A coil is a passive electronic component. It is also called an Inductor. The SMD (Surface Mount Device) Coils are used in Mobile phones Circuit Boards.
SMD coils are soldered directly on the circuit board.
It looks black in color and has a metal pad on both sides. The shape of the SMD coil is cuboid.
Inside the coil, there is an insulated copper wire wrapped around in the shape of a pressed spring.
Use of Coil in Mobile Phones
Coils are used in the Backlight section, Buck line Power Section, and Network Section in Mobile Circuit Board.
In the Backlight section, the backlight coil helps in boosting voltage from Battery voltage (4.2V) up to 30V.
If there is no Backlight in Mobile phones, you can check Backlight Coil first. Because most of cases, the backlight coil is found to be faulty.
A Mobile Phone requires different types of voltages i.e., CPU core voltages, VPH voltage, Ram voltage, Modem voltage, etc. For this purpose, a dedicated Power IC is mounted on the circuit board and there are 5 to 10 SMD coils soldered around the power IC. Different SMD coils help in generating different bulk voltages ranging from 0.5V to 3.3V.
In the network section, Coils are used in electronic filters to separate different network signals of different bands (High Band, Low Band & Medium Band)
SMD coils used in Backlight & Buck lines sections are bigger in size than that of the Network section.
A coil passes DC & blocks AC
Testing of Coil or Inductors
A coil or inductor is measured by its electrical inductance. The unit of Electrical Inductance is Henry.
1Henry (1H) = 1000000 microhenry (μH)
The coils used in Mobile Phone Circuit Board are ranging from 1μH to 15μH.
The electrical inductance of Buck Coil is between 2μH to 5μH.
But the electrical inductance of Boost Coil is between 10μH to 15μH.
You can use a normal Multimeter to test a Coil. Put the multimeter in continuity mode or Diode Mode. Connect the probes to either ends or terminals (metal pads) of the Coil
If you measure a good Coil in the Continuity Mode or the Diode Mode of a Multimeter, it will beep and show 0Ω to 3Ω in Continuity Mode and 0V in Diode Mode.
While testing with Multimeter, if there is no value shown in Multimeter, then the coil must be a bad one. Because it is open internally as the wire is disconnected or burnt.
But if the coil is shorted internally, it will also beep & show 0 Ω to 3 Ω in Continuity Mode while testing in Multimeter.
Hence Common Multimeter is not ideal for measuring a Coil or Inductor.
LC Meter or LCR Meter is actually used to measure or test the Coil.
You can use LCR Meter to measure resistance, capacitance & inductance.
In LCR meter, it will show the actual value of the Coil
You can check the onboard-mounted coil without de-soldering.
First, put the knob in the 200µH range. Just connect the 2 probes of the LCR Meter with both ends of the coil, and you will see the value of the coil.
I have illustrated how to check a backlight coil in Mobile Phone Circuit Board below. Please watch the video and learn.
Backlight Coil testing in LCR Meter
When you want to purchase an LC Meter or LCR Meter, please make sure it can measure a minimum value of 1µH Coil.