Copper Flexed Printed Circuits
A flex circuit has many design considerations, including layer stack and parts placement. There are also gnarly material quirks such as copper’s work hardening and fatigue.
One key consideration is to determine up front how much flexing will occur. This will help to determine copper thickness, type and other material specifications.
Base Layer
The base layer is the thin, flexible substrate used in a flex circuit. It flexible printed circuit is typically made of polyimide. The copper film and the overlay holding film layers are on top of it. The conductive copper layer is the part of the flex circuit that conducts electronic signals. The copper is etched through the base layer. The base layer also includes a dielectric material to prevent electrical shorts from occurring.
The layered construction of a flex circuit creates a number of differences from that of a rigid printed circuit board (PCB). Differences in registration tolerances, chemical and thermal properties of the materials as well as dimensional variations make designing for a flex circuit much more complex.
A flex circuit can have either a single-sided or double-sided copper foil. Single-sided flex circuits have the copper foil on one side of the base polyimide layer. The copper is accessible from the flex surface through metalized, plated-through holes.
A double-sided flex circuit has a conductive layer on both sides of the base polyimide layer. The conductive copper is accessible from the flex surface through holes in the plated-through hole (PTH) and through-hole assembly. The PTH in a flex circuit may be tented or Plugged, depending on whether the solder mask on the rigid portion of the circuit spans the drilled via opening.
Laminates
The base film or laminate of a flexible circuit provides most of the primary physical and electrical properties. It can be single-sided, a double-access flex or multilayer.
The most common insulator for a flex is polyimide. It has a glass transition temperature of more than 300degC and offers excellent chemical resistance. It also has high thermal stability and low expansion coefficient. It is also a good choice for high-speed digital circuits as it has low loss tangent and dielectric constant, which minimize jitter, cut signal rise times and reduce intersymbol interference.
Conductors for a flex are typically aluminum, electrodeposited copper or rolled annealed copper. These are bonded to the insulator using an adhesive layer. They may also be covered with solder mask or an etching resistant coating to prevent contamination and corrosion.
Some flex circuits are designed to be dynamically flexed, while others are designed for static bending or no flexing. The flex design is determined by the end use of the product. If the flex will be dynamically flexed, the circuit board must be designed to meet the required flexing specifications. This will include consideration of the copper thickness. It must be thin enough to allow the conductors to flex properly and still remain rigid. This will help avoid damage to the circuit during flexing and reduce assembly errors, test time and failure rates.
Conductors
The conductors of a flexible printed circuit allow current to flow freely. Copper is one of the primary conductor materials used in this type of circuit. Other materials include silver ink, Constantaneg and aluminum. Conductors are bonded to the base layer with an adhesive material such as epoxy or acrylic. In some cases, a pressure sensitive adhesive (PSA) is used.
The base layer of a flex circuit contains an electrical insulation material called polyimide. This material provides a significant amount of stability, thermal resistance and mechanical strength. Its ability to bend, however, is limited.
A flex circuit’s conductors are made of a metal foil that is either rolled and annealed or electrodeposited. The thickness of the copper is important, as it determines the flexibility and reliability of a flex circuit. A flex circuit should be designed with the smallest width possible, as a thicker copper foil is less flexible and requires more space to bend.
To ensure a flex circuit is reliable, manufacturers test each individual component before they complete the assembly process. These tests include bending the flexible circuit and electrical testing. A delamination test is also done to determine if any layers are detaching from the base. This is a significant problem that can cause the circuit to fail, so it is important to prevent these problems from occurring.
Copper Film
Copper is the most common metal conductor used in PCBs and comes in varying thicknesses for different preferences. Other conductor options include aluminum, silver ink, and conductive flexible printed circuit factory carbon. The conductive layers on flexible circuits need to be thin enough to accommodate the circuit’s bend radius. Otherwise, the traces may become damaged and short circuit.
These conductive paths are created using additive and subtractive processes. Subtractive methods involve removing solid areas of copper to define the desired circuitry pattern, while additive methods start with a bare dielectric layer and then add metallic traces to the surface. The latter have higher current-carrying capacity and environmental resistance than those produced through subtractive techniques.
The thickness of the traces is also critical. Thicker traces can carry more current, but are less flexible. Thinner traces have lower resistance to bending and are more suitable for dynamic flex circuits.
The insulators that encapsulate and separate the copper layered circuits are usually polyamide, polyester, and solder mask materials. They serve as insulation for the conductive circuits and protect them from damage, as well as enhance its flexibility. Aside from the insulators, copper-plated through holes are another important feature of flexible printed circuits. They enable electrical connection between conductive layers and are essential for minimizing crosstalk and meeting impedance specifications. Moreover, they increase the component density.
