Rigid-Flex PCB: DFM and Design Rule Considerations

15 January 2023
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Rigid-Flex Printed Circuit Boards (PCBs) are becoming increasingly popular in various industries, from consumer electronics to aerospace and defense. While flex PCBs have been around for many years, the addition of rigid components to the mix opens up a whole new world of design possibilities.

However, designing and manufacturing rigid-flex PCBs is not without its challenges. In this blog post, we'll take a look at some of the key design and manufacturing considerations for rigid-flex PCBs, including:

Designing for manufacturability (DFM)
Ensuring compliance with design rules
Managing thermal issues

We'll also provide tips on overcoming some of the challenges associated with rigid-flex PCB design and manufacturing.

Rigid-Flex PCB Designing for Manufacturability (DFM)

One of the most important considerations when designing rigid-flex PCBs is manufacturability. Due to the unique construction of rigid-flex PCBs, several challenges must be considered during the design process.

Some of the key challenges associated with rigid-flex PCB manufacturability include:

Ensuring correct registration between the Rigid and Flex layers
Avoiding voids in the solder joints
Minimizing warpage and bow

Let's take a closer look at each of these challenges.

Ensuring Correct Registration between the Rigid and Flex Layers

One of the most important aspects of rigid-flex PCB design is ensuring correct registration between the rigid and flex layers. This can be a challenge, as the two layers need to be aligned perfectly to avoid problems during assembly.

The most popular way to ensure correct registration between the rigid and flex layers is to use alignment holes. Alignment holes are small openings in the PCB that allow for alignment pins to be inserted during assembly. This ensures that the rigid and flex layers are correctly aligned before soldering.

Another way to ensure correct registration is to use fiducials. Fiducials are small, round features typically placed in the corners of the PCB. They can be used for alignment during assembly, but they can also be used for calibration during automated optical inspection (AOI).

Avoiding Voids in the Solder Joints

Another challenge associated with rigid-flex PCB design is avoiding voids in the solder joints. This can be a problem when using lead-free solders, as they tend to have a lower melting temperature and are more prone to forming voids.

There are a few different ways to avoid voids in the solder joints. The most common method is to use staggered vias. Staggered vias are small holes that are offset from one another. This helps distribute the heat evenly, preventing voids from forming in the solder joints.

In addition to this, copper pillars can also be used to avoid voids in the solder joint. Copper pillars are small posts of copper placed around the PCB's perimeter. They help conduct heat away from the solder joint, preventing voids from forming.

Minimizing Warpage and Bow

Another challenge when designing rigid-flex PCBs is minimizing warpage and bow. This can be an issue with larger boards, as they are more likely to warp during assembly.

There are more than one approaches that minimize warpage and bow. However, the most commonly used are stiffeners. Stiffeners are small pieces of material placed on the PCB's underside. They help to support the board and prevent it from warping during assembly.

One more way to minimize warpage and bow is to use copper fill. Copper fill is a technique that involves filling the unused areas of the PCB with copper. This helps to add rigidity to the board and prevent it from warping during assembly.

Ensuring Compliance with Design Rules

Another important consideration when designing rigid-flex PCBs is ensuring compliance with design rules. This is important as rigid-flex PCBs are subject to different design rules than traditional PCBs.

Some of the key considerations when designing rigid-flex PCBs include:

Trace width and spacing
Hole size and annular ring
Solder mask clearance
Silkscreen clearance

Let's discuss each of these considerations in detail:

Trace Width and Spacing

One of the key considerations when designing rigid-flex PCBs is trace width and spacing. This is important as the traces need to withstand the mechanical stress of being bent during assembly. The recommended trace width for rigid-flex PCBs is 6 mils. The recommended trace spacing is 10 mils.

Hole Size and Annular Ring

Another important consideration when designing rigid-flex PCBs is hole size and annular ring. It is essential to consider the hole size as the smaller holes are not enough to accommodate the alignment pins during assembly. So, the optimal hole size for rigid-flex PCBs is 0.040 inches, whereas for annular ring its 0.015 inches.

Solder Mask Clearance

Solder mask clearance is the space between the copper trace and the solder mask. This space is important to ensure that there is enough room for the solder to flow during assembly, and to prevent shorts from occurring between the traces.

The width of the solder mask clearance depends on the width of the trace, the thickness of the solder mask, and the thickness of the copper. The clearance should be at least 0.007 inches for narrow traces, and at least 0.010 inches for wide traces.

Silk Screen Clearance

A silk screen clearance is simply a small amount of space between the edge of the printing surface and the beginning of the design. This clearance ensures that the design will be printed in the correct location on the flex circuit, regardless of how thin or delicate the circuit may be. In most cases, a silk screen clearance of 0.010 inches will be sufficient to ensure accurate printing.

Managing Thermal Issues

Thermal management is a critical consideration in the design of any electronic device, but it is especially important in the case of flexible and rigid-flex printed circuit boards. There are a number of factors that must be considered when designing a flex or rigid-flex PCB, including the thermal characteristics of the materials used, the impact of thermal expansion on the circuit board, and the need for adequate cooling.

With a few simple design considerations, you can help to ensure that your PCBs don't overheat and cause damage to sensitive components.

Thermal Vias

One way to help manage heat build-up on your flex rigid PCBs is to use thermal vias. Vias are small holes that connect different layers of the PCB together. By placing thermal vias in strategic locations, you can help to dissipate heat away from critical components.

Lightweight Material

Another way to manage heat build-up on your flex rigid PCBs is to use a lightweight material for the substrate. A lighter substrate will have less mass, which means that it will heat up more slowly than a heavier substrate.

Heat Sink

A third way to manage heat build-up on your flex rigid PCBs is to use a heat sink. A heat sink is a piece of metal that helps to dissipate heat away from the PCB. By attaching a heat sink to your PCB, you can help to keep critical components cool.

Thermal Tape

A fourth way to manage heat build-up on your flex rigid PCBs is to use thermal tape. Thermal tape is a special type of tape that is designed to dissipate heat. These taope ensure to keep components cool after being placed between layers of PCB.

Final Words

To ensure that your rigid-flex PCB design is manufacturable, it is important to consider the DFM and design rules early in the process. By working with a qualified manufacturer, you can avoid common problems and get your product to market quickly and efficiently.