A brief Analysis on a PCB Copper Trace Capacity to Carry Current

It is extremely vital to study the importance of copper traces in PCB. This is because these traces are the connecting tracks between the two components on a PCB. In high current application PCBs have high width copper traces. Hence it is very important for PCB layout design engineer to know what trace width should be selected for a particular power circuit or device. This would increase the current carrying capacity of the PCB board.

 

Mathematically speaking, the capability of trace to carry current is directly proportional to the cross sectional area of trace and temperature rise. As can be seen from the table below, that under the temperature conditions of 20OC, a trace width of 10mil with 1. oz of copper can withstand 1. 2A current. So if talking linearly and simple multiplication, the current carrying capacity of 50mil trace at 20OC and 1.0 oz copper will be 6.0A, which is not actually the case. According to MIL-STD-275 specifications a 50mil trace can withstand only 3.6A

 

PCB Copper Trace Capacity to Carry Current

 

Anyhow, the table shown above has been replaced by the IPC-2221 standard. This generic standard is a reference for PCB designers to design accurately.

 

Copper Thickness Measurement Unit

 

In the terminology of PCB design, the thickness term given to copper is measured in ounce (oz). Although this unit is for the weight, but in circuit board design this is the measurement unit of copper thickness.

 

The copper thickness in terms of ounce is measure in such a way that 1 ounce means 1 square feet of area covered with 1 ounce of copper. It means that the more the weight of copper the thicker the copper surface will become. So thickness is directly proportional to weight of copper. Hence the unit of copper thickness is ounce.

 

Unit Conversion:

 

1.0oz = 0.0014inch = 1.4mils = 0.035mm

 

Two Parameters Defining the maximum limit of Copper Trace current passing Capacity

 

The generic standard of IPC-2221 in section 6.2 shows the two types of conductors. Internal and External. According to it, the limit of external conductor to carry current is double as that of internal conductor. In this standard, table # 6.4 demonstrates the relationship among the copper sheet cross section area, temperature condition and current limit of both internal and external conductors. Above all this a simplified formula for determining the above relation is give below

 

Parameters Defining the maximum limit of Copper Trace current passing Capacity

 

Here

I = Current

K = Constant = 0.048 for outer layers and K = 0.024 for inner layers

= Maximum Temperature Change

A = Cross Section Area of copper unit (mil2)

Hence it is concluded that the two significant factors that govern the limit of current a conductor can carry are 1- Cross Sectional Area and 2- Temperature Change

There are also online tools that just according to the IPC 2221 standard, gives the accurate trace width calculation both for the internal and external conductors. These tools are convenient for the design engineers in designing PCB layout.

Diagrammatic Representation of Copper Trace:

 

Diagrammatic Representation of Copper Trace

 

IPC-2221 Section 6.2

 

Conductor width to cross-section relationship

 

External Conductors PCB design

 

Internal Conductors PCB design

 

Other Factors to Affect:

 

There are other factors also that contribute to the determination of current limit that a conductor can carry. In practical situations, when the PCB is fabricated, apart from the temperature and cross sectional area, some other factors like PCB pads, via and components also affect.

 

There are PCBs that have traces that are connected to many vias and pads. In this situation a strong trace with proper tinning will perform the current carrying capacity with good efficiency. While on the other hand, an ordinary trace cannot bear the current and can damage or burn out. This is because a large current flow from that trace or a spike current can burn the track. The reason why this happens is that the solder paste is applied too much on the components and pads resulting in increased cross section area while the trace width is still the same and hence as soon as power is applied to the board, a high spike or surge of current flows through the trace between pads, and damages it.

 

Dust and Contamination Factors:

 

The dust particles and contaminants can reduce the current carrying capacity and partial trace damage, hence a careful consideration has to be taken. The protection circuit can be implemented to avoid overloading / overcurrent issues.

 

Curved Traces:

 

The above discussion done in light of IPC 2221 standard was for the straight traces. For the curved traces with acute angles, it can affect little towards the wider traces high current flow. But for the low current flow the curved traces can be a problem.

 

PCB Base Material Another Factor:

 

The PCB board material is also another factor that can affect the internal and external conductor current carrying capacity. According to IPC-2221A standard, the testing was performed using 5 different materials as shown in the graph below. The electrical current is applied to the same width/height of trace in each PCB material type. The only difference was board material thermal properties. The material XXXP is the phenolic used as PCB base material.  This shows how different materials can increase the current flow ability of copper trace. This issue of thermal effect can be overcome using thermal management by means of conductor sizing.

 

Different PCB materials can increase the current flow ability of copper trace