printed board assembly
A printed circuit board (PCB) is a key component in most electronic devices. It is responsible for the complex tasks that these devices perform, including generating power and transmitting signals to other components. For this reason, it is important that PCBs are assembled correctly to ensure that they perform properly. One of the most critical parts of the assembly process is impedance control. This is when the PCB trace’s characteristics are adjusted to meet certain specifications and tolerances. The goal of impedance control is to prevent the transmission of unwanted frequencies, which can cause issues like signal attenuation and EMI interference.
Ideally, a signal will leave a component output pin and travel through a printed board assembly trace until it reaches the other component’s input pin. Along the way, the signal will encounter changes in impedance caused by things like a change in copper layer or geometry. If the new impedance is significantly different from the original, some of the signal will be reflected and lost. This can result in performance issues, such as noise and distortion, that could reduce the integrity of the circuit.
In order to avoid this, the impedance of the signal should be matched to the source and load’s impedance at each point in the circuit. This can be accomplished by carefully analyzing the circuit’s requirements and then designing the traces accordingly. For example, the signal’s height, width, and distance from other copper features on the PCB will all play a role in its impedance. The dielectric material’s thickness and dielectric constant will also be a factor. Most industry-standard CAD software has built-in impedance control and differential pair features that will do these calculations for you, but it’s always good to have a thorough understanding of how these factors work together.
What is the role of impedance control in printed board assembly?
The best way to communicate your desired impedance values to a PCB manufacturer is by marking them appropriately within the schematic. You should annotate the information with special notes and symbols, incorporate it into your layered stack-up design, and describe it on the fabrication drawing. Then, the PCB manufacturer will be able to fabricate the traces according to your specification.
Another method of ensuring that your PCB meets your impedance specifications is by performing a DFM check with an electronic manufacturing services company, such as Altest. DFM checks will validate a client’s design based on the manufacturer’s guidelines, and can include techniques such as part spacing, part indications, footprint corroboration, BOM verification, and more.
In addition to implementing proper impedance control techniques during the design stage, it is important that you highlight these areas within your schematic by assigning them a specific net class so that they are easily distinguishable during PCB layout. Additionally, if you have a pair of differential signals, be sure that they are routed symmetrically and avoid splitting them around components or via holes, as these can create discontinuities in the impedance value that can lead to unwanted signal reflections. This will help minimize your potential EMI concerns and improve the overall performance of your circuit.
