Controlled impedance in high-density interconnect (HDI) printed circuit boards (PCBs) refers to the characteristic impedance of an electrical trace or transmission line in the PCB. The characteristic impedance is the ratio of the voltage to the current in an electrical transmission line, and it determines the electrical behavior of the trace.
In HDI PCBs, controlled impedance is critical for maintaining signal integrity in high-speed digital and RF circuits. It helps ensure that signals are transmitted with minimal waveform distortion and reflection, preserving signal quality, and reducing the chance of errors. The characteristic impedance is typically specified in ohms and is determined by the trace width, spacing, material properties, and layer stack-up of the PCB.
Controlled impedance is important in HDI PCBs for several reasons:
There are several challenges in manufacturing PCBs with controlled impedance:
Differential impedance is a characteristic of a differential pair of signals in an HDI circuit board. It refers to the impedance of a pair of signals, typically two electrical signals that are transmitted in opposite phases on two separate conductors. The differential impedance is the difference in impedance between the two signals and is an important factor in determining the signal quality of the differential pair.
In HDI PCBs, differential impedance is critical for maintaining signal integrity in high-speed digital and RF circuits. Controlled differential impedance helps ensure that the differential pair signals are transmitted with minimal skew and crosstalk, preserving signal quality, and reducing the chance of errors. The differential impedance is typically specified in ohms and is measured as the difference in impedance between the two conductors in a differential pair.
To achieve controlled differential impedance, the trace width, spacing, and material properties of the differential pair must be carefully optimized. The layer stack-up and manufacturing process must also be considered to ensure that the desired differential impedance is achieved. Verifying the differential impedance of the trace is a critical step in the manufacturing process, and specialized test equipment and techniques are typically used to ensure that the final product meets the desired differential impedance specifications.
The challenges in manufacturing PCBs with controlled impedance include design complexity, manufacturing tolerance, layer stack-up, material properties, and test and verification. As the features get smaller that need impedance control, it becomes more important that the PCB fabricator has invested in the proper equipment to ensure that the circuit board can be produced reliably. Specifically, laser direct imaging, laser drills, high-end etching equipment, AOI capacity, and the proper test equipment are critical and expensive for PCB manufacturers to have in operation.
Despite these challenges, controlled impedance is an important aspect of high-quality PCB manufacturing, and these challenges must be carefully managed to ensure that the final product meets the desired performance specifications. The Netvia Group facility has many years of experience with HDI-printed circuit boards.