cutout muffles calls and audio, which people often mistake for a signal problem.
Check the buttons and ports
A tight case pressing a volume or power button causes odd behaviour that looks like a software fault.
Signal strength in bars is almost useless for comparison, because the mapping between bars and actual signal is arbitrary and differs between phones. Most phones can display the real figure in dBm somewhere in the network settings or a field test screen. Closer to zero is stronger; anything worse than about -110 dBm is very poor.
The magnetic mount problem
Magnetic car mounts are convenient and they cause three separate problems at once. The steel plate blocks wireless charging entirely. The magnets disturb the compass, which makes navigation point the wrong way. And the plate can reduce antenna performance depending on where it sits.
If you want a magnetic mount and wireless charging, phones with built-in magnetic alignment solve this properly, because the magnets are designed into the phone around the charging coil rather than being a plate stuck over it. Otherwise a clamp-style mount avoids all three problems.
Never put a bank card, hotel key card or travel pass between your phone and a wireless charger, including inside a wallet case. The field can wipe a magnetic stripe and heat an embedded chip. Some chargers detect this and refuse to charge, which is the system protecting you rather than a fault.
Where cases matter most
In a strong signal area, a case that costs you some antenna performance makes no practical difference; you have plenty of margin. In a weak signal area, the same case can be the difference between a call connecting and not.
That is why this problem appears inconsistent. People conclude the case cannot be responsible because it works fine most places, when in fact it is responsible in exactly the places where the margin was already thin.
Do phone cases block signal?
Ordinary plastic, silicone and leather cases have essentially no effect. Cases with metal plates or substantial metal content can reduce signal measurably, most noticeably in areas where reception was already weak.
Why did wireless charging stop after I changed case?
Either the case is too thick, over about three millimetres, or it contains a metal plate for a magnetic mount. Both block the field.
Why will my phone not tap to pay in its case?
A card in a wallet case, or a metal plate, sitting between the phone and the reader. Remove them and try again.
Can a case affect GPS or the compass?
GPS rarely. The compass frequently, because magnets in mounts and closures disturb it directly. A map pointing the wrong way is usually this.
How do I measure signal properly?
Bars are arbitrary. Find the dBm figure in your phone’s network settings or field test screen and compare that with and without the case in the same spot.
The bottom line
Take the case off and repeat the failing task, because that settles it in a minute. Look specifically for a metal plate, cards in a wallet case, or a case over about three millimetres thick, since those three cause nearly all genuine problems. And compare actual signal figures rather than bars, which tell you almost nothing.
