How do you handle impedance control in pcb assemblers?

pcb assemblers

Impedance control is critical to PCBs that use high-speed signals. Without impedance control, these signals can become distorted, which causes signal loss or data error and can ultimately interrupt the flow of energy that powers circuits, leading to product failures. Fortunately, PCB assemblers can handle these problems with proper planning and careful design.

The first step is to understand the impedance concept itself. Impedance is the product of resistance and reactance, measured in ohms. As the frequency of a signal increases, its impedance also rises. Impedance control is achieved by reducing the resistance and reactance of signal lines through various design changes to the board. This includes changing the line width, copper thickness, dielectric layer thickness, and insulating material.

A pcb assemblers can also alter the layout of components to keep impedance in check. This can be done by adding or removing spacers between the traces, adjusting their pitch (how far apart they are), and changing the trace width. It is important to note that a change in the line width must be applied to all lines of the same width in the layer. Otherwise, the impedance value will not match the design goal.

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How do you handle impedance control in pcb assemblers?

It is also important to consider the impact of the etch factor, which is how much the line width is reduced during the etching process. This is especially true with etched copper. However, this is not as big of a concern with conductive epoxy based coatings, which typically do not reduce the line width to the same extent as etched copper.

Another important consideration is the effect of the insulating material on the controlled impedance. Increasing the thickness of the dielectric material has a logarithmic impact on impedance. This is why it is generally recommended that designers choose a thicker dielectric material when designing a PCB to maintain controlled impedance.

Finally, it is crucial to ensure that a continuous reference plane exists underneath the signal traces. This helps prevent cross-talk, which occurs when signals from one signal interfere with neighboring traces. This is particularly critical in densely packed circuit boards with high-speed signal lines.

In order to specify impedance control requirements, a designer should clearly communicate them to the fabricator. This can be done by including them in the documentation materials that the designer prepares to send to the fabrication company, such as stackup tables and available materials. By doing this, the fabricator will have a clear understanding of the desired impedance and will be able to ensure that the board can be produced reliably. This will save time and effort for both the fabricator and the pcb assembler.

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