The Basic Design Rule for PCB

pcb Basic Design Rule


The Basic Principles of Printed Circuit Board Design

The design of a printed circuit board (PCB) determines its inherent characteristics and, to some extent, influences the complexity of manufacturing, assembly, and repair. It also impacts the reliability and cost of the PCB. Despite the numerous types of PCBs available, there are common principles to follow during design, which are used as universal guidelines. When designing, to achieve optimal results, it is crucial to adhere to these principles by taking into account various factors like circuit design requirements, PCB manufacturing, electronic assembly processes, and so on.

Accuracy and Aesthetics of Electrical Connections:
The traces’ connection to conductive patterns in the PCB layout should align with the circuit schematics and logic diagrams. If traces can not be made due to mechanical or electrical performance requirements, the assembly diagram should specify the traces’ connection method and requirements. The width and spacing of traces’ should meet the electrical requirements. If possible the traces’ layout should be reasonable and aesthetically pleasing.

The reliability of a PCB is the fundamental guarantee for its intended use. Different performance levels of PCBs have varying reliability requirements, measured by mean time between failures or mean time to work. Reliability is influenced by factors such as the structure of the PCB, the operating environment, the choice of base materials, the width and spacing of traces, as well as the manufacturing and assembly processes. Any change in these factors can impact the reliability of the PCB. These factors should be considered comprehensively in the design such as to reasonably determine the PCB structure (number of PCB layers), traces width and spacing, interconnection methods as well as to select suitable base materials. In general, PCBs with fewer layers and lower trace density exhibit higher reliability. However, in specific applications where high-speed digital or microwave circuits are requested and electrical performance and electromagnetic compatibility (EMC) are crucial, so using a multi-layer PCB design may achieve better results than single or double-sided PCB.

Manufacturability is a critical factor that determines whether the PCB design can be manufactured, tested, and repaired or not and has a great impact on production quality and costs. When choosing the PCB structure, trace width and spacing, interconnection methods, hole sizes, and the ratio of board thickness to hole diameter, designers should consider how well the design aligns with the current manufacturing and electronic assembly processes. While meeting electrical design requirements, the design should be favorable for manufacturing, assembly, and repair. Generally, these elements including higher trace density, greater trace precision, more layers, complex structures, and smaller hole sizes increase manufacturing difficulty. Poor manufacturability or design requirements exceeding current manufacturing capabilities can lead to challenges such as increasing production costs, longer production time, decreased quality, and even impossibility of manufacturing.

Different types of PCBs, including single-sided, double-sided, and multilayer PCBs, with varying base materials, processing precision requirements, and design methods show significant cost differences. Generally, multilayer PCBs are more expensive than single-sided and double-sided ones, high-density PCBs cost more than low-density ones, while higher-performance levels requested also contribute to increased costs. Design should prioritize the principles of low cost and optimal cost-effectiveness to get the best possible savings under the premise of ensuring safety and reliability.

Environmental Adaptability:
Based on where the PCBs are used, selecting appropriate base materials and coating layers is essential to meet the needs of the environment and extend the PCBs’ lifespan. For PCBs with high-reliability requirements, electromagnetic compatibility must be considered to prevent electromagnetic interference with other electronic devices. The PCBs should be designed to have interference resistance themselves. Materials chosen should aim for minimal or low pollution: the base material of the PCB should be halogen-free because flame-retardant baseboards containing halogens produce toxic gases during incineration. The solder-resist coatings and solder used for PCB assembly should be lead-free (excluding high-reliability military products). Currently, EU countries have clear regulations (such as ROHS). China has corresponding regulations, too. All these regulations prohibit the use of lead-containing coatings or lead-based soldering for PCBs in some civilian products. Now halogen-free base materials for PCBs are in production but the price is much higher than normal materials. However, the performance of these materials will continuously improve with technological advancements so cost will decrease, which surely leads to increasingly widespread applications.

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