What is the unit of mil in PCB?
The term "mil" is frequently mentioned in PCB manufacturing. But what exactly does "mil" refer to? In fact, mil is a crucial unit of length in the PCBA process, commonly used to specify line width, spacing, hole diameter, and even copper foil thickness. Let’s take a closer look at its definition and applications.
Definition and Conversion of Mil
Mil is an imperial unit of length, defined as:
1 mil = 1/1000 inch = 0.0254 mm
In PCB design and manufacturing, mil is widely used. For instance, in circuit routing design, typical line widths and spacing are often measured in mils, such as 6mil, 8mil, or 10mil. Similarly, when specifying via hole diameters, common sizes include 12mil, 16mil, and 24mil.
Applications of Mil in PCB Design
1. Line Width and Spacing
Conductive traces in a PCB are used to connect various electronic components. The width of these traces (line width) and the spacing between adjacent traces (line spacing) directly affect signal integrity and current-carrying capacity. For example, high-speed signal lines may require wider traces to minimize signal loss, while high-density circuit designs demand smaller spacing to save space.
2. Via Holes (Vias)
Vias are holes in a PCB used to connect different layers. Their diameters are typically measured in mils. For example, a standard via might have a diameter of 12mil, while microvias can be as small as 6mil or less to accommodate HDI (High-Density Interconnect) technology.
3. Copper Foil Thickness
The conductive layers of a PCB are made of copper foil. While copper thickness is commonly expressed in ounces (oz), it can also be converted into mils. For instance, 1oz of copper corresponds to approximately 1.4mil (35μm) in thickness. Common copper thickness specifications include 1oz (1.4mil), 2oz (2.8mil), and so on.