Surface Mount on Flex Circuitry

Estimated reading time: 8 minutes

Flexible circuitry has been around for decades. For most of this time, components populated on flexible circuitry have been thru-hole mounted. In the past 10 years, there has been a shift toward surface mount components on both rigid and flexible PCBs. This article details advances and hurdles that must be overcome when implementing this technology.

By Mark Finstad, Minco

For decades, most components populated on flexible circuitry have been thru-hole-mounted. Recently, however, there has been a shift toward surface mount components on rigid and flexible PCBs. Industry demands to shrink package sizes and technology advances in the flex-circuit manufacturing industry have driven this shift. Some advances include large improvements in photo-imagable coverlay (PIC) products, cost-effective and readily available adhesive-less substrates, improved design practices, and a better understanding of flex-circuit material limitations.

Figure 1. PIC materials provide tightly spaced, irregular-shaped openings for SMT components, while maintaining flexibility.

When SMT components started becoming mainstream in the rigid PCB industry, there was a push to develop ways to mount them on flexible circuitry. The first obstacle was defining the irregular SMT access openings in the coverlay material. Unlike rigid PCBs, which use a photo-imagable solder mask, flex circuits primarily use polyimide film coverlays to encapsulate outer circuitry layers. These films must be mechanically drilled to define access openings. Because no one has invented a square drill bit, flex-circuit manufacturers were forced to use a combination of round holes and slots in the cover to provide access to SMT pads. These holes and slots usually had to be oversized to expose the SMT pads completely. Oversized openings reduced the amount of web material between the openings - leading to this material tearing during the cover lay-up operation. To eliminate these problems, several PIC products were introduced to allow flex-circuit manufacturers to define irregular-shaped access openings. While these early PIC materials were more flexible than solder mask, they were more brittle than a polyimide film cover. The brittle nature of these materials significantly reduced the amount a flex circuit could be bent before it would crack. PIC materials have evolved, and there now are products that will perform nearly as well as polyimide film. These PIC materials allow flex-circuit manufacturers to provide tightly spaced, irregular-shaped openings for SMT components, while maintaining flexibility (Figure 1).

Another factor that has had a positive effect on SMT components on flexible circuitry is the introduction of adhesive-less copper-clad polyimide film. For years, the mainstay base material for flex-circuit manufacturing has been copper foil bonded to polyimide film using acrylic adhesive. Currently, a large percentage of flexible circuits are fabricated using this adhesive-based material. While acrylic adhesive has many good qualities, one poor quality is the degradation of some mechanical properties at elevated temperatures. For example, bond strength often suffers at elevated temperatures. As the temperature rises to soldering levels, acrylic adhesives soften, and bond strength is reduced significantly. While this causes few problems with thru-hole-mounted components, it is especially troublesome to SMT components, which require a conductive pad with no insulating material on the mounting surface. When designing a flex circuit for thru-hole-mounted components, the designer would make the access opening in the cover material smaller than the pad it exposed. This would allow the cover to capture the pad and anchor it to the substrate. Because this practice is not possible in SMT, mounting pads would be anchored to the substrate with only the acrylic adhesive to hold them in place. When a circuit was exposed to soldering temperatures, pads to lift off the substrate, resulting in a rejectable condition. The advent of adhesiveless material eliminated most pad-lifting problems because there was no acrylic adhesive that could fail. However, the cost of this new material was several times the cost of adhesive-based material, making it difficult to justify its use. Over the years, the cost of adhesiveless material has dropped, making it competitive with adhesive-based materials. It is becoming the material of choice for flex-circuit applications requiring SMT components.

Figure 2. FR-4 stiffeners are incorporated into the carrier pallet, which will provide a rigid assembly to travel through the component placement and reflow processes.

Perhaps the most important advancement in mounting SMT components to flexible circuits has been improved design practices for SMT applications. Flex-circuit manufacturers have learned that there are critical features that must be incorporated into a design to ensure that SMT components can be mounted successfully on their products. A good design should start with the use of PIC for the insulating material around SMT pads, and adhesiveless copper-clad material as a base. In addition, it also is important to rigidize the SMT area with some type of stiffener. If a flex circuit is allowed to flex in the SMT area after components have been installed, associated stresses can cause components to rip the mounting pads off the circuit. For that reason, it is important to ensure that the SMT area remains flat during and after component installation. This is most commonly accomplished by laminating an FR-4/glass stiffener to the side opposite the components. FR-4 stiffeners often are incorporated into the carrier pallet, which will also provide a rigid assembly to travel though the component placement and reflow processes (Figure 2).

While the stiffener keeps the flex circuit flat during processing, it also makes it difficult to place any SMT devices on the stiffener side of the flex circuit. Small cutouts can be drilled or routed in the stiffener to allow the placement of a few isolated SMT components on the stiffener side, but these components would have to be soldered by hand. Stiffener thickness would essentially put the SMT pads in a deep hole, making it impossible to apply solder paste with a stencil. This is why it is rare to see flex circuits with SMT components on both sides. When a design requires SMT components on both sides of the flex circuit, it is best to move to a rigid-flex construction.

Rigid-flex Circuits

Rigid-flex construction combines the robust mechanical properties of rigid PCBs with the 3-D packaging options of flexible circuitry (Figure 3). What sets a rigid-flex circuit apart from a rigidized flex circuit is that the hardboard material typically is laminated to both sides of the circuit, outer hardboard layers have conductors, and plated thru-holes go through both hardboard and flex materials. Because the outer hardboard layers of a rigid-flex circuit have conductors, SMT components can be mounted on both sides. This increases the packaging density, but not without cost. A rigid-flex circuit can easily cost two or three times that of a comparable multi-layer flex with stiffeners. Designers must carefully weigh packaging-density needs against cost ramifications. Generally, if SMT components were required only on one side, the best option would be a multi-layer flex with stiffeners.

Figure 3. Rigid-flex boards combine the robustness of rigid PCBs with 3-D packaging options of flex circuitry.

Contract manufacturers (CMs) and assembly houses have made advances in SMT component placement on flexible circuitry. These companies have gained a better understanding of flex-circuit material limitations. As noted, acrylic adhesive softens and loses bond strength when nearing soldering temperatures. Even if adhesiveless base materials are used in the construction of the flex circuit, acrylic adhesive still will be used to bond the layers together. Therefore, it is important to monitor the temperature profile during reflow carefully to ensure that maximum temperatures, as well as the duration at those temperatures, do not exceed material limitations.

Another factor that has a major effect on the successful population of SMT components is moisture removal. Both the polyimide film and acrylic adhesive are extremely hydroscopic. They saturate in humid environments quickly, and are difficult to dry out completely. This moisture can cause serious problems when the circuit is brought up to soldering temperatures. As the moisture turns to a gas, it expands rapidly, possibly causing delamination between layers. The stress caused by expanding water vapor can easily tear a circuit apart. The way to eliminate this problem is to bake circuits thoroughly prior to component population at a temperature of about 250°F for a minimum of one hour (thicker multi-layer and rigid-flex circuits may require several hours). After baking, the circuit should be either processed immediately (less than 30 minutes), or stored in a desiccant chamber or dry box until it is processed.

Surface Finishes

A final topic that should be covered in any discussion of SMT is conductor surface finishes. The most common solder surface finish used in the flex circuit arena is eutectic tin/lead solder. The low melting point of eutectic tin/lead solder allows a lower, less-damaging temperature profile in the reflow process. The flex circuit can have solder paste and components added, and can be reflowed similarly to a rigid PCB, with the exception of a few tweaks in reflow temperatures.

Industry trends toward lead-free finishes and RoHS compliancy have complicated SMT flex-circuit processing. When lead-free requirements are placed on a flex-circuit design, the most common finish used by flex-circuit manufacturers is electroless nickel/immersion gold (ENIG). A few manufacturers opt for immersion silver, with the balance using immersion tin. All of these finishes are relatively easy to apply, and provide a solderable surface. However, problems rise when these flexible boards are exposed to the higher reflow temperatures of lead-free solders. The higher temperatures needed to reflow lead-free solders will further aggravate problems encountered when soldering to flex circuitry at tin/lead solder temperatures. In these applications, the process of removing moisture from the flex circuit, and keeping the moisture from being re-absorbed, is critical and can mean the difference between shipping good assemblies and filling the scrap barrels.

Conclusion

As SMT on flexible substrates become mainstream, it will be important for CMs and assemblers to include flex-circuit manufacturers in the process. Achieving a successful flex-circuit SMT project requires the knowledge and experience of manufacturers, coupled with proper processing techniques of the assemblers. Good communication between all parties is the first step to ensure that such projects are successful.

Mark Finstad, principal applications engineer, Flex Circuit Division, Minco, may be contacted at (763) 571-3121.