Journal of SMT Article


Author: Dwayne R. Shirley et al.
Company: University of Toronto
Date Published: 1/1/2004   Volume: 17-1

Abstract: As wireless local area networks (WLAN) and other electronic assemblies exceed operating frequencies of 5 GHz, the need for a technique to characterize no-clean solder paste compatibility with RF circuit assemblies operating at these frequencies requires the development of a test method that is not limited by lumped circuit parameter modelling. Furthermore, it demands a test method that is sophisticated and robust enough to discern differences between the effects of several no-clean paste residues and characterize their respective electrical properties.

This paper presents a new method of characterizing the effect of solder paste residues on RF signals between 5 and 10 GHz with the use of Ansoft HFSS full-wave electromagnetic simulations and insertion loss measurements. A microstrip T-Resonator circuit was designed as the test vehicle of choice because resonant frequencies near 5 and 10 GHz can be achieved without unwanted coupling effects. The test vehicle was built with a low-loss substrate (Rogers 4350) and featured an ENIG surface finish.

Optical and scanning electron microscopy techniques were used to obtain solder fillet and residue dimensions needed as input to the model. Insertion loss measurements were obtained for samples processed with several different solder pastes, both after processing and after aging at 85oC and 85% RH for 14 days. Five different no-clean solder paste formulations were tested, and the effect of their residues on RF signal propagation was characterized.

Key words: solder paste, residues, RF, signal propagation.

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