Characterizing the weak organic acids used in low solids fluxes
The elimination of chlorofluorocarbons (CFCs) and other chlorinated cleaning solvents due to their long-term environmental impact has lead electronic assemblers to examine soldering fluxes that reduce or eliminate the need for post-solder cleaning. Today, low solids fluxes are replacing more traditional rosin-based and water-soluble fluxes because many of them can be used in a no-clean process. Most low solids fluxes use weak organic acids as active ingredient. It has been reported that some of these weak organic acids leave behind residues that are corrosive to copper. Surface Insulation Resistance (SIR) measurements of flux-processed comb patterns have been the main test method used to determine the corrosivity of flux residues. This test has been performed with test samples exposed to accelerated temperature and humidity conditions of 85°C and 85%RH and a 50 V bias. Recent data on some weak organic acids suggests that they slowly disappear at this temperature and a lower test temperature of 65°C has been introduced into the new Bellco Standard. In Europe, this test is normally performed at 40°C and 93% RH. This paper reports on the application of SIR tests to study the corrosive behavior of three carboxylic acids (succinic, glutaric, and adipic acids) that are commonly used as the active ingredients in soldering fluxes. Coupons treated with equi-molar solutions of the acids were either exposed to reflow-soldering conditions or wave soldered face-up to create partially heated residues. Both tests were run under two different accelerating conditions, 85°C/85%RH for 7 days or 40°C/93%RH for 20 days. This latter condition is being considered for inclusion in an IPC standard. At the end of the test period, both corrosion and SIR test samples were examined under a microscope and any residues or dendritic growth were documented. SEM and EDX characterization was also performed to determine the residue and dendrite composition.