PCB Tech - What spacing should be considered in PCB design

There are many areas in PCB design that need to be considered for safe spacing. In this, can be temporarily classified into two categories: one is electrical related safety spacing, one is non-electrical related safety spacing.

1. Electrical safety spacing:

Wire spacing

The spacing between conductors shall not be less than 4mil according to the processing capacity of the leading PCB manufacturer. Line distance is also line to line, line to pad distance. From the perspective of production, the larger the better if conditions are available. Generally, 10mil is more common.

Pad aperture and pad width

According to the processing capacity of mainstream PCB manufacturers, the aperture of pads shall not be less than 0.2mm if drilled mechanically, and 4mil if drilled by laser. The aperture tolerance is slightly different according to the plate. Generally can be controlled within 0.05mm. Pad width shall not be lower than 0.2mm.

The spacing between pads

According to the processing capacity of mainstream PCB manufacturers, the spacing between pads should not be less than 0.2mm.

The spacing between the copper skin and the edge of the plate

The distance between charged copper skin and PCB board edge should not be less than 0.3mm. Set the spacing rule on the Design-rules-Board Outline page, as shown in the figure above.

If it is a large area of copper, usually with the plate edge need to have a shrinkage distance, generally set to 20mil. In PCB design and manufacturing industry, generally, for the consideration of finished circuit board machinery, or to avoid the coiling or electrical short circuit caused by copper skin exposed to the edge of the board, engineers often spread the copper block in a large area relative to the edge of the board shrink 20mil, rather than all the way to the edge of the board. There are many ways to treat this indented copper skin. For example, draw a keepout layer on the edge of the board, and then set the distance between the copper and keepout. A simple method is introduced here, that is, set different safety distance for the copper laying object, for example, the safety distance of the whole plate is set to 10mil, and the copper laying is set to 20mil. It can achieve the effect of 20mil edge shrinkage. At the same time, the dead copper that may appear in the device is also removed.

2. Non-electrical safety spacing:

Character width Height and spacing

No changes can be made to the text film during processing, except that the line width of characters with D-code less than 0.22mm(8.66mil) will be increased to 0.22mm. That is, character line width L0.22mm(8.66mil). The width of the entire character is W1.0mm. The height of the entire character is H1.2mm. The spacing between characters is D0.2mm. When the text is less than the above standard processing printing will be blurred.

Through hole to through hole spacing (hole edge to hole edge)

The through-hole (VIA) to through-hole spacing (edge to edge) is greater than 8mil.

Distance from screen to pad

Screen printing is not allowed on the pad. Because if the silk screen is covered by the pad, when the tin screen will not be on the tin, so as to affect the installation of components. General plate factory requirements reserved 8mil spacing as well. If the PCB board is solid in a limited area, 4mil spacing is barely acceptable. If the screen screen accidentally covers the pad during design, the board manufacturer will automatically remove the screen screen part left on the pad during manufacturing to ensure tin on the pad.

Of course, in the design of specific circumstances specific analysis. Sometimes the screen is intentionally close to the pad, because when the two pads are close together, the screen in the middle can effectively prevent the solder connection from short-circuiting during welding. This case is another matter.

3D height and horizontal spacing on mechanical structures

PCB components should be installed in the horizontal direction and space height will not conflict with other mechanical structures. Therefore, in the design, full consideration should be given to the suitability of spatial structure between components, as well as between PCB finished products and product shell, and the safe spacing should be reserved for each target object. The spacing is considered in order to ensure that they do not conflict in space.

How to solve the problem of insufficient spacing?

The spacing is measured in the air (line of sight), so it can be properly laid out at the layout level to reduce the required spacing. A significant reduction in spacing can be achieved by using insulation and, where possible, by bilateral assembly. The insulation may be a sheet barrier between high voltage nodes. Since the tall parts are surface mounted, circuits that require spacing can be placed on opposite sides of the board. Nodes in the same high-voltage circuit at the same potential usually need to be carefully spaced from the low-voltage circuit. A good method is to place a high voltage circuit at the top of the board and a low voltage circuit at the bottom for control and monitoring. Low-voltage circuits generally do not have the boundary surface (housing) creepage requirements required by high-voltage circuits.

How to solve the problem of insufficient creepage distance?

We know that creepage distance is the distance between electrical nodes on an insulating surface. In our discussion, this means the space between the conductors on the surface or inner layer of the PCB. However, further expansion of components will be constrained by the volume of the product package, so some other strategy will be required to meet the required creepage distance while allowing higher package density.

Standard for calculating conductor spacing at various voltage levels

Proper distance between PCB wires is essential to avoid short circuit between conductors. Unfortunately, there is no single solution to this problem. Various industrial and safety standards exist that specify different spacing requirements depending on voltage, application, and other factors. Here are some considerations to help you determine the proper distance between PCB leads.

When a product must pass through a safety facility, each safety facility has a set of standards to meet specific insulation requirements. In this case, it is convenient to find the desired spacing. For example, in the United States, for most mains or battery-powered information technology equipment, the allowed PCB spacing should be determined according to tables 2K, 2L, 2M, or 2N of the standard UL IEC60950-1, Edition 2. These tables specify the so-called safe distances and "creepage distances" for various insulation classes.

The level required depends on the location of the circuit. When considering the spacing and creepage requirements of a given design, consider the combination of pollution levels and insulation types.Pollution levels usually refer to the amount of dust, moisture and other particulate matter in the surrounding air or on surfaces between high-pressure nodes. The standard specifies functional, basic, complementary, double and reinforced insulation. These definitions of insulation are quite complex. Creepage distance standards also vary with these insulation classes. The following figure shows the creepage distance required by IEC60950-1. The creepage distance required for different voltage levels. The data in the following table are used for basic insulation classes. if it is double or reinforced insulation class, the data should be doubled.