Lead-Free Reliability: Building It Right the First Time
This article is based on an original publication by Kester.
The recent rise in lead-free assembly and RoHS compliancy introduces a higher risk of defects and reduced product reliability. As technicians transfer to lead-free soldering, the ensuing changes will impact every aspect of the assembly process, including solder alloy, component termination, board finish, board and component thermal limits, component moisture sensitivity levels (MSD), flux chemistry, equipment changes as well as changes in the cleaning and inspection processes.
Final reliability of the product is dependent on the optimization of the production process. Reduced wetting speeds of lead-free solders present a challenge in this regard. Wetting may be reduced in reflow soldering in which SnAgCu (SAC) alloys are used if the profile is not optimized to the manufacturer's parameters.
Assemblies with more active, water-washable flux systems experience the least issues with lead-free builds. In wave soldering, higher activity flux systems improve hole-fill; in reflow soldering, higher activity flux systems improve wicking and spread properties. The added activity creates better joints and promotes reliability. However, these flux systems also create situations in which complete residue removal is difficult.
During wave soldering, the activity of no-clean fluxes and solder pastes must be meticulously designed for lead-free assembly in order to sustain higher reflow profiles and longer contact times. Defects such as lack of hole-fill, de-wetting of leads, voids in the barrel and bridges may develop in wave soldering that could impact joint reliability.
Poor hole-fill is likely if flux activity is depleted prior to entry into the solder wave. De-wetting of leads may be caused by prolonged contact with the molten solder or too high solder temperature. Voids may be caused by solder properties as well as the flux and board finish. Bridging may result from excessive contact time at the wave solder and inactive or insufficient flux.
Poor wetting, de-wetting, solder balls, bridges, tombstoning and voids appear to impact reliability in lead-free reflow soldering. Improper thermal profile for the paste can cause insufficient wetting. Insufficient activity of the paste flux or board and component solderability can also contribute to poor wetting. Excessive times above liquidus contribute to de-wetting. If a paste has poor hot slump characteristics, bridges and solder balls can be solder paste related. Flux chemistry, an incorrect profile or termination geometries may cause voids.
In hand-soldering, poor wetting, de-wetting, cold solder, component damage and board damage reduce reliability. Low temperatures, insufficient activity and low flux in the solder wire contribute to poor wetting and cold solder joints. Technicians see de-wetting when tip temperatures are too high. The transition to lead-free soldering from hand-soldering has proven to be the most challenging.