For example, in a tin plated PCB circuit board with a line width of 10 mil, a thickness of 0.035mm, a line length of 10cm, and a maximum allowable voltage drop of 0.1V, what is the maximum current that can pass through?

The calculation formula used is as follows (this calculation formula does not consider heat dissipation and other situations, and is applicable to situations where the circuit is not burned out and the temperature of the circuit board is relatively stable):

Ohm's Law: Current=Voltage/Resistance, i.e. I=U/R,

Resistance formula: R=P * L/S, which means resistance=impedance of the conductor P * length of the conductor L/cross-sectional area of the conductor S.

The impedance P is approximately a fixed constant, but there may be slight variations at different temperatures. Generally, we use components in environments around 20 ℃, so we take the impedance P20 of the conductor at 20 ℃. Due to the fact that the conductor of FFC cable is electrolytic copper with a copper content of 99.99%, which is approximately pure copper, the impedance rate of pure copper at 20 ℃ is P20=0.017241 Ω mm ²/m. Friends in the FFC ribbon coil know that the conductor of conventional FFC ribbon wire is plated with a layer of tin on pure copper, and the thickness of the tin plating layer is generally greater than 1 μ. Based on industry experience and our company's years of practice, the impedance rate of this tinned flat copper wire at 20 ℃ is P20=0.0208 Ω mm ²/m. Now let's go back to the original resistance formula: R=P20 * L/S, the impedance ratio P20 is a constant, and L is taken as a unit length of 1M. Therefore, the magnitude of the resistance R depends only on the cross-sectional area S of the conductor.

The size specifications of the copper wire used for FFC flexible ribbon cables with different spacing are different. Below, we will explain the most commonly used 0.5mm spacing and 1.0mm spacing for FFC flat ribbon cables. The standard copper wire specification for FFC ribbon cables with a spacing of 0.5mm is thickness * width=0.035mm * 0.3mm, while the standard copper wire specification for FFC ribbon cables with a spacing of 1.0mm is thickness * width=0.035mm * 0.65mm. First, calculate the resistance R=0.0208 * 1/(0.035 * 0.3)=1.98 Ω≈ 2.0 Ω of the FFC ribbon cable with a spacing of 0.5mm. Then, take the unit voltage of 1V and obtain the current I=U/R=1/2=0.5A. Calculate the resistance of the 1.0mm spacing FFC ribbon cable R=0.0208 * 1/(0.035 * 0.65)=0.914 Ω≈ 1.0 Ω, then the voltage is also taken as the unit voltage 1V, resulting in a current I=U/R=1/1=1.0A. According to the above calculation, the rated current that a single conductor of FFC flexible ribbon cable (tin plated) with a conventional spacing of 0.5mm and 1.0mm can withstand is 0.5A and 1.0A.

Since we have calculated the current passing through a single conductor of a tinned copper wire FFC cable, let me also calculate the current that a gold-plated FFC cable can pass through. Since the FFC cable is gold-plated, that is, the gold fingers on both ends are gold-plated, the proportion of gold in the copper wire can be ignored and can be considered as pure copper wire. Therefore, its impedance ratio P20=0.017241 Ω mm ²/m. Taking the 0.5mm pitch gold-plated FFC ribbon cable as an example, the resistance of the 0.5mm pitch gold-plated FFC flexible ribbon cable is R=0.017241 * 1/(0.035 * 0.3)=1.642 Ω≈ 1.7 Ω. Therefore, the voltage is also taken as a unit voltage of 1V, and the current I=U/R=1/1.7=0.6A. Therefore, it is easy to conclude that gold-plated FFC wires with the same spacing can pass a larger current than a single conductor of tinned FFC wires. That is to say, after being gold-plated, the impedance value of the FFC wire decreases, the current passing through it increases, and it is more resistant to oxidation. This is also why many customers prefer to use gold-plated FFC ribbon cables in some special situations.

The impedance of pure copper wire is P20=0.017241 Ω mm ²/m.

The impedance of tinned copper wire is P20=0.0208 Ω mm ²/m.

The impedance of gold-plated copper wire is P20=0.017241 Ω mm ²/m.

Tin plated FFC ribbon cable with a single copper wire passing through the current (allowing a voltage drop of 1V/m, P20=0.0208 Ω mm ²/m)
Copper wire specification/mm	0.035*0.3	0.035*0.32	0.035*0.65	0.05*0.65	0.1*0.65	0.1*0.8	0.1*1.27
Through current/A	0.50481	0.53846	1.09375	1.5625	3.125	3.84615	6.10577

 

Gold plated FFC ribbon cable with a single copper wire passing through current (P20=0.017241 Ω mm ²/m when a voltage drop of 1V/m is allowed)
Copper wire specification/mm	0.035*0.3	0.035*0.32	0.035*0.65	0.05*0.65	0.1*0.65	0.1*0.8	0.1*1.27
Through current/A	0.60901	0.64961	1.31953	1.88504	3.77008	4.6401	7.36616

The above is the calculation result of the current passing through a single copper wire of FFC ribbon according to the above formula, where the length L=1m and the allowable voltage drop is 1V. In practical applications, the calculation can be based on the actual length.

For example: PCB pure copper wire [P20=0.017241 Ω mm ²/m], cable length L=10cm=0.1m, line width 10mil, allowable voltage drop 0.25V, maximum current that a single wire can pass through? The result is 1.28908 A.

Resistance R (m Ω, Ω)	193.93701 mΩ, 0.19394 Ω
Allowable voltage drop U (mV, V)	250 mV , 0.25 V
Maximum current I (mA, A)	1289.07836 mA ,1.28908A
Cross sectional area S (mm ^ 2, m ^ 2)	0.00889 mm^2 , 8.89e-9 m^2
Line width W (mm, mil)	0.254 mm , 10 mil
Line length L (mm, m)	100 mm , 0.1 m

Reprinted from: Electrician Home Network