Epec's Blog | Electronics Manufacturing Solutions

Flexible circuits have two material options available to encapsulate the exposed outer layer circuitry: polyimide coverlay and flexible solder mask. While both perform the same basic function of insulating the external layer circuitry, each has different characteristics and capabilities that address specific design requirements.

Flexible circuit boards are necessary in numerous applications where a design requires the circuit to be bent within the electrical equipment or electronic device. However, it is not desired to have the flexible circuit board bend adjacent to connectors, mounted components, solder joints, and hole patterns. In these instances, a stiffener needs to be designed in to add rigidity and stability so that the flex circuit performs reliably.

Selecting the optimum flex circuit board material is a key element to the success of a flexible circuit design. A wide variety of materials and configurations are available to address the needs of today’s design applications.

When developing a flex PCB based design, one of the most common early decisions is whether a flex circuit with stiffener(s) will meet the design requirements or if a rigid-flex construction is necessary or more effective. While there is some overlap between the two methodologies, there are significant capability, performance, and cost differences that require review to ensure a successful design.

The integration of a flex circuit(s) with rigid PCBs into a rigid-flex configuration can solve many of today’s design challenges. The combination of the mechanical capabilities of flex circuits with the functionality of rigid PCBs is a solution that provides many benefits, including improved reliability, tighter packaging capabilities, high speed signal performance, reduced assembly costs, and opportunities for further overall design packaging reductions.

Stiffeners are a key design element in most flex designs and have a significant impact on both the performance and reliability of the finished flex circuits. As a result, stiffeners need to be fully and accurately defined in the data set. Not doing so may result in a finished part that does not meet your requirements.

Many of today’s rigid-flex circuit designs utilize the same high-density components found in rigid PCB designs. This requires the use of blind and or buried vias to allow the signal lines to be routed out from within the high-density components or the high-density areas of the design. The most common component that we see driving this today is the 0.4mm pitch BGA package.

The flex PCB stack-up documentation is an important component of the data set of a flexible printed circuit board design. It consists of a description of a flex or rigid-flex circuit board that defines in detail the specific material requirements and construction of the design.

Many electronics assemblies that utilize flexible printed circuit boards are sensitive to either absorbing or emitting electromagnetic interference (EMI). If EMI is left uncontrolled it can negatively impact the performance of the design and in extreme cases completely prevent it from functioning.

The pre-baking of flex printed circuit boards (PCB) immediately prior to assembly is an industry standard requirement that is documented in IPC2223 sec 5.3.5, IPC-FA-251 sec. 3.2.1.1.2 and by material suppliers (i.e. DuPont Pyralux Technical Manual sec. 5.23). This applies to all polyimide-based flex and rigid–flex designs. But why is pre-baking done prior to assembly, rather than earlier in the circuit board manufacturing stage?