Impact of PCB manufacturing process variations on trace impedance

This paper demonstrates statistically the impact of PCB manufacturing variations on the characteristic impedance. Moreover, it shows that the characteristics of the PCBs vary across different suppliers. These differences cannot be tolerated in some applications where the characteristic impedance is restricted to be within a specific range. We sampled 3 x 20 PCBs, each batch of twenty is ordered from a different manufacturer: The sampling consisted of measuring the phase shift between the reflected and the incident signals when injecting a ISO MHz sinewave into a PCB trace. The trace is selected to be the same for all samples. All the PCBs are ordered to be identical and designed for 50 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 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 manufacturers. 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 showed 2.6 % to 3.5 % difference above the mean.Godkänd; 2012; 20121221 (andbra)