In PCB design, the rules governing safety distances are a crucial and critical indicator. They affect both the aesthetics and functionality of the PCB design. From a functional perspective, safety distances are categorized into electrical safety distances (violations of these distances can cause short circuits and other functional malfunctions, damaging the circuit board and the entire product design) and mechanical safety distances (violations of these distances can prevent component installation or cause incompatibility between the circuit board and the product casing).
What spacing needs to be considered in PCB design?
There are many areas in PCB design where safety clearances need to be considered. Here, we will categorize them into two types: electrical safety clearances and non-electrical safety clearances.
1. Electrical safety clearances:
Spacing between conductors
According to the processing capabilities of mainstream PCB manufacturers, the spacing between conductors must not be less than 4 mil. This minimum spacing is also the distance between conductors and between conductors and pads. From a production perspective, the larger the better, but 10 mil is commonly used.
Pad diameter and pad width
According to the processing capabilities of mainstream PCB manufacturers, the minimum diameter of solder pad holes should not be less than 0.2mm if mechanically drilled, and not less than 4mil if laser drilled. Hole diameter tolerances vary slightly depending on the board material, but are generally controllable within 0.05mm. The minimum solder pad width should not be less than 0.2mm.
Spacing between pads
According to the processing capabilities of mainstream PCB manufacturers, the spacing between pads should not be less than 0.2mm.
Spacing between copper sheet and plate edge
The distance between the charged copper trace and the edge of the PCB board should ideally be no less than 0.3mm. As shown in the image above, this spacing rule can be set on the Design-Rules-Boardoutline page.
For large-area copper pours, a recessed distance from the board edge is usually required, typically set to 20 mil. In the PCB design and manufacturing industry, generally, for mechanical reasons related to the finished circuit board, or to avoid potential issues like edge curling or electrical short circuits caused by exposed copper at the board edge, engineers often recess large copper pours by 20 mil relative to the board edge, rather than extending the copper to the entire edge. There are many methods for this copper recess. For example, a keepout layer can be drawn at the board edge, and the distance between the copper pour and the keepout layer can be set. Here, we introduce a simple method: setting different safety distances for the copper pour object. For example, the safety distance for the entire board can be set to 10 mil, while the safety distance for the copper pour itself can be set to 20 mil. This achieves the effect of a 20 mil recess from the board edge and also eliminates any dead copper that may appear within components.
2. Non-electrical safety distances:
Character width, height, and spacing
No changes can be made to the text film during processing; only the line width of characters with a D-CODE smaller than 0.22mm (8.66mil) will be thickened to 0.22mm. That is, the character line width L is 0.22mm (8.66mil). The overall character width W is 1.0mm, the overall character height H is 1.2mm, and the spacing between characters D is 0.2mm. Text smaller than these standards will result in blurry printing.
Via-to-via spacing (edge-to-edge)
The via-to-via spacing (edge-to-edge) should ideally be greater than 8 mil.
Distance between silkscreen and pad
Silkscreen printing is not allowed on solder pads. If silkscreen printing covers the solder pads, solder cannot be applied to the silkscreen area during soldering, affecting component placement. Generally, PCB manufacturers recommend an 8mil spacing. If the PCB area is extremely limited, a 4mil spacing is barely acceptable. If silkscreen printing accidentally covers the solder pads during design, the manufacturer will automatically remove the silkscreen portion from the pads during manufacturing to ensure proper soldering.
Of course, each case needs to be analyzed individually during the design process. Sometimes, the silkscreen is intentionally placed close to the solder pads because when two solder pads are very close, the silkscreen in the middle can effectively prevent short circuits in the solder connection during soldering. This situation is a different matter.
3D height and horizontal spacing on mechanical structures
When mounting components on a PCB, it's crucial to consider whether they will conflict with other mechanical structures horizontally and vertically. Therefore, the design must fully consider the spatial compatibility between components and between the PCB and the product casing, reserving safe distances for each target object. These distances should be determined to ensure they do not conflict spatially.
How to solve the problem of insufficient spacing?
Spacing is measured in air (line of sight), so it can be optimized at the layout level to reduce the required spacing. Significant reductions in spacing can be achieved through the use of insulating materials and, where possible, by double-sided assembly. The insulating material can be a sheet-like barrier between high-voltage nodes. Since taller components are surface-mounted, circuits requiring spacing can be placed on opposite sides of the board. Nodes within the same high-voltage circuit at the same potential typically require careful spacing from low-voltage circuits. A good approach is to place high-voltage circuits at the top of the board and low-voltage circuits at the bottom for control and monitoring. Low-voltage circuits generally do not have the same creepage requirements on boundary surfaces (housing) as high-voltage circuits.
How to solve the problem of insufficient creepage distance?
We know that creepage distance is the spacing between electrical nodes on an insulating surface. In our discussion, this means the space between conductors on the surface or inner layers of a PCB. However, further expansion of components will be constrained by product package volume, thus requiring alternative strategies to meet the required creepage distance while allowing for higher package density.
1. Calculate the standard for conductor spacing at each voltage level.
Proper spacing between PCB traces is crucial to preventing short circuits between electrical conductors. Unfortunately, there is no single solution to this problem. Various industry and safety standards exist, specifying different spacing requirements based on voltage, application, and other factors. Here are some considerations to help you determine the appropriate spacing between PCB traces.
When a product must be certified by a safety agency, each agency has a set of standards to meet specific insulation requirements. In this case, finding the required spacing is straightforward. For example, in the United States, for most AC or battery-powered IT equipment, the minimum permissible PCB spacing should be determined according to Tables 2K, 2L, 2M, or 2N of standard UL/IEC 60950-1, 2nd edition. These tables specify the so-called safe spacing and "creepage distance" for various insulation classes.
The required creepage distance depends on the location of the circuit. When considering the spacing and creepage requirements of a given design, the combination of contamination level and insulation type must be taken into account. Contamination level typically refers to the amount of dust, moisture, and other particulate matter in the surrounding air or on surfaces between high-voltage nodes. The standard specifies functional, basic, supplementary, double, and reinforced insulation. These insulation definitions are quite complex. Creepage distance standards also vary depending on these insulation classes. The following figure shows the creepage distances required by standard IEC 60950-1. It outlines the minimum creepage distances required at different voltage levels. The data in the table below uses basic insulation classes; for double or reinforced insulation classes, the data should be doubled.
