Investigation of the fundamental interactions among the ingredients of flux by the group contribution method

Abstract

A clear understanding of the complicated interactions among various key ingredients of flux will be very beneficial for developing future high performance solder pastes. In this work, the group contribution method was introduced to estimate and predict the properties of compound and their interactions, especially, the chemical interactions. Based on the assumption that the effects of the individual groups are additive, the properties of a compound are calculated as a function of structurally dependent parameters, which can be determined by summing the number frequency of each group multiplied by its contribution. To be accurate, some corrections such as interaction between the neighbor groups, vaporization enthalpy contribution, and symmetrical effect are considered in the estimation. The thermodynamic properties of the formation enthalpy (Δ f H), the molar entropy (ΔS), the boiling temperature (T b ), the vaporization enthalpy (Δ V H) and the heat capacities (C p ) were calculated individually. Then, the chemical interaction properties such as the reaction enthalpy (Δ r H), the reaction entropy (Δ r S) can be calculated based on the chemical reaction which might happen among the various ingredients. And hence, the reaction Gibbs energy (Δ r G) and reaction constant (K) also can be estimated by the equations of ΔG = ΔH-TΔS and Δ r G =-RTlnK, respectively. When the parameters a, b, c in the relations of the heat capacity and the temperature of C p = a+ bT+cT2 were determined, the dependence of Δ r G and K with the temperature also can be determined through the formation enthalpy. The flux media generally contains several chemicals: acids, bases, and additional compounds which can help to improve solder wettability. Usually, the main interactions between acids and bases are the neutralization when the temperature is low. For the cases in which acids have -COOH groups and bases have -OH groups, the calculation results suggested that Δ r G of the esterification between these acids and bases were all smaller than zero in the temperature range, which meant the esterification were spontaneous reaction. Meanwhile, because Δ r G decreased and K increased with the increasing temperature, it predicted that the higher the temperature was, the higher degree the esterification was. So the interaction of the neutralization changed into the esterification when the temperature is high enough. Containing multi-carboxylic groups in acids or multi-alkaline groups in bases, not only monoester but also diester, triester ect., multi-ester compounds or even polyester may be possibly formed at higher temperature due to the negative values of Δ r G and the negative slopes of Δ r G with temperature of multi-esterification. Most of the calculation results were coincide with the experimental phenomens, this investigation may develop a method to evaluate the effects of various functional groups to the solder paste stability and wettability at different processing temperature ranges.