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Ning-Cheng Lee: Indium Corporation of America, Utica, NY, USA
ACKNOWLEDGMENT: Received August 1998 Revised November 1998
Introduction
Reflow solder paste is the primary method of forming solder joints during SMT printed circuit board assembly. In general, when properly done, the reflow process provides the advantages of a high yield, high reliability, and low cost. Among all of the process considerations, the reflow profile is one of the most important factors in determining the soldering defect rate. The types of defects affected by the reflow profile include component cracking, tombstoning, wicking, solder balling, bridging, solder beading, cold joints, excessive intermetallic formation, poor wetting, voiding, skewing, charring, delamination, leaching, dewetting, solder or pad detachment, etc. (Lee, 1997). It is therefore extremely important to have the reflow profile engineered properly in order to achieve both high yield and high reliability.
In general, a reflow profile can be roughly divided into three major elements: the peak temperature, the heating stage, and the cooling stage. Each element has its impact on the reflow results. Based on the understanding of defect formation mechanisms, the discussion will focus on how each part of the profile can be engineered to minimize the defect rate and to maximize reliability.
Fluxing reaction
Soldering normally starts with fluxing to clean up the metal oxides, which is then followed by solder wetting to form the solder joints. Therefore, before discussing any profile setting, it is essential to understand the time and temperature requirement for the fluxing reaction.
1. Time/temperature requirement for fluxing reaction
The fluxing reaction usually can be monitored by determining the wetting time S with the use of a wetting balance, as illustrated by Figure 1. A short wetting time normally reflects a fast fluxing reaction. Table I shows the wetting time of four fluxes A, B, C, and D from several commercially available Sn63Pb37 solder pastes at 200 and 240[degrees]C respectively. These four fluxes A, B, C, and D, represent four major flux categories, i.e. water washable, halide-free no-clean, halide-containing no-clean, and amine-containing RMA respectively. The copper coupons tested were pre-baked in a forced air convection oven at 100[degrees]C for three hours to simulate a situation where the surface is difficult to solder. The solders used in this study are Sn63Pb37 and Sn62Pb36Ag2. Results...