STEP 7: Soldering
December 31, 1969 |Estimated reading time: 5 minutes
Based on volumes of research, test data, and at the recommendation of industry associations, most companies have chosen one of the SAC alloys for lead-free processes. While these alloys have price and performance benefits, some critical process issues remain.
By Gavin Jackson, Ph.D. and Brian Toleno, Ph.D.
A widely known characteristic of SAC alloys is their increased tendency to produce solder joints with more voids than their tin/lead predecessors. Most experts agree that a certain degree of voiding is acceptable, but minimizing the amount and size of voids in lead-free solder joints is the best formula to increase reliability.
Before one can attempt to reduce the number of voids successfully in a lead-free process, they must understand the variation between SAC and lead-based solder materials. While there are numerous process- and material-related variables that can cause void formations, three primary physical differences between SAC and tin/lead systems account for these increased void instances.
SAC alloys have a higher surface tension than tin/lead alloys. Therefore, any trapped gasses have difficulty escaping from the molten SAC alloy, compared to molten tin/lead alloy. Inherent processing requirements of lead-free mean that there are potentially more trapped gasses that must escape. Because the melting temperature of SAC alloys is about 217°C and tin/lead is 183°C, more volatile compounds are released from the substrate and components - leading to the likelihood that more gasses will be trapped in the lead-free solder joint. Compared to tin/lead solder, SAC materials also have decreased wetting characteristics and larger wetting angles, which means that voids that may form must travel a longer distance before they can escape.
Optimizing Material Formulations
SAC materials will produce more voids than tin/lead materials. But just as there are differences in the performance of various tin/lead solder pastes, optimizing the formulation of SAC solder pastes can result in a lower number of SAC alloy-related voids. By using a SAC materials-based solution, void formation can be reduced. In this case, the effect of flux materials on void minimization is examined.
While the majority of the solder paste chemical system is composed of the solder alloy (90% on average), the balance of the system is flux chemistry. Flux content, concentration of solvent in the flux, solvent boiling temperature, and the flux-activator concentration play a role in solder paste generation of volatile compounds and void formation.
To understand how changes in flux formulations affect void formation in lead-free solder joints, a research study was conducted that altered flux conditions. In addition to varying the flux content, solvent concentration, solvent boiling temperature, and activator concentration, a reflow process factor was also added to understand the effect of process variation. The study then evaluated the number of joints with voids, the percentage area of voiding, void size, and solder paste processability. Not only was the testing designed to evaluate the effect of flux on void formation, but also to analyze the solder paste’s ability to perform in a manufacturing environment. Results were evaluated based on material performance when processing a BGA and a SOIC.
Figure 1. Parameter effect on hot slump.
Based on the results and variations in flux formulation and process parameters, a SAC material was developed and compared to another lead-free solder paste material. Solvent choice, flux-activator concentration, flux-solvent concentration, paste-flux content, reflow profile, paste viscosity, and hot-slump performance were assessed. Results revealed that the choice and concentration of solvent affected viscosity significantly, and solvent choice and activator concentration most affected hot slump, which causes bridging (Figure 1). To maximize print performance with lead-free solder pastes, it must be printable and cannot lead to excessive bridging. Variations in solvent choice, concentration of the solvent, and activator concentration must be controlled tightly to ensure a printable, hot-slump-resistant paste.
Voiding occurrence was evaluated next. Typical voiding on a BGA was plotted as an index of void size vs. the number of voids. Results show that the majority of voids were small in number and size, with some locations indicating larger voids. There are a variety of factors that can affect lead-free voiding; determining these can be somewhat complex. However, a few conditions have been identified to show the most effect on voids. Reflow profile is of great significance. The concentration and choice of solvent in the flux system also affected void formation. For BGAs, the worst voiding performance was observed when a linear reflow profile was used with a volatile solvent.
Primary factors affecting void formation were reflow profile and flux volatility. While the combination of both reflow profile and solvent volatility influenced voiding performance in BGAs, solvent choice affected void occurrence in SOICs more than reflow profile (Figure 2). One can conclude that tight control over flux-system volatility, combined with optimized reflow profiles, can reduce voids in lead-free solder joints.
Figure 2. Void formation based on volatilization rate of different solder joints.
Taking measures to reduce voids in lead-free solder joints is the best mechanism by which to increase long-term reliability. Ensuring that the solder paste used has a robust flux system and reflow profiles are ideal has been proven to lessen void formation. However, it is unlikely that these factors will reduce all lead-free-related voids, which may not necessarily be an insurmountable process obstacle.
The latest IPC Solder Products Value Council (SPVC) study confirms what other organizations have noted - certain levels of voiding do not affect solder joint reliability adversely. While the industry has adhered to a 25% rule for BGA voiding, it is a somewhat subjective measurement based on X-ray system parameters. Whether justified or not, this voiding specification has been applied to lead-free assemblies as well. But the 25% rule may not be appropriate for lead-free, and some voiding may be acceptable. The IPC SPVC study concluded: “The presence of process-related voids in the interconnections formed using the SAC alloys has been found to have no statistically significant effect on solder interconnection reliability as tested by accepted thermal-cycling methods.”
Conclusion
By incorporating process-optimization tools and materials that minimize the effect of adverse conditions, end-product reliability is improved substantially and can exist around the process. SAC alloys will produce more voids, and necessary steps must be taken to reduce voids and increase the probability of long-term viability. Using optimized reflow profiles in combination with SAC pastes that contain ideal flux formulations, lead-free solder voids can be minimized; remaining voids will most likely have no significant impact on long-term reliability.
Gavin Jackson, Ph.D., product development manager, the electronics group of Henkel, may be contacted at 44 1442 278162. Brian Toleno, Ph.D., application engineering team leader, the electronics group of Henkel, may be contacted at (949) 789-2554.