Flex Circuits at High Temperatures

Apart from being used at regular room temperatures, many industrial applications require exposing or continuously operating flex circuits at elevated temperatures. Most materials for flexible circuit boards can withstand reflow temperature profiles for RoHS solder. But sometimes, the elevated temperature may be well beyond the requirements of connector or component assembly.

These applications may range from short-term exposure of the flex circuit to elevated temperature, such as those the board faces during autoclaving of medical instruments, to requirements of continuous exposure to extreme temperatures, such as those faced by the board in drilling equipment. Some applications can expose the flex circuits to temperatures in excess of 200 °C for extended periods.

Rush PCB makes various types of flex circuits for operating at room temperatures as well as in elevated temperatures. For addressing the requirements of operation at elevated temperatures, we use a variety of flex materials with different maximum operating temperature capabilities. We provide dependable flexible circuits that we build from these special materials.

Materials for Flex Circuits

We typically use three standard materials for flexible circuit boards:

• Flex cores — these form the base of the circuit
• Copper films — these form the circuit board patterns for conducting electricity
• Coverlays — these form a protective covering over the external circuit
• Flexible Adhesives — these form a bond between various materials

Some flex circuits may also require stiffeners. These primarily act as support for components and/or connector areas. To attach the flex circuit to the enclosure wall, we use double-sided pressure sensitive adhesive or PSA. For EMI sensitive applications, we have shielding films that cover the flex circuit entirely.

For each of the above materials, we offer multiple options with maximum operating temperature capabilities. All our standard flex materials are capable of withstanding temperatures from -40 °C to 85 °C, and no special materials or construction-related considerations are necessary. However, for temperatures higher than 85 °C, the construction as well as materials for flex circuits must be different.

Materials for Flex Cores

We use flex cores in two types of construction. The difference is in the manner of attachment of the copper film to the polyimide core. The standard flex core material uses a layer of adhesive to bond the copper film to the core. In the other type, the manufacturer binds the polyimide directly to the copper film without using any adhesive, forming the adhesiveless flex core. We use the adhesiveless flex core for applications that require temperature withstanding capability above 85 °C. The regular polyimide core can withstand temperatures up to 400 °C.

Materials for Coverlays

Coverlays are also a layer of polyimide bonded on to the external flex circuit with a flexible adhesive. Application of heat and pressure laminates the adhesive and coverlay on to the flex circuit. Usually, the adhesive servers a dual purpose—it bonds the coverlay to the outer surface of the circuit, and it also serves to encapsulate the flex circuit. We use three types of adhesives suitable for three temperature ranges.

Materials for Stiffeners

We use several types of stiffeners made from various types of materials—aluminum, stainless steel, FR4, and polyimide. We use the same adhesive that we use for coverlays to attach stiffeners also. Sometimes we also use PSAs for attaching the stiffeners.

Among the materials for stiffeners, FR4 has the lowest temperature withstanding capability. Other stiffener materials being metal and polyimide, have a far greater temperature withstanding capability.

Materials for PSAs

Multiple suppliers offer various types of PSAs. We commonly use PSA materials supplied by TESA and 3M. Before deciding to use a specific type of PSA, we review its performance for meeting the temperature and adhesion requirements.

Most PSAs can withstand high operating temperatures, but their adhesion strength is temperature dependent. However, as the PSA is not subject to high forces, the adhesion strength of the PSA is not an important concern.

Materials for Shielding

We use various types of shielding materials depending on the application. We have shielding made of copper layers or dedicated shielding materials. The copper shielding can be made from films, cross-hatched or solid copper. Copper shielding has the highest temperature withstanding capability.

However, copper shielding can increase the thickness of the flex circuit significantly, along with a drastic reduction in flexibility and bending capability. The cost also increases with the introduction of a copper shielding.

Dedicated shielding materials made of films have the advantage of being cost-effective and not reducing the flexibility and bending ability to the extent copper shields do. However, their temperature withstanding capability is limited to 125 °C continuous use.

We typically laminate the EMI shield to the top surface just as we do for coverlays. The adhesive we use for attaching the shielding servers two purposes. It attaches the film to the flex circuit and it also electrically connects the EMI shield to the ground net of the flex circuit.

As the operating temperature exceeds the specified limit for the adhesive, it increases the electrical resistance of the contact and thereby compromises the effectiveness of the shield.

Materials for Flexible Adhesives

There are three specific areas where we use flexible adhesives:

• Layer to layer lamination
• Attaching Coverlays
• Attaching Stiffeners

We typically use three types of flexible adhesives for various temperature ranges:

• Acrylic-based Adhesives — for temperature ranging from -40 °C to +85 °C.
• Epoxy-based Adhesives — for temperatures ranging from +130 °C to +140 °C.
• Polyimide-based Adhesives — for temperatures around +220 °C.

We use acrylic-based adhesives for general-purpose flex circuits. We use epoxy-based adhesives for medical applications such as for autoclaving. The polyimide-based adhesive is best suited for high temperature applications.

While it is possible to laminate acrylic-based and epoxy-based adhesives at around 180 °C to 200 °C range, polyimide-based adhesives require very high temperatures for lamination, typically in the range 300 °C to 315 °C. Of the three types of adhesives we use, polyimide-based adhesives can withstand the highest range of temperatures, but is significantly more expensive as compared to the other two.

Read More About: RISK FACTORS FOR FLEXIBLE PRINTED CIRCUIT BOARDS

Conclusion

Rush PCB is an expert in the area of flex circuits and we design high-quality boards for high-temperature operation. If you have an application for flex circuits that must operate continuously at elevated temperatures, get in touch with us, and we will design a flex circuit that will satisfy your operating temperature requirements.