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A PCB designer’s balancing act—selecting the right board thickness

Choosing wisely is critical for PCB quality and performance, but it can be tricky depending on size constraints, functional requirements, and environmental factors.

While we sometimes have a general idea about assembly requirements or how the board will be used, there can still be a lot of unanswered questions as we begin the manufacturing process. After all, there’s a big difference between “This PCB will go in a drone.” and “This PCB will be part of a submersible drone the size of a tennis ball, need to withstand intense heat or cold, and function forty fathoms below the surface.”

When we receive a design, there’s a lot we can evaluate before production to ensure its manufacturability and functionality. We are, however, limited in our ability to judge whether a board, as designed, is too thick or too thin.

In general, you might think thicker is better, because then the board is less brittle and won’t break as easily. But thicker is also heavier, hotter, and not appropriate for many applications or assemblies. As you design your PCB, you should ask yourself:

  • How thick does it need to be to work?
  • How thin must it be to fit?
  • What will it have to do?

To avoid producing boards that don’t fit into the assembly, or fail to perform reliably, choose your thickness carefully. Here are some tips and guidelines for ensuring proper fit and function.

Test your design assumptions

Keep in mind, PCB design is often as much an art form as it is a science. It’s not as straightforward as plugging a bunch of numbers into an equation and receiving an answer.

So, before you lean towards a thicker or thinner board design, or accept your design tool’s default thickness, consider as many relevant factors as possible.

  • Do you need to prioritize thermal transfer or insulation?
  • Are there weight restrictions?
  • How flexible or stiff does the board need to be?
  • Will it be well protected inside the assembly?
  • How much space do you have to work with?
  • Are component lengths long enough for thicker boards?
  • Is drill aspect ratio acceptable? 8:1 is a good rule of thumb.
  • Can your design be manufactured? Does it fall within manufacturer tolerances for very thick or thin boards?

Guidelines for choosing PCB thickness

Determining the answers to those questions will go a long way towards establishing peace of mind during the transition from design to prototyping. At a high level, thicker boards make sense when:

  • There is a high layer count.
  • The board needs to be stiff because of its size or lack of protection in the assembly.
  • Durability is a priority.

Thinner boards are preferable or necessary when:

  • The board must be light.
  • In rare instances, flexibility is required to facilitate installation, or solve interconnection issues. In general, you don’t want to bend your boards.
  • The design calls for tiny vias. If the board is too thick when drilling through holes, drill bits will break. Consider keeping aspect ratios below 10:1, preferably 8:1.
  • The assembly is smaller with support for a board sandwiched into a tight space.
  • The device runs hot. A thinner board can help dissipate heat, because thermal vias are shorter and pass through a smaller volume of insulating material.

We recognize cost can influence decision-making. Your fabricator will have a standard board thickness, probably .062”. Optional thicknesses may be more expensive, but savings evaporate quickly if the PCBs aren’t durable enough for their intended use. Thinner boards can be damaged and may warp during solder processes.

Checklist for determining thickness

As you consider thickness in your design, we encourage an easily replicated, checklist approach. Here are some factors to consider as you design your board:

How big is your device? Thinner PCBs take up less space, so if you’re building a device that will fit in your hand, go as thin as necessary to accommodate the space.

It ain’t heavy, it’s my board. PCB weight can be an issue more often than you might think. Is your board part of a cell phone, Bluetooth headset, or tablet device? Thinner boards are lighter and can help give your device that trim figure we all appreciate.

Know your connections. PCB edge connectors or PCB trace-based connectors, for example, require a board with thickness matched to fit in the mating portion of the connector. Some through hole parts have pin length that limits board thickness.

What’s your frequency? Layer-to-layer thickness will affect signal integrity, crosstalk, impedance calculation, and signal loss. For high frequency signals, impedance control is a factor in determining the right thickness of your board.

Be aware of the side effects of board flexibility. Will your product be subjected to excessive shock and vibration? Thin boards that flex can cause broken connections or components, so be sure it’s properly supported in your assembly.

You can usually just choose the default…

When it matters, the rest of your design can be absolutely perfect, but if it’s the wrong thickness it will fail, electrically or mechanically. Before you send your design to be manufactured, take a few moments to be sure your board is the right thickness for your needs.

 

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