VIA HOLES AND FPC

Benefits And Drawbacks Of Using Vias
      There are several benefits and drawbacks to using vias on flexible circuit boards. One of the additional advantages is that vias can be used to control the temperature within the circuit board. They allow heat from components to pass through the via holes to a heat-dissipating plane layer located below.
      A drawback is that a via placed in a bendable area can be prone to cracking and should never be placed near the interface of a rigid area. The structural stability of the circuit board can be compromised when too many vias are added. Below we will dive into more about the types of vias available, the design considerations, and the pros and cons of using them for printed circuit boards.

Types Of Vias

      Through-hole/Thru-hole: The term via and through-hole are sometimes used interchangeably. However, a through-hole is a via that goes all the way through the layers of the printed circuit board to the other side.
      Buried Via: A buried via is an internal via. It does not meet to an external layer of the surface of the board and instead connects internal layers. Example: via that connects only layers 2 and 3 in a 4-layer design.
      Blind Via: A blind via only goes partway through the board as it starts at the external layer. Example: via connects only Layers 2 and 3 in a 4-layer design.
      Microvias: A microvia is a via with a diameter of less than 0.006 inches. Unlike the other vias that are made with a mechanical drill, microvias are drilled using a laser.

Design Considerations For Vias In Flexible Areas

      Flexible circuit boards can go through large amounts of mechanical stress due to their bending and flexing. Traces and via placed in these bendable sections can experience damage over time that cracks the electrical signal traces along the circuit board. Here are several design considerations to be aware of when selecting a flexible printed circuit board.

Location
      Location is important so that the vias are not subjected to damage. Vias should not be placed in a bend area if possible. They should be within areas supported by mechanical stiffeners. A via also cannot be located on the edge of a stiffener as it may crack the via plating during the stiffener lamination process. Place vias at least 20 mils from any stiffener edge and 50 mils from rigid to flex interfaces.

Pad Size
      Pad plating is designed to meet the mechanical bend requirements for flexible circuits. This method limits the thickness of the finished copper which improves flexibility, mechanical bend reliability, and improves controlled impedance signal performance. Typical design requirements for pad plated via pads are that the pad size should be a minimum of 0.010” larger in diameter in relation to the size of the via holes and through-holes.

Vias in Flexible Areas
      If via in a flexed area are necessary, a minimum bend requirement review is required to ensure the reliability of the finished parts. The presence of vias in a flexed area will limit the minimum bend radius capability. Providing additional information such as bend radius, location of bends, and application use can ensure PCB reliability.

Additional Manufacturing Considerations
      While you may have set specifications that will need to be made for the flexible PCB to work with the application, the manufacturer that you partner with has design standards for you to be aware of based on their capabilities and the manufacturing equipment that is used.

Some of these design standards are the following:
      Vias in flex areas are not recommended by IPC 2223 design standard. IPC 2223 covers the standards for flexible PCBs, various component mountings, interconnection systems, and electrical constrain instructions.
      Vias in flexible areas can create mechanical stress concentrators that when the flex circuit is bent can lead to potential breakage. This may crack the plating in via(s). If necessary, locate away from the bend area in flex.
      In a rigid-flex PCB design, flex area vias add cost due to additional drilling and plating processes (similar to a buried via configuration in a rigid PCB) required. These costs need to be factored in and if possible, it is recommended that they be avoided if the design allows.
Shielded designs, with Faraday cage requirements, require vias in the flex area(s) which cannot be avoided without compromising the shield effectiveness.

Summary
      When designing and manufacturing flexible printed circuit boards, you want to ensure high electrical signal reliability no matter the number of laminated layers. Vias are the only solution to connect between layers and achieve the circuit routing requirements. However, there are technical items to be aware of to avoid damaging the flex circuit and or increasing your material and production costs.
Here at Epec, we manufacture flexible and rigid-flex printed circuit boards for a very wide variety of applications. Please feel free contact us to discuss the details of your specific design.