Impedance control
Impedance control is the practice of specifying and manufacturing PCB traces so their characteristic impedance meets a precise target — typically 50 Ω single-ended or 100 Ω differential. Required for high-speed digital, RF, and signal integrity-critical designs, it depends on trace width, dielectric thickness, copper weight, and material properties.
What it is
For signals above roughly 100 MHz, a PCB trace behaves as a transmission line rather than a simple wire. If the trace's characteristic impedance does not match the source and load impedances, signal reflections occur — causing distortion, ringing, and bit errors in digital interfaces. Controlled impedance design ensures the trace impedance stays within a defined tolerance, typically ±10% (±5% for premium applications, ±3% for military and aerospace).
Standard target impedances are well established by industry interface specifications. Single-ended traces are usually 50 Ω, used for RF, single-ended high-speed signals, and most test equipment. Differential pairs use 100 Ω for Ethernet and LVDS, 90 Ω for USB, and 85 Ω for some PCIe variants. The values are not arbitrary — they are locked into the IEEE and JEDEC standards governing those interfaces.
Achieving the target impedance depends on the stack-up. The fabricator calculates trace width, spacing (for differential pairs), dielectric thickness, and material dielectric constant to hit the target, then verifies on each panel using test coupons. Modifying the stack-up after design typically requires recalculating all impedance values.
When it matters
Impedance control affects which projects need it, what it costs, and how to specify it. Any high-speed digital interface (USB above 2.0, Ethernet 100 Mbps and above, DDR memory, PCIe, HDMI, MIPI) requires controlled impedance to function reliably. RF designs above roughly 100 MHz always need it. Specifying impedance control adds typically 10-25% to fabrication cost compared to standard multilayer due to material requirements and test coupon manufacturing. Specifying the wrong target impedance, or omitting it for high-speed designs, causes signal integrity failures that are only discovered at electrical test or in the field.
At Nordic PCB
For controlled-impedance designs, specify the target impedances and which layers they apply to in your RFQ. Our certified suppliers calculate the required stack-up, return it with the quote for approval, and manufacture test coupons on each panel to verify actual impedance values. Standard tolerance is ±10%; ±5% is available for high-speed and RF designs. For complex stack-ups, our DFM review includes impedance feasibility against the requested layer count and dielectric requirements.
Related terms
- Stack-up
A PCB stack-up is the cross-sectional arrangement of copper layers, dielectric materials, and bonding films that make up a multilayer board. It defines layer thickness, copper weight, dielectric properties, and is critical for impedance control, signal integrity, and manufacturability.
- FR-4
FR-4 is a flame-retardant woven glass-reinforced epoxy laminate. It is the default base material for most rigid PCBs, balancing mechanical strength, electrical insulation, thermal performance, and cost. Variants exist for high-Tg, halogen-free, and high-speed applications.
- Trace width
Trace width is the lateral dimension of a copper conductor on a PCB. It determines current-carrying capacity, signal integrity, and impedance. Standard fabricators support down to 100 µm (4 mil) on outer layers with 35 µm copper; advanced and HDI processes support narrower traces.
- HDI
HDI (High Density Interconnect) describes PCBs with significantly higher routing density than conventional boards, achieved through microvias, thinner traces, and sequential lamination. HDI is essential for high pin-count BGAs, compact electronics, and applications in smartphones, medical devices, automotive ECUs, and aerospace systems.
- Multilayer PCB
A multilayer PCB has more than two copper layers separated by dielectric material and bonded together under heat and pressure. Common configurations are 4, 6, 8, 10, and 12 layers. Multilayer designs enable higher routing density, dedicated power and ground planes, and better signal integrity for complex circuits.