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54 DESIGN007 MAGAZINE I OCTOBER 2021 Most electronic products today are assem- bled using the no-clean soldering process. e need for no-clean solder pastes emerged in response to legislation against the use of ozone-depleting chemicals, and the appeal of removing the costly flux cleaning operations in the assembly of PCBs. However, the impact of no-clean flux residue on signal integrity is a concern for many high-frequency products such as 5G and other high-speed communica- tions. No-clean flux residue can lower the char- acteristic impedance of an interconnect, create an alternative path to the signal, and degrade the signal integrity performance, especially at high frequency. With the predominance of no-clean solder- ing processes and the ever-decreasing compo- nent standoff, the electronics industry has had to consider the reliability of what may be par- tially activated flux residues under component bodies. Similarly, questions have also arisen about the reliability of flux residues resulting from the reflow of no-clean solder pastes that are entrapped under RF shields, where the escape of the volatile ingredients of the flux is greatly hindered. e residue le behind is generally clear in appearance and tends to flow uniformly away from the joint. And due to the small quantity of solids, this residue is extremely thin in most places. is allows for a camouflage effect; upon visual inspection, the joint looks to be free of virtually all residues. Little information exists about the effects of flux residues on electrical circuit functionality. But, with requirements for increased rise times and higher operating frequencies, there is par- ticular concern about the magnitude of such effects on high-frequency, digital, microwave, and RF circuits. e effects of flux residues are measured in terms of their effective dielectric properties on microstrip structures. In this way, the change in microstrip circuit perfor- mance caused by the presence of flux residues can be deduced. Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA The Impact of No-Clean Flux Residue on Signal Integrity

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