The need for no-clean solder pastes emerged in response to legislation against the use of ozone depleting chemicals and the desire to remove the costly flux cleaning operation from the prir ted circuit assembly process. Much data exists on the electrical reliability of boards containing flux residue. However, little information exists regarding the effects of flux residues and SMT materials in general on the electrical functionality of circuits. With requirements for increased circuit speeds and higher operating frequencies, there is particular concern about the magnitude of such effects on RF circuit performance. We have developed a method for charac terizing SMT materials by measuring the change in response of RF resonator structures caused by their presence. Microstrip resonator structures are widely used to measure the dielectric constant of substrate materials at high frequency. Results are applied to the design of controlled impedance lines and RF circuit elements. In the same way, by measuring the response of resonators which have had no-clean paste applied and reflowed, the change in RF circuit performance caused by the presence of the residues can be deduced and incorporated in RF design. Using a similar methodology other SMT assembly materials were evaluated for HF performance - namely wave solder fluxes, solder mask, epoxy-molding compounds, adhesives and underfills. In all cases results are referenced to bare FR4 boards so as to facilitate comparison. This work targets the frequency range 0.5 ― 10 GHz, which includes current wireless frequencies, Bluetooth and the C/X bands.
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