The approach to designing flex PCBs is drastically different from that of rigid PCBs. Flex PCBs offer many benefits compared to rigid PCBs, such as ease of installation, durability, and being lightweight. In applications where real estate comes at a premium, flex PCB circuits provide major breakthroughs to designers.
In this article, we’ll look at design tips for Flex PCBs.
Know Which Flex PCB Materials You Are Working With
The industry’s preferred material for flex core layers is polyamide. You could choose any other material as long as it provides superior material properties compared to rigid PCBs. Flex materials are uniform in thickness with minimal variations in Dk values (between 3.2 and 3.4). This is because PCB boards do not have woven glass reinforcements applied to them.
As a result, the thickness for each layer ranges from 0.5 to 4 mils.
The two major types of flex material you will work with include:
Adhesive-based materials: This is where the copper is bonded to the polyimide with the help of an acrylic adhesive.
Adhesive-less materials: This is where the copper is applied directly to the polyimide.
Although adhesives can be used to laminate copper layers with the polyimide core, they can cause cracks in the copper plating within the via holes because adhesives become softer upon exposure to heat. This is why it is recommended to use more teardrops and anchors in PCB flexible circuit design.
Using Tear-Dropped Vias
Tear-dropped vias are useful in flex PCB design because it guards against drill breakout. This is because the use of a drill bit can create holes outside the pad or via. Teardrops are useful because they provide more copper for connection. This is true even if the breakout occurs at the back of a via or pad. Teardrops are thick enough to ensure that the connection doesn’t get broken.
Using Curved Traces
Major design consideration for flex PCBs is to use copper traces at right angles to flex circuit bends. This may be unavoidable in certain situations. In these cases, try to keep the curve as gentle as possible and use conical radius bends where possible. Also, it is recommended to avoid hard-right angel trackworks.
If possible, try to route your tracks with arc corner modes instead of 45 degree hard corners. This will reduce the stresses in the copper when it bends.
Coverfilm or Coverlay Requirements
Coverlays play an important role in designing flexible PCBs by encapsulating and protecting the board’s external circuitry. Coverlays are analogous to solder masks that are used on rigid printed circuit boards. The main difference is the need for flexibility and durability as it applies to flex PCB design.
Coverlays are composed of two part sheets with a layer of polyimide and a layer of flexible adhesive (made out of acrylic or epoxy). The layer of polyimide is about 0.001” thick and the layer of adhesive is about 0.001” thick. However, you can get varying degrees of thicknesses including 0.002” and 0.0005” depending on your design requirements.
A thickness of 0.001” ensures that enough adhesive is available to ensure proper lamination and makes the circuit appreciably thick. It is worth noting that adhesive thickness is reduced during the lamination process because it flows to encapsulate the flex circuit. The extent of reduction depends on the ratio of line width to spacing in the given area. Thickness reduction will be higher in circuits with large spacings compared to higher density designs or designs with copper fill areas.
Component to Component Spacing
An important design consideration in flex PCB manufacturing is the space between components. Placing components too close to each other can create a range of issues that may require re-fabrication and re-design, which could affect the effectiveness of the circuit.
This is why the circuit design should create a significant gap between component boundaries to alleviate any issues that could arise from close proximities. As a general rule, place-bound component shapes should not overlap each other.
Consult your design software to ensure that component rules, proximity regulations, and requirements are properly established before designing the flex PCB. It is worth noting that the minimum space between discrete components like resistors and caps should be 10 mils apart at the least, with 30 mils being a better option. This can alleviate many proximity issues that could disrupt circuit assembly down the line.
Components Should be Selected in the Design Phase
Circuit designers should choose their components as early in the design process as possible. This is the best way of ensuring that there are no conflicts between the actual design of the flex PCB and component assembly. By choosing components at the design phase, component size and space will no longer be a concern and the flex PCB assembly process will proceed without any problems.
Try to see if smaller components can be selected to optimize the flex PCBs real estate. Smaller parts will reduce the flex circuit’s overall footprint.
Insert Cutouts and Slots to Minimum Bend Radius
Designers can minimize the bend radius by inserting slots and cutouts in the bend region. Doing so will reduce the amount of bend in the flex circuit. An alternative is to remove sections of the flex board that doesn’t have any circuitry, just make sure to remove these sections lengthwise. If you prefer to go down this route, do keep in mind that you will have to re-route the circuit.
Heat Forming for a Tighter Bend Radius
Heat forming is a great way to optimize the placement of the board. It requires a steel jig that is inserted into an oven. A major benefit of heat forming PCBs is the together bend radius. However, most designers use heat forming to make installation easier, and a tighter bend radius just happens to be the cherry on top.
Make sure that the flex PCB manufacturer you choose produces the boards on a regular basis. The more experience they have, the superior their product. Make sure to ask them about the materials they work with and learn about the manufacturing process. For more information, get in touch with the experts at Hemeixin HDI PCB.