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Impact of PCB Manufacturing Process Variations on TraceImpedance
Keywords: PCB, Process, Variations
The characteristic impedance has a significant impact on signal integrity and power loss while transmitting asignal from source to destination. For sending signals over a line, both transmitter and receiver must be matched with the characteristic impedance of the line. In a microstrip line the ratio W/h plays an important role in controlling the characteristic impedance where W is the width of the trace and h is the distance from trace to the ground plane. εr which is the dielectric constant of the material that separates the trace from the ground plane is also an important factor. These two work together to form the characteristic impedance. There exist several variations that can occur during volume production of PCBs and many of these variations will affect the characteristic impedance of the trace to fall outside the accepted range. Apart from material tolerances, variations in etching, variations while creating the core FR-4 and while pressing the pre-preg layers of FR-4. This paper demonstrates statistically the impact of PCB manufacturing variations on the characteristic impedance. It also shows that different PCB supplier can lead to different PCB characteristics. These differences cannot be tolerated in some applications where the characteristic impedance is restricted to be within a specific range. We sampled 3x20 PCBs, each batch of twenty is ordered from a different manufacturer. The sampling consist of measuring the phase shift between the reflected and the incident signals when injecting a 180MHz sinewave. The trace is selected to be the same for all samples. All the PCBs are ordered to be identical and designed for 50 Ohms devices. Our conclusion was drawn after running the T-Tests to assess statistically the significance of the difference occurring between the PCBs. Based on the computed P-Values all the three batches are different from each other in the mean of the measured phase shift with 95% confidence. The difference between the measured and the expected characteristic impedance is found as 3%, 10% and 20% for these three manufactures. We also witnessed board-to-board variations even within the same batch and from the same supplier due to the process instability by looking at the probability density of having the same phase shift that is equal to the mean. Some samples shown 2.6% to 3.5% difference above the mean.
Abdelghani Renbi, Ph.D. Student
Lulea University of Technology; EISLAB, Department of Computer Science, Electrical and Space Engineering
Lulea 971 87,
Sweden


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