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 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 not be placed too close to the interface of a rigid area. The structural stability of the circuit board can also be compromised when too many vias are added. Below we will look further into the types of vias available, design considerations, and the pros and cons of using them for printed circuit boards.

Types Of Vias

Through-hole/Thru-hole: The terms via and through-hole are sometimes used interchangeably. However, a through-hole via 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 connect to an external surface layer of the board and instead connects internal layers. Example: a via that connects only Layers 2 and 3 in a 4-layer design.

Blind Via: A blind via only goes partway through the board, starting at an external layer and connecting to one or more internal layers. Example: a via that connects Layer 1 to Layer 2 in a 4-layer design.

Microvias: A microvia is a via with a diameter of less than approximately 0.006 inches. Unlike conventional mechanically drilled vias, microvias are typically drilled using a laser.

Design Considerations For Vias In Flexible Areas

Flexible circuit boards can experience significant mechanical stress due to bending and flexing. Traces and vias placed in these bendable sections can experience damage over time, potentially cracking electrical signal traces along the circuit board. Here are several design considerations to be aware of when designing a flexible printed circuit board.

Location

Location is important so that vias are not subjected to unnecessary mechanical stress. Vias should not be placed in a bend area if possible. They should preferably be located within areas supported by mechanical stiffeners.

A via also should not be located directly on the edge of a stiffener, as this may increase the risk of cracking the via plating during the stiffener lamination process. A commonly used design guideline is to place vias at least 20 mils from a stiffener edge and 50 mils from rigid-to-flex interfaces. Final clearance requirements should be confirmed for the specific PCB construction.

Pad Size

Pad plating is designed to meet the mechanical bend requirements of flexible circuits. Limiting finished copper thickness can improve flexibility, mechanical bend reliability, and controlled impedance performance.

A common design guideline for pad-plated vias is that the pad diameter should be at least 0.010 inches larger than the via or through-hole diameter. Actual pad requirements should be determined according to the specific PCB design and fabrication requirements.

Vias in Flexible Areas

If vias in a flexed area are necessary, the minimum bend requirements should be reviewed to help ensure the reliability of the finished PCB. The presence of vias in a flexed area can limit the minimum bend radius capability.

Providing additional information such as bend radius, location of bends, and application requirements can help ensure PCB reliability.

Additional Manufacturing Considerations

While a flexible PCB may have specific design requirements for its intended application, the PCB manufacturer will also have design and fabrication requirements based on the materials, processes, and manufacturing methods being used.

Some important design considerations include the following:

Vias in flex areas are generally not recommended in IPC-2223 design guidance. IPC-2223 addresses design requirements for flexible and rigid-flex printed boards, including component mounting, interconnection systems, and electrical and mechanical design considerations.

Vias in flexible areas can create mechanical stress concentrations. When the flex circuit is bent, this additional stress can potentially lead to cracking of the via plating. If vias are necessary, they should be located away from the primary bend area whenever possible.

In a rigid-flex PCB design, vias in flex areas can add manufacturing complexity and cost because additional drilling and plating processes may be required. These factors should be considered, and flex-area vias should be avoided when the design allows.

Shielded designs with Faraday cage requirements may require vias in flex areas to maintain shielding effectiveness.

For current fabrication parameters and design limits, refer to Gold Phoenix PCB Flex and Flex-Rigid Capacity.

Summary

When designing and manufacturing flexible printed circuit boards, maintaining reliable electrical connections is important regardless of the number of laminated layers. Vias provide electrical connections between layers and help achieve circuit routing requirements.

However, via placement, bend location, pad design, bend radius, and manufacturing requirements must all be considered to avoid damaging the flex circuit or unnecessarily increasing manufacturing complexity and cost.

For flexible and rigid-flex PCB designs intended for fabrication by Gold Phoenix PCB, designers should review the current Flex and Flex-Rigid Capacity requirements when finalizing the design.