The practical PCB design guide and Rigid-Flex PCBs

PCBs (Printed Circuit Board) form an elemental part of electronic products. They serve as bridges to connect electronic components, enable signal transmission, and integrated circuits in most modern electronic devices.

Practical Design

Good PCB design can increase efficiency of transmission, ensure the functionality and quality of product. Meanwhile, it can reduce manufacturing cost. So how should a good PCB design look like? A simple design checklist can be generated by adapting some principles of design for manufacturing (DFM).

Components count and modular design

Less components often means 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 less layers that can handle complex functionalities and meet the power requirements, without degrading performance in 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 software, but is best done manually. A simpler design will help reduce manufacturing costs and minimise the chance of rework.

A profitable product should always go through a rigorous testing procedure. While the development of a 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.

Standard components and material selection

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 more customised manufacturing process, which means a high cost. Additionally, selection of a standard component will ease company in the future when they need to find alternative vendors.

Materials used to build PCB is no longer limited to traditional FR4. 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 the 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 PCB with high tolerance design will 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 angular rings in the PCB design when enough space is given.

Design for connectivity

According to an IT priorities survey, 44% percent of 1300 professional IT enterprise respondents indicated that the top priority is upgrade their connecting foundations. The change of demand shows that information and data transformation are clearly volumizing as technology goes up. This high demand will no doubt affect the PCB design directions – design for connectivity.

Generally, some constraints such as temperature, power durability should be considered. Running many functional parts in a small chip will incur thermal problems, which add burdens to power consumption. Other issues such as change of via and signal peaking are critical as well. Companies which aims at automation of production line will definitely want a fast and stable signal connection to ensure performance.

Designers’ roles integration

The roles of conventional PCB designers are more than a traditional one may have. Experienced designers would take more aspects than achieving functional deliverables into account. These designers should also understand the mechanical rules for product integration, apply DFM and DFMA principle in the design checklist for mass production purpose. Apart from that, nature of new materials and packaging technologies may also be valued in design procedure. Finally, they need to be proficient in 2D/3D design tool especially when tight and small design is required, since 3D diagram gives clearer illustrations.

Another option can be recruiting a whole design team. Professional team members carry these different roles. Long-term teamwork can shorten the real-time communication latency and minimise misunderstanding. An integral design will be given and with report and specifications in solutions.

In short, the reliable PCB designers should consider more aspects than before to create the quality IoT devices. The viable PCB design for IoTs should be faster and more stable connectivity to hold volumising data and signal transformation, achieve more complex functionalities in a miniaturised product.

Rigid-flex PCB

Rigid-flex PCBs have played important roles within defense 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 for the design trend. Rigid-flex PCB design has since increased in popularity, with the wearable technology industry market value projected to exceed USD 150 million in 2026. A more flexible, reliable, and miniaturised PCB design is no doubt the trend in wearable devices.

Rigid-flex PCB integrates flexible circuits and rigid ones. Traditional PCB design usually choose 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.

Unique Benefits

One of the initial challenges that rigid-flex PCBs design aims to overcome is the 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 smart watches 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 of 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 the 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, nature of materials and components placements allow technologies such as high density interconnect (HDI) microvia to be more practical.

Minor Drawbacks

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 of 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.

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