EFFECT OF FLUX SYSTEMS ON ELECTROCHEMICAL MIGRATION OF LEAD-FREE ASSEMBLY
The reliability concerns regarding surface insulation resistance (SIR) and electrochemical migration (ECM) have been significantly raised in the printed circuit board (PCB) assembly with wide adoption of lead-free assembly materials and process. Moreover, the miniaturization trend of the electronic industry also increases the sensitivity of the functional assemblies to surface contamination and flux residues. There are numerous SIR reliability testing methods and standards, such as IPC, JIS, Bellcore, and Bono, whose predictive power will depend on the field of application including temperature, relative humidity and electric field. Each standard is limited in essence, because the failure mechanism is strongly affected by the chemistry of the assembly materials, e.g. soldering flux, paste, and substrate materials, which will perform differently under various conditions. This paper presents a summary and comparison of the major SIR reliability standards currently in use in the industry, and discusses the impact of the different flux systems on the failure mechanisms.
“ PARTIALLY-ACTIVATED ” FLUX RESIDUE IMPACTS ON ELECTRONIC ASSEMBLY RELIABILITIES
The reliabilities of the flux residue of electronic assemblies and semiconductor packages are attracting more and more attention with the adoption of no-clean fluxes by majority of the industry. In recent years, the concern of “partiallyactivated” flux residue and their influence on reliability have been significantly raised due to the miniaturization along with high density design trend, selective soldering process adoption, and the expanded use of pallets in wave soldering process. When flux residue becomes trapped under low stand-off devices, pallets or unsoldered areas (e.g. selective process), it may contain unevaporated solvent, “live” activators and metal complex intermediates with different chemical composition and concentration levels depending on the thermal profiles. These partially-activated residues can directly impact the corrosion, surface insulation and electrochemical migration of the final assembly. In this study, a few application tests were developed internally to understand this issue. Two traditional liquid flux and two newly developed fluxes were selected to build up the basic models. The preliminary results also provide a scientific approach to design highly reliable products with the goal to minimize the reliability risk for the complex PCB designs and assembly processes.
Reliability of Printed Circuit Boards Processed Using No-Clean Flux Technology in Temperature–Humidity–Bias Conditions
Printed circuit board (PCB) specimens containing three different IPC-B-25 test structures were exposed to temperature-humidity-bias conditions in order to evaluate the effects of no-clean flux chemistry, conductor spacing, voltage bias, and test environment on surface insulation resistance (SIR). Comb patterns on the PCBs were coated with a eutectic (63Sn/37Pb) solder applied by a hot air solder leveling and processed by using no-clean aqueous-based and rosin-based fluxes. The SIR failure rate with rosin-based no-clean flux was observed to be greater than that with aqueous-based no-clean flux. This was explained by the more corrosive nature of the flux residues and the larger concentration of hygroscopic weak organic acids in the rosin-based flux residues. A characteristic of the SIR failures for PCBs processed with rosin-based flux was a series of intermittent SIR drops, which could severely affect the reliability of electronic assemblies. It was hypothesized that flux residues combined with adsorbed moisture from the environment form an acidic medium, occasionally breaking through the tin oxide passivation layer on the electrodes. Penetration of the passivation layer combined with conductive flux residues bridging the electrodes caused the resistance to decrease, and rehealing of the passivation layer resulted in the intermittent behavior. Conductor spacing was observed to represent a factor in the electrochemical migration process that is independent of electric field. Since conductor spacings in electronic products continue to decrease, the experimental results support recommendations to replace 25-mil (0.64-mm) comb structures on industry standard test boards with those having smaller spacings, below 12.5 mil (0.32 mm), that accurately reflect the greater risk for SIR drops of today's higher density assemblies.