Industrial audio equipment flexible PCB

Name: Industrial audio equipment flexible PCB
Material: FASB0520
Layers: 2L
Thickness: 0.12mm
Surface Finish: Gold fingers
Min Trace/Width: 2.5/2.5mil
Application: Industrial audio equipment

DESIGNING FLEXIBLE PCB FOR HIGH-TEMPERATURE APPLICATIONS

Many flexible pcb applications require designs to be exposed to and or operate continuously at elevated temperatures well beyond that of standard room temperature. These requirements are above and beyond the requirements of component or connector assembly.

Materials for flexible pcb are rated to withstand a RoHS solder reflow temp profile. These applications vary from short-term elevated temperature exposures, such as autoclaving of medical devices, to designs that operate continuously at extreme temperatures, such as down hole drilling electronics and applications that can see greater than 200°C for extended lengths of times.

To address these requirements, the industry has a variety of flex materials that have different maximum operating temperature capabilities. The end result is a dependable flexible PCB design that will be built on the materials specified throughout the construction.

In this blog post, we will review the properties of the available flexible PCB materials and resulting flex circuit constructions that will meet your operating temperature requirements.

Download Our Flex Circuits Design Guide

Flexible PCB Material & Construction Overview

Flexible PCB are constructed of 3 standard materials: copper for the printed circuit board patterns, flex core(s) to which the copper circuitry is attached, and coverlays, which encapsulate the external circuit layers serving the same function as soldermask on a rigid-flex printed circuit board.

Some flex circuit designs may also require stiffeners, to support component/connector areas, PSAs (double-sided, pressure-sensitive adhesive) to attach the flex within the enclosure or shielding films for EMI-sensitive applications. For each of the above materials, there are multiple options that have different maximum operating temperature capabilities. For a typical -40°C to 85°C requirement, all standard flex materials are capable, and no additional material or construction-related considerations are necessary. At higher temperatures, the materials and constructions will need to change.

Flex cores are available in two different types of constructions. They differ in how the copper circuit layer(s) are attached to the central polyimide core. The original flex core materials use a layer of adhesive to bond the copper to the polyimide core. The later material type has the Polyimide cast directly onto the copper thus eliminating all adhesive layers. This type of flex core is referred to as an “Adhesiveless Flex Core”. The standard practice, once the temp requirements rise above the common 85°C requirement, is to use adhesiveless flex cores. We will discuss the specifics as to why further along in this blog. The polyimide layer as a maximum temp in the 400°C range and is not of concern.

Coverlays consist of an external polyimide layer and a layer of flexible adhesive. They are laminated, under heat and pressure, onto the surface of the circuits with the adhesive serving both to attach the coverlay and to encapsulate the circuitry. There are 3 different types of adhesive available all with different temp rates which we will discuss in detail further along in this blog.

Stiffeners are available in polyimide, FR4, stainless steel, and aluminum. Stiffeners are attached using either the same adhesives as used with coverlays or in some cases a PSA. The only temperature-related considerations are if FR4 is used, and the type of adhesive used for attachment.

PSAs are available in an extremely wide range from multiple suppliers. 3M and TESA are the most commonly used material brands. The PSA selected will need review to ensure it meets both the temp and adhesion requirements of the design specification.

Flex circuits can be EMI shielded using either additional copper layers, dedicated shielding films, or silver ink layers. The use of silver ink has diminished significantly over the last 10 years and is not commonly recommended anymore.

2-layer flexible printed circuit board construction

3-layer flexible printed circuit board construction

Flexible Adhesives

Flexible adhesives are used in three specific areas: coverlay attachment, layer-to-layer lamination in 3-layer or greater designs, and stiffener attachment. There are three types of adhesives that can be used: acrylic-based, modified flexible epoxy, and polyimide coating. Each of these have different continuous operation temperature ratings. Acrylic adhesives have the lowest temp rating and are used for standard applications without any significant elevated temperature requirements. Epoxy-based adhesives are rated in the 130 – 140°C range and polyimide base adhesives are rated at 220°C+. Epoxy adhesives, as an example, are suitable for medical applications that require autoclaving.

Polyimide-based adhesives, as an example, are required for down-hole drilling applications that have a 200°C continuous temp specification. Polyimide adhesives require very high temperatures for lamination. While acrylic and epoxy adhesives are laminated in the 180 – 200°C range, polyimide adhesives require 307- 316°C. Not all flex circuit manufacturers may have high-temp lamination capabilities. Polyimide-based adhesives are also significantly more expensive than acrylic or epoxy adhesives which are cost comparable.

We recommend that the adhesive requirements be specified in the fabrication drawing notes per IPCF specifications as follows:

Acrylic Adhesive: IPC4203/18
Epoxy Adhesive: IPC4203/19
Polyimide Adhesive: IPC4203A/24

Flexible Circuit Board PSAs & Stiffeners

This an extremely wide variety of PSAs are available. Most have fairly high operating temperatures, but the adhesion strength will vary depending upon the temperature. For a flex application adhesion strength is usually not a significant requirement as the PSA on the flex is not subjected to any high forces. The following is the specifications for the most commonly used 3M 467MP PSA as an example:

For stiffener applications, beyond the previously discussed adhesive requirements, the only potential temperature issue may occur with FR4 stiffeners. Stainless steel, aluminum and Polyimide stiffeners have temperature ratings way beyond what will be required. We recommend following the same FR4 requirements for a comparable 2-layer rigid-flex PCB operating at the same temperature for the FR4 stiffeners.

Flex Circuit EMI Shielding

EMI shielding can be achieved using either EMI shielding films or additional solid, or cross-hatched, copper layers. We will not cover silver ink shielding as this method is rarely used in modern designs. Copper layer shields provide the highest possible operating temps and will be temp-rated based upon the adhesive used as this will require a flex construction of 3-layer or more.

The negative impacts of copper layer shielding are the significant increase in flex thickness, and the resulting reduction in flexibility and bend capability, and the increase in part cost. EMI shielding films are the preferred solution for flexibility/bend capability and cost-effectiveness but is limited to 125°C max. continuous operating temp.

EMI shielding films are laminated to the surface of the part in a similar method as coverlays. The adhesive used in the shielding film is used to both attach the film and electrically connect it to the ground net within the flex circuit. Exceeding the operating temperature will increase the electrical contact resistance and potentially compromise the effectiveness of the shield. For temperatures greater than 125°C the only viable option is additional copper shielding layers.

Summary

We hope this blog has answered many of the questions that may arise when developing a high operating temperature flex circuit board design. Feel free to contact QFPCB if you have any questions. We look forward to the opportunity to discuss your specifications and review your design to ensure that it will meet your operating temperature requirements.

QFPCB has 15 years of experience in the field of Flexible PCB production, choose QFPCB, you will get a reliable supplier in China. Contact us today for your Flexible PCB solutions.

Flex and rigid-flex circuit boards are a combination of both electrical and mechanical requirements that allow for solutions to many tight packaging requirements. However, this combination is also the potential source of design challenges as some electrical requirements can have a negative impact on the mechanical bend capabilities of flex circuits. If not, correctly addressed the reliability of the finished design may be compromised.

In this blog, we’ll discuss the three most common electrical requirements that have a negative impact on the mechanical capabilities of a flex or rigid-flex circuit board and the potential solutions that can be applied to achieve a successful design.

Controlled Impedance

Controlled impedance designs, and their inherent requirement of thicker cores to achieve the required impedance values increase the flex thickness and negatively impacts both the flexibility and bend capabilities. The goal is to minimize the flex thickness as much as possible to allow for the greatest degree of bend capability and reliability.

In combination with the improved dielectric constant of polyimide, thinner copper weights and thinner trace widths/spacing than typically used in rigid PCB designs allow for a higher degree of flexibility and bend capability.

A common preferred configuration utilizes a 2-3 mil flex core, 0.5 oz copper and an approx. 0.004” line width. With minor adjustments, this will allow for the common 50-ohm single-ended and 90- to 120-ohm differential pair requirements.

An additional element is the configuration of the controlled impedance. The two most common being surface micro strip and stripline.

The surface micro strip allows for a thinner flexible PCB design (2 layers with a thinner core), while a stripline significantly increases the flex thickness (3 layers with 2 thicker cores). A stripline configuration is approximately 75% thicker than a surface microstrip. See images below.

Shielding requirements will also need to be evaluated to determine which configuration is needed.

To summarize, thinner controlled impedance line widths allow for thinner copper, thinner cores, and, in turn, improved flexibility and mechanical bend reliability.

Flexible PCB surface micro strip controlled impedance stack-up

3 Layers flexible PCB stripline controlled impedance stack-up

Higher Current Carrying

Higher current carrying requirements pose a challenge due to the thicker copper needed. Increasing the copper thickness, however, has the greatest negative impact on flexibility and bend capability.

The preferred methodology is to utilize wider traces combined with thinner copper to achieve the current carrying capacity. This may require one or more of the following techniques:

Increasing the width of the flex area to allow for wider traces.
Using thinner copper combined with an additional layer to double up the current carrying circuits.
Divorcing the current carrying layer from the signal layers in a rigid-flex design with an air-gap construction. (See image below with layer 4 dedicated to the higher current lines).

A careful review of the bend requirements is a necessary part of the design process to ensure the mechanical reliability of the finished parts. The bend capabilities of a flex circuit are determined by the design, construction, and materials only. The manufacturing process does not have an impact on flexibility.

                        5 Layers Rigid-flex PCB stack-up for higher current carrying requirements

High Density/Interconnects

Designs with many interconnects, traversing the flex section(s), can lead to a higher layer count requirement which, in turn, constrains the bend capabilities. The goal is to either minimize the layer count and/or use a construction method that improves the flexibility.

Reducing the layer count by using a combination of the above is the optimum solution if the design allows. A 0.004” line and space with 0.5 oz copper may result in eliminating a flex layer, which will have the biggest positive impact on flexibility and bend capability/reliability.

If the flex layer count cannot be reduced, a rigid-flex design can use an air-gap construction where the flex layers are configured as independent sets or pairs from one another. This reduces the “I-Beam” effect and allows the flex layer pairs to bend independently from one another, which significantly improves the flexibility of the design.

This method can be applied to designs with 3 or more layers, impedance control, and or higher current carrying requirements. The copper weight and flex core thickness can vary between flex layer pairs as necessary.

We recommend a design review and bend capability input from your supplier to determine the optimum construction for high density designs.

8-layer rigid-flex PCB with a 6-layer flex stack-up for a high density / interconnect design.

Summary

A successful design meets both the electrical and, equally important, the mechanical requirements. The electrical provides the functionality and performance while the mechanical addresses the packaging requirements but also has the greatest impact on the reliability. As discussed above, controlled impedance, higher current, and high circuit density can have a negative impact on the mechanical capabilities and the reliability. There are many options available in materials and construction that will resolve this challenge and result in a successful and reliable design.

Related Products

10-Layer HDI Rigid-flex PCB

Name: 10-Layer HDI Rigid-flex PCB
Material: NPG-170N+PI
Layers: 10L(1+1+6+1+1)
Thickness: 1.0mm
Surface Finish: ENIG
Min Trace/Width: 2.5/2.5mil
Application: Smart Wear

Smart Wear Rigid-flex Circuit Board

Name: Smart Wear Rigid-flex Circuit Board
Material: IT-180A+PI
Layers: 12L
Thickness: 1.0mm
Surface Finish: ENEPIG
Min Trace/Width: 3/3mil
Application: Smart Wear