Flex PCB Coverlay Guide to Assist You Apply It Better

The copper traces on the PCB as pathways for the flow of current in an entire circuit. However, these exposed traces may be susceptible to damage due to various external factors. In contrast to rigid PCB, which incorporate a solder mask layer made of solder resist ink to shield the circuit, the flexible nature of flexible PCB poses a challenge in terms of protection. How then can flexible PCB safeguard their circuits? In this article, TechSparks introduces a solution – coverlays, a protective layer made of flexible materials applied to the surface of flexible PCB!

Table of Contents

Flexible PCB and Coverlay

The placement of traditional PCB poses challenges due to the limited internal space of small devices. In such cases, customizing a PCB that adapts to the device’s spatial structure can be the optimal solution, and flexible PCB are precisely designed to address this challenge. This enhanced circuitry can bend, fold, and conform to various shapes, fitting perfectly within the tight confines of external casing curves.

Now, while the tricky spatial issues have been resolved, new challenges arise. The thickness of flexible PCB typically ranges from 0.1 millimeters to 0.5 millimeters, and they cannot use solder mask layers. In the event of device jolting, flexible PCB are prone to collisions and friction within cramped spaces, leading to circuit malfunctions.

Rigid PCB can mitigate this potential risk by using solder mask layers, but this approach is not applicable to flexible PCB. Following the design approach of rigid PCB, applying a protective film layer on the surface of flexible PCB can address this issue.

One commonly used material for the coverlay is polyimide, a high-performance material often used to construct flexible PCB substrates and serves as a coverlay. Applying this protective film ensures the reliability of flexible PCB, preventing direct contact between the circuit and external elements such as air, moisture, and objects, all while retaining their flexible characteristics.

Fundamentally, solder mask layers and cover layers serve the same purpose for PCBs – protecting the circuit. The difference lies in their application to different types of circuits, hence the term “flex PCB solder mask.”

Characteristics of Flex PCB Coverlay

Although there are various materials that can be used as the covering layer for flexible PCB, polyimide is the most commonly used material. This material appears as a yellow transparent film, with a layer of white release paper in the middle. Based on its characteristics, it is highly suitable for use as a flexible PCB substrate and various high-temperature-resistant electrical insulation materials.

Heat Resistance:

Thanks to the abundance of aromatic rings in its molecular chain, polyimide stands out in terms of thermal performance among many organic polymers. It can operate in the range of -269°C to 280°C for long periods, with a short-term working temperature reaching up to 400°C. The glass transition temperature can even exceed 500°C.

Mechanical Properties:

The relative density of polyimide ranges from 1.39 to 1.45. At room temperature (20°C), its tensile strength reaches 200 MPa, maintaining above 100 MPa even in high-temperature environments (200°C). Compared to unreinforced materials, polyimide exhibits excellent tensile strength. For example, the tensile strength of Kapton film is 170 MPa, while polyimide reaches up to 400 MPa. These properties mean that the flexible coverlay can consistently protect circuits in dynamic applications.

Environmental Resistance:

Although the coverlay is thinner and lighter compared to solder mask layers, it still excels in resistance to external environmental factors. Polyimide film is insoluble in organic solvents, corrosion-resistant, and hydrolysis-resistant. It can even be modified in molecular design to achieve better resistance in extreme environments. In addition to these fundamental environmental resistance capabilities, polyimide film can maintain its strength at a high level even after exposure to a dose of 5×10^9 rad radiation.

Flex PCB Coverlay Opening

Burrs

Due to the inherent fragility of flexible PCB, the occurrence of burrs is a common phenomenon when creating openings in the cover layer. To avoid this potential defect, the following measures are recommended:

  • Use a drill bit with a minimum increment of 0.05mm as the reference. If the drill bit specifications cannot meet the design requirements, choose the closest available option.
  • Implement a skip-drilling strategy when the hole arrangement is too dense.
  • Minimize the occurrence of overlapping holes. If overlapping is unavoidable due to design constraints or drill bit sizes, consider modifying the design or changing the overlapping sections to slot holes.
  • Maintain a spacing between holes of greater than 0.2mm, with a minimum distance not less than 0.15mm.

Covering layer drilling distance

Pads

When it comes to the layout of pads and coverlays in flexible PCB design, two configurations are commonly encountered: “Opening Solder Pads” and “Opening Non-Pressure Pads.”

Opening Solder Pads:

Opening Solder Pads

In this configuration, a opening is created on the solder pad, exposing a portion of it, and the coverlay is then opened in this area. This layout requires consideration of issues such as solder overflow and alignment offset. The following are suggested solutions:

  • Design the edge of the coverlay opening to be at least 6 mil larger than one side of the solder pad.
  • Opt for a holistic design approach, making the overall coverlay 2 mil larger than the original solder pad, with an additional 6 mil compensation for increased margin.

Opening Non-Pressure Pads:

Opening Non-Pressure Pads

In areas without solder pads, opening are created, leaving the positions of the solder pads uncovered. This is applicable when there are soldered components on the pads, preventing pad detachment. The following guidelines should be followed:

  • Ensure that the coverlay opening hole is at least 0.1mm larger than one side of the solder pad.
  • If space allows, consider increasing it to a distance of 0.15mm from the edge of the trace.

Flexible PCB cover drilling distance

Overlap

When the opening positions of the coverlay on both sides of a flexible PCB completely overlap, it can result in stress concentration in that area. Ultimately, this leads to substrate fatigue and fracture, particularly pronounced in the edge regions. To mitigate this issue, TechSparks recommends slightly offsetting the opening positions (typically by 0.5-1mm), thereby dispersing the stress and alleviating the load in specific areas. This practice enhances the lifespan and reliability of the PCB.

Flexible PCB overlay overlap

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