Printed flex circuits and rigid-flex PCBs provide unparalleled design flexibility for electronics. Designed to bend and flex, they reduce space requirements and wiring considerations.
Flex PCBs start with a base layer of polyimide (PI) film that remains flexible after thermosetting. Photosensitive etch resist is applied and patterned with masks to expose and develop copper areas.
Surface Treatment
Surface treatment is an essential component of flexible circuit fabrication. It protects copper traces from corrosion and other environmental exposure, and helps them remain functional for their intended lifetime. Without this protection, oxidation may cause increased resistance in the trace, which can compromise signal and power transmission. The chosen surface treatment should also be able to resist thermal cycling, humidity, and chemical exposure.
Most flex circuits use a polyimide (PI) base for their core material. This material does not soften during heating and stays flexible after thermosetting. It can be combined with rigid sections to create a hybrid board, or it can be used as the basis for a single-sided or double-sided flex circuit.
The PI is then either coated with metal or laminated to it, depending on the type of circuit. A metal foil is typically used for the conductor layer, and it can be etched to form a specific pattern of conductive paths. Copper is the most commonly used metal for this purpose due to its balance of cost, physical performance, and electrical properties. Other metals can be used, but they often require more expensive processing to ensure good conductivity.
Lamination
The conductive materials that make up the electric paths are vital to the flex circuit’s performance. These materials are printed using a variety of techniques. In addition, the flex circuit may be coated with a material to prevent damage and oxidation. These coatings are also available in different thicknesses. They are applied to the copper pads of the circuit boards. These layers are essential for preventing solder bridges and improving the strength of the flex circuit’s connection points.
During the lamination process, multiple layers of the PCB are stacked together and bonded using an adhesive. After the lamination, the flex circuit flex circuit manufacturer is exposed to heat to solidify the adhesive and provide mechanical stability. This step is important for the flex circuit’s reliability.
Double-sided flex circuits have two conductor layers and can be fabricated with or without plated through holes. The plated through-holes are accessed from either side of the completed circuit. The copper is deposited by electroplating. Before this, the flex circuit is coated with an activator. Then, the laminate is immersed in an etching solution. This removes the photoresist and exposes the copper.
Thermoforming
Thermoforming is a complex process that requires precise temperature control, careful handling and rigorous quality assurance. If any of these steps are compromised, the resulting flex circuit will not meet the demands of its end-use product.
The metal foil that forms the conductive path is typically copper, which provides a good balance of cost and physical/electrical performance attributes. This one or two-sided flex section is then drilled, chemically plated, and etched in much the same way as rigid PCB cores.
Once the copper has been etched and plated, the exposed surfaces are covered with an insulating material known as coverlay. For high-volume manufacturing, this can be an additional layer of polyimide film with adhesive, or in an adhesive-less process a photo-imageable solder mask that essentially prints the coverlay onto the bare flex circuit.
This is an important step, as it protects the flex circuit from scratches and contamination. It also allows the fabrication to be cut into its final shape by using a hydraulic punch and die set. For prototype and low-volume production, a blanking knife is used instead of a hydraulic press.
Stiffeners
Stiffeners are non-electrical components added flex circuit to flex circuits that function as a mechanical support. They can be attached using thermal bonding or pressure sensitive adhesives (PSAs). Flex circuit manufacturers typically add stiffeners to areas of the board where plated-through holes are located to prevent flexing that could rip away copper and damage solder joints.
Stiffness requirements and operating environment drive the choice of material for stiffeners. Popular choices include FR-4, polyimide, and aluminum.
Depending on the design, the location of stiffeners is also important. They should not interfere with component placement, clamping and machine handling, or block routing. They should also allow for visual inspections and rework access.
In addition to preventing flexing, stiffeners help with heat dissipation. Aluminum stiffeners, for example, can be used to increase the thickness of ZIF connectors and other areas where flex pcbs are prone to wear resistance. They can also be added to flex pcbs that are exposed to harsh environments, such as automotive products and consumer electronics. These stiffeners can prevent damage due to repeated vibrations and environmental stresses.
Packaging
There are many ways to package a flex circuit. Some manufacturers will add stiffeners to selected areas of the PCB to provide extra mechanical stability. This can be useful in places like the edge connector and mounting holes. These stiffeners can be made of FR4 or metal and will increase the cost of the PCB.
The etched and plated copper is then combined with any rigid sections of the PCB using heat, adhesives and pressure. This is called lamination and is a key step in the production process. The flex section is usually separated from the rigid pre-pregs and cores or PI bonding sheets by blanking.
For high-volume orders, the flex is typically cut into its final form with a hydraulic punch and die system. For low-volume and prototype work, the flex is simply pressed into a routed slot in a backing board, usually MDF or plywood. The resulting blanked flex panel is then packaged according to customer requirements and external dimensions. A final electrical test is performed to ensure that the flex circuit meets the original design requirements.