Tips to simplify designs
Printed circuit boards are at the heart of virtually every modern product – from a toaster to a car, from a cellular network to the electricity grid management system. They embody and make functional all of the microprocessor, data, communications and automation that defines the functions of virtually every sector. On that basis, the design of PCBs is both critical and highly sensitive.
Devices are expected to simply work, but behind that simple need is layered complexity that begins with the chemical etching and insulation laminating of essentially simple materials.
These PCBs are then populated with a huge range of devices – both simple and impossibly complex – to make the functional sub-assembly devices that are referred to as PBCa (printed circuit board assemblies).
Relative advantages of SMT, PTH, and hybrid PCBs
Surface-mount technology (SMT) PCBs:
- Size and weight reduction: SMT components are typically smaller and lighter than through-hole components, allowing for more compact and lightweight designs.
- Higher component density: SMT enables the placement of more components on both sides of the PCB, increasing the circuit’s complexity and functionality without increasing its size.
- Automated assembly: SMT is well-suited for automated assembly processes, which improves manufacturing speed, consistency, and reduces labor costs.
- Improved electrical performance: Shorter lead lengths reduce parasitic inductance and capacitance, enhancing the electrical performance of high-frequency circuits.
- Cost efficiency: SMT components and assembly processes are generally less expensive than through-hole due to lower material and labor costs.
Plated-through-hole (PTH) PCBs:
- Mechanical strength: PTH components provide robust mechanical connections, making them ideal for applications requiring high mechanical stability and durability.
- Thermal resistance: PTH components can handle higher thermal loads, making them suitable for high-power applications.
- Ease of prototyping and repair: Through-hole components are easier to handle, making them more suitable for prototyping, testing, and manual repairs.
- Reliable connections: The solder joints of PTH components offer more reliable connections in harsh environmental conditions, such as high vibration or mechanical stress.
Hybrid PCBs (Combination of SMT and PTH):
- Optimized performance: Hybrid PCBs combine the benefits of both SMT and PTH, leveraging the high density and automation capabilities of SMT with the mechanical robustness of PTH.
- Flexible design: Hybrid designs allow for greater flexibility in component selection and placement, optimizing the board for both electrical performance and mechanical strength.
- Cost-effective solutions: By strategically using PTH for components requiring mechanical strength and SMT for others, hybrid PCBs can achieve a cost-effective balance between performance and manufacturing costs.
- Enhanced functionality: Hybrid PCBs enable designers to use a wide variety of components, enhancing the overall functionality and versatility of the final product.
- Adaptability to diverse applications: Hybrid PCBs are well-suited for applications that demand both high electrical performance and mechanical durability, making them versatile across various industries.
The choice between SMT, PTH, and hybrid PCBs depends on the specific requirements of the application. SMT is ideal for high-density, high-performance designs with automated assembly, while PTH is preferred for applications needing robust mechanical connections. Hybrid PCBs offer a balanced approach, optimizing both performance and cost for diverse applications.
Tony K
Senior Mechanical Engineer
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DFM principles
Printed circuit board layout/design is a very regulated science that is typically performed using specialized CAD tools with powerful in-built tools. It is entirely possible to automatically generate the layout design files (GERBERS) from a schematic of the electronics and a series of adjustable design rules. In reality, good PCB designers don’t stop at this point, as there is a wide spectrum of finesse and expert knowledge that the automation tools cannot really encompass.
Component placement
Group components in a logical physical sequence, to minimize trace lengths for interconnect where possible. Ensure adequate spacing between components to avoid shorts and allow for heat dissipation.
Trace width, design allowances and spacing
Design traces with appropriate width and spacing based on current-carrying requirements and manufacturing capabilities. Follow IPC standards for trace width and spacing to ensure reliability and manufacturability.
Implement thermal reliefs for pads connected to large copper areas to facilitate soldering. Use teardrops at the junction of traces and pads or vias to improve mechanical strength and reduce stress concentration.
Maintain proper clearance and creepage distances between conductive elements, especially for high-voltage circuits. Follow industry standards to prevent electrical arcing and ensure safety.
Design with manufacturing tolerances in mind, considering factors such as drill wander, etching precision, and layer alignment. Ensure that your design can accommodate these variations without performance degradation.
Design the PCB layout to facilitate panelization for mass production. Include fiducial marks for accurate alignment during assembly.
Incorporate test points for critical signals and power rails to facilitate testing and debugging. Ensure these test points are easily accessible without obstructing other components.
Via and pad sizes
Use standard via and pad sizes that match the fabrication capabilities of your PCB manufacturer. Avoid using excessively small vias or pads that may lead to manufacturing defects.
Layer stack-up
Optimize the layer stack-up for signal integrity, power distribution, and thermal management. Ensure that power and ground planes are continuous and adequately connected to reduce impedance and noise.
Design rule check (DRC)
Perform thorough DRC to catch potential issues related to spacing, trace widths, and other manufacturability constraints. Use the PCB manufacturer’s DFM guidelines as a reference to ensure compliance.
Benjamin S
Mechanical Engineer
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Jiga is the best way to get the parts you need, when you need them.
Conclusion
The capabilities required in advanced PCB design are wide spectrum and hard to master. Outcome functionality in the finished assembly is deeply affected by these DFM skills, however;
- Radio frequency susceptibility is a major issue in high frequency circuits, so managing the antenna potential in PCB design is paramount.
- Radio frequency emissions are a major issue in many products, so prevention and choking of potential emitter points on a board are critical.
- Solderability is overwhelmingly important and can easily be disrupted by poor layout.
- Thermal management in power circuits is often an issue.
While the PCD design/CAD software in widespread use has increasingly clever and sensitive automation that can handle the bulk of the layout quite well, reliance on these tools is the downfall of many, many products. Performance can be come marginal, solderability can be challenging and RF issues can be highly disruptive.
The manufacturer of the PCB and PCBa – i.e. the board maker and the board populator – can have a great influence in terms of the quality of outcome. The client relies heavily on the precise interpretation of the design data and the error free manufacture of the board. All aspects can be heavily aided by applying good DFM principles in the board layout, with many layers of production reliant upon the designer’s skill and understanding.
