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56 DESIGN007 MAGAZINE I OCTOBER 2021 rately design a target characteristic impedance. It will affect the characteristic impedance of a transmission line and the propagation delay of the line. e strength and quality of the signal are augmented by a low dielectric con- stant of the materials. Low-loss and ultra low- loss materials, while being a bit more costly, can dramatically improve the propagation and quality of the signal at high frequencies. However, the presence of flux residue tends to increase the dielectric constant thereby reduc- ing the signal quality. Figure 3 shows how the change in flux residue affects the impedance of the microstrip trace. For a 15-µm residue, the impedance can drop by about 2.8 ohms, depending on the dielectric constant of the particular flux used. e impact of flux residue on microstrip impedance is not very large. However, the tolerance for con- trolled impedance boards is now regularly set at ±5%, making the use of no-clean fluxes bor- derline for high-density interconnects (HDI). Changes in a PCB material's dielectric con- stant can adversely affect the performance of broadband high-frequency analog circuits as well as high-speed digital circuits, because it will change the impedances of transmission lines in unexpected ways. In particular, these undesirable changes in dielectric constant and impedance result in distortion to the higher- order harmonics making up a high-speed digi- tal signal, with loss of digital signal integrity. In general, PCB materials with low and stable Dk values (with frequency and temperature) will support high-speed digital circuits with low distortion of the higher-order harmonic signal components 2 . Generally, we try to use low-Dk materials in the construction of HDI boards, which con- tributes to reduced interconnect delays and loss in the circuit. Consequently, an effective higher Dk caused by the no-clean flux is det- rimental to high-speed design. Also, keep in mind that flux tends to congregate in the inner corners of right-angle bends which will add to this issue—another reason not to use right- angle bends when routing. RF and microwave signal interconnects are typically routed on the Figure 3: Residual flux vs. effective impedance (simulated by iCD Stackup Planner).