Flexible PCB High Current: Everything You Should Know About Current and Voltage Handling Capability

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Oakley Mae

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Electronic devices seem to be evolving by the day into more and more complex products, necessitating flexible PCB high currents, and designing these boards entails a lot of precision and care.

Remember, too, that it’s a system of components, each of which demands design considerations.

Read on to learn about applications and design considerations for flexible PCBs for high currents. 

Table of Contents

Flexible PCB for High Current Application

The following are some examples of applications that require high current loads:

  • Welding
  • Power converters
  • Automotive
  • Aviation
  • Power supplies
  • Heavy machinery
  • Solar arrays

Flexible PCB High Current Design Considerations

Generally, high current impacts the flexibility of a PCB. The following guidelines are for designing flexible PCBs for high-current applications to mitigate the negative impact.  

Materials

Generally, the standard copper thickness available from suppliers is 2oz.

But often, you might find that your design requires the copper to be thicker when it comes to materials.

When that happens, you have three options.

  • Special-order: Usually, it costs much more and entails a longer waiting time when you make a special order. But the good thing is that special-order materials have an adhesive-based flex core consisting of epoxy or acrylic, which ensures higher copper peel strengths and reduced flex thickness.  
  • Surface plating: Granted, adding surface plating offers a feasible way of increasing the thickness to 3oz. However, the added plate often comprises electro-deposited  (ED) copper, which lowers bend capability despite its good conductivity. 
  • Manufacture it in-house: You’ll purchase the copper thickness you need and then laminate each copper layer to a polyamide layer using adhesive. While the adhesive layer(s) means an increased core thickness, the design will have no additional surface plating and, thus, no reduced bend capability.

High and Low Current Signals

Some designs combine high currents with standard-level signal-carrying capability.

For such designs, some special design elements are necessary.

For example, the wider the line width, the thicker the copper.

So because generally, wider wires can carry more current, the design must consist of wider wire and thicker copper. 

Further, a flexible PCB for high current must have a thicker core for controlled impedance, which requires a high current application. 

Coverlays

Generally, for every 1 oz of copper thickness, you need at least 0.001 inches of coverlay adhesive.

That way, you ensure the adhesive is sufficient for void-free lamination.

But the problem is the adhesive adds some thickness to the design, which by extension affects bend capability.

That said, during lamination, the adhesive’s thickness reduces depending on the ratio of the width of the signal line to the spacing of a given area. 

Therefore, the larger the spacings a circuit has, the higher the thickness reduction.

On the other hand, circuits with thick traces and or copper fill areas, the less thickness reduction.

Thickness

It’s important to understand the thickness suitable and the number of components your design will hold.

See, although boards come in varying thicknesses, depending on the number of layers, there’s a specific thickness that each layer must maintain. 

For example, a thickness of 14 mils works well with many projects. This is the standard thickness for a 2-layer board. The thickness ranges between 14 mils and 32 mils. And 32 mils is ideal for a project requiring a 4-layer board.

Factors That Influence PCB Thickness

  • Copper thickness: The thicker the copper, the thicker the board, and the more current can pass through the PCB.
  • Weight and flexibility: Thicker boards are less flexible and weigh more. And the final application is what will determine a PCB’s size, weight, and flexibility.
  • Board materials: Without a doubt, the choice of board material will determine the board’s longevity and functionality. And out of all the materials a board comprises, the laminate and substrate impact the heaviest on a board’s overall thickness.
The substrate polyamide

(Caption: The substrate polyamide) 

  • Signal: Often, the signal type determines the materials you’ll select for the board, which by extension, will affect PCB thickness. Because as mentioned earlier, a board that a high current will pass through should be thicker.
  • On the other hand, a board intended for a low current environment will be thinner.
  • Vias: The type and density of vias will determine how thick the board must be.
  • For example, manufacturers build micro vias for high-density connections, so you’ll use them on a thinner board. 
  • PCB layers: The more layers a PCB has, the thicker the board needs to be. However, if you don’t intend to use your design.
flex PCB layers

(Caption: flex PCB layers)

  • Operational conditions: Generally, different board thicknesses work best in different environments since it impacts the PCB’s conductivity and resistance. 

Strategies to Reduce Thickness and Improve Flexibility

Generally, PCBs carrying high currents require more copper layers.

But as mentioned earlier, copper thickness has a negative influence on flexibility.

Fortunately, there are design strategies you can employ to help mitigate the impact. These are:

1. Increase Flex Area Width

Increasing the flex area allows the use of wider circuits.

And when you combine this with thinner copper, it enables the board to carry current safely. As a result, you’ll notice the following benefits:

  • Reduced overlay adhesive thickness
  • More flexibility
  • Less flex thickness 

2. Split High Current Lines

You can split the high current lines on a 2 layer circuit to make them run side by side on the layers with thinner copper.

However, note that such splitting is only possible if the board has some space on the secondary layer. 

3. Separate the High and Low Current Needs

Another strategy you can employ is to separate the high and low current flex layer needs.

That means rather than combine both needs, separate the two so that they’re independent flex layer pairs using the air gap construction technique.

That way, each pair can utilize the materials and construction appropriate for its specific requirements. 

FAQs

What Is Heavy Copper PCB Current?

This is a high-current PCB board with outer and inner layers of 3 oz copper or more. However, such a board may sometimes have less than 3 oz of copper but a copper thickness exceeding 4 oz per ft2.   

Which Conductor Is Used in Flexible PCBs?

Often, copper is used as the conductor in flexible PCBs.

That’s because it has high conductivity and heat dissipation. Also, compared with other conductor materials, it costs less and is readily available.

What Are the Characteristics of Flexible PCBs?

Often, flexible PCBs consist of the substrate polyamide, which has excellent thermal resistance, meaning you can use the board in extreme temperatures without risking damaging it.

Also, flex PCBs are more reliable, offer more consistent performance, and last longer. 

Further, they’re lightweight and compact. 

What Are the Disadvantages of Flexible PCBs?

First, compared to standard PCBs, these boards entail a higher initial cost, mainly due to poor utilization of panels.

Secondly, there’s a high chance of damage during handling primarily because often the user has more experience working with rigid boards.

Lastly, the assembly process is complex, and in case of any faults or errors, reworking or repairing is challenging.

Conclusion

There you have it. Now you know some flexible PCB high current applications. Also, design considerations to keep in mind when designing these boards.

As we’ve learned, PCBs carrying high currents need to be thicker, and we’ve also outlined factors that influence the thickness and some strategies to reduce thickness and increase flexibility.