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Friday, June 5, 2020

Practical modern PCB design with rigid-flex

PCBs form an integral part of modern products and serve as bridges between components. Now, they are evolving.
Written by
Jane Lai
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Good PCB (Printed Circuit Board) design can increase the efficiency of transmission, ensure the functionality and quality of the product. Meanwhile, it can reduce manufacturing cost. So how should a good PCB design look like? A simple design checklist can be generated by adopting some principles of design for manufacturing (DFM).

Components, shape, and material selection

Fewer components often mean less cost. One consideration can be the count of layers on the PCB. Additional layers can hold more tracks and integrate more circuits, but increase production cost. Choose fewer layers that can handle complex functionalities and meet the power requirements, without degrading performance in the early stages. This will also prevent the high reject rate caused by layer clearance issues. Another consideration is to optimise electronic components placement. This can be done by utilising automatic features in the software, but is best done manually. A simpler design will help reduce manufacturing costs and minimise the chance of rework.

The shape of a standard PCB is either rectangular or square. Generally, PCB can have various shapes depending on the size and shape of the final product. But a standard shape is always preferable than the non-standard ones whenever necessary. This is because PCBs with irregular shape design will often require a more customised manufacturing process, which means a high cost. Additionally, the selection of a standard component will ease the company in the future when they need to find alternative vendors.

Materials used to build PCB is no longer limited to traditional FR4, a composite fibre-glass cloth in epoxy that is flame resistant. More often, designers may turn their eyes to build a rigid-flex PCB with the flex-rigid copper, and even deploy plastics for other part design. These materials are chosen for enhancing capabilities such as connectivity and data transformation. They are adding values when smaller parts are needed.

Design for manufacturing and assembly

Design for manufacturing and assembly (DFMA) It is very common to have various sub-assemblies when designing the PCB. In order to improve the overall efficiency, it is critical to choose optimal combinations of materials and fabrication processes for these components. For example, if the PCB is aimed for achieving complicated tasks, it is better to choose a thicker copper base in the initial board design.

The orders and directions of assembly should be considered as early as possible. It is suggested that all components (especially ICs) should be aligned in one direction. These components are broken down into minimal functional groups and the sequence of assembly is decided first within the smaller groups, then the larger. Sometimes designers can utilise the effect of gravity to reach better outcomes.

A profitable product should always go through a rigorous testing procedure. While the development of modular design will always be beneficial to this. Nowadays devices are usually formed by small fabricated groups of components to achieve complex functionalities. Break down the complicated system into smaller sub-assemblies will shorten the testing procedures. It helps designers to effectively find which modules need optimisations as well.

A PCB with high tolerance design will also accelerate the production, reduce errors of mounting, and decrease the chance of rework. Compliance decreases when a requirement of higher precision should be met. Therefore, the occurrence of errors will increase. For instance, if PCBs consist of components that are thicker than 10 mm, it is reasonable to leave at least 0.5 mm extra space between adjacent courtyards. This design consideration will benefit for the assembly process. Another example is to deploy larger holes and annular rings in the PCB design when enough space is given.

Rigid-flex PCB

Rigid-flex PCB integrates flexible circuits and rigid ones. Traditional PCB design usually chooses epoxy resins sandwiched in copper foils as base materials, while rigid-flex PCB varies. The flexible circuits are based on more flexible plastics such as Polyethylene Terephthalate (PET) or Polyimide (PI). The degree of flexibility affects the choice of copper foil as well. For instance, Rolled Annealed copper (RA copper) is preferred when circuits are constantly bending or rolling in movements.

Rigid-flex PCBs have played important roles within defence and aerospace industries for decades. They are long known for their reliable performance with tolerance for extreme environment. In recent years, companies are trying to adopt the technology into commercial industries. The advancement of wearable technology is the most notable cause of the design trend.

One of the initial challenges that rigid-flex PCBs design aims to overcome is space utilisation. Rigid-flex PCBs have fewer interconnecting cables compared to rigid board stacks. They allow more layers to be stacked within similar thicknesses to rigid PCBs, are mostly foldable, as well.

Nowadays wearable devices like smartwatches tend to go smaller and lighter. Customers do not want to wear a giant. In the case of medical wearables, some patients may prefer not to be noticed by others, and this drives the demand for unnoticeable medical monitors. These characteristics can both be reached by rigid-flex PCB design, since the reduction of connecting cables and parts also reduce the weight of devices. Companies can then create miniaturised products for their customers.

Different base materials enable rigid-flex circuits to reach higher performance, when dealing with various temperatures and stresses during operation. Products utilising the technology also become less vulnerable in the process of manufacturing, assembly, shipping, and after selling to end-users. In other words, it also improves long-term reliability. In rigid-flex PCBs, data and signal transmission can be performed partially or entirely within a homogeneous system structure. The partial approach enables the possibility of modular system design, while the latter integrates the flex layer throughout the system with high-density wiring. Both systems can increase fault tolerance rate, since less soldering is involved.

Possibility of more complex design. The facts that rigid-flex circuits eliminate some connectors and cables and require smaller space also enables the company to build more complex functionalities in their products. Additionally, the nature of materials and components placements allow technologies such as high-density interconnect (HDI) micro-via to be more practical.

In general, the price of manufacturing rigid-flex PCBs is lower than building fully flex circuits, but it is still higher than traditional rigid PCBs. The increase in cost mainly comes from slightly added bill of materials (BOM) complexity and higher precision required within manufacturing and assembly.

Rigid-flex PCB can be crucial for the development of complex product designs. It may require testing under additional scenarios and constraints than alternative technologies. Rigid-flex PCBs also rely more on 3D design than usual, requiring a higher level of CAD experience and collaboration.