More and more of today’s PCB designs require faster processing of information.
That means the bare printed circuit board must have what is known as controlled impedance—the elimination or minimizing of discontinuities in the signal that a trace will deliver.
It is no longer a trace or track that connects a plated through-hole to a via or to another device on the PCB. Rather, transmission lines are designed to transport energy at high speeds, with little loss in signal shape, magnitude, or speed.
This high-speed circuit is similar to a high-speed highway where controlled impedance prevents the formation of potholes or speed bumps in the road.
The possible distortion of the original signal intended to be sent along a particular transmission line may cause the PCB to not perform as desired. A uniform controlled impedance is required for PCB signal traces to minimize signal distortions caused by reflections.
Controlled impedance adds another level to the PCB’s design, artwork, material selection and manufacturing processes. Even solder mask, with its insulation properties, will affect impedance values.
Upon receipt of an order, the PCB vendor will conduct a simulation to verify that the design will allow for the final impedance values, with a usual tolerance of +/- 10%.
If the design calculations do not agree, the supplier will notify the customer to allow a change in the stack-up to meet the design’s needs.
Artwork being plotted has to compensate for the plating tolerances of the PCB manufacturer. Material consistency is also important—glass style, resin content and resin flow affect the dielectric constant, which will affect the impedance results. Usually, the same materials will need to be used on repeat orders.
All controlled impedance boards will require TDR (Time Domain Reflectometry) measurements that confirm the impedance values are within tolerance.
TDR traces are placed on a coupon that is located on the manufacturing panel. The coupon usually contains several traces of some length (usually up to 8 inches) that run parallel to each other, with a plated through-hole at either end.
Using a TDR measuring device, the coupon verifies the impedance values are met. The testing is done on the coupon and is considered the benchmark that both designer and fabricator agree upon.
The PCB manufacturer has no control over discontinuities in the design, meaning if the coupon is correct, so–it is assumed–is the PCB.
Upon request, the TDR report may be included in the shipment, along with the coupons.
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