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Designing Wave Flux Chemistries
December 31, 1969 |Estimated reading time: 8 minutes
By Eli Westerlaken, Gerjan Diepstraten, Bob Silveri, and Harry Trip, Cobar SOLDER PRODUCTS
Lead– and silver–free solder alloys with additives are finding increased acceptance globally and hold promise as a mainstream solution. However, like virtually all lead–free solders, they melt at a higher temperature than lead–based solders, driving the industry towards demanding thermal profiles. This paper describes the development and implementation of water–based no–clean wave fluxes that use VOC–free technology, for use in lead–/silver–free and nitrogen–free wave soldering processes.
A silver– and lead–free solder metallurgy* offers superior characteristics to SAC alloys in terms of reduced copper erosion of assemblies, better fluidity, wetting, and drainage in wave and selective soldering. However, like virtually all lead–free solders, silver–free formulations melt at a higher temperature and therefore drive the industry toward thermal profiles that are more demanding for the materials used.
As temperatures rise, the flux materials undergo changes in their physical and chemical properties: evaporation of volatile fractions, surface activity, melt viscosity, etc. The consequence for the solder flux is early displacement, caused by the scrubbing action of the solder wave and thermal breakdown of the material. This results in reduced functionality as a protective blanket and insulating film over the liquid solder when it wicks up the barrel of a via or thru–hole aperture.
In conjunction with the larger ΔT in lead–free processes between the bottom and top sides of the board, passing through the solder wave results in early solidification before the liquid solder is able to wick up the barrel and wet the top side of the pad. This defect is commonly referred to as inferior topside wicking. Whereas the use of an N2 blanket over the solder wave prevents oxidation and thereby assists in wetting and wicking, it does not impact the melt viscosity, and thus the displacement, of the organic materials in the solder wave.
Unlike ordinary rosins, modern fluxes may consist of multiple polymer species and property–modifying additives. These additives affect the system’s mobility. The key to maintaining all desired product attributes, as well as maximizing topside fillet performance, lies in a thorough understanding of the interactions between these polymers and certain properties of the additives.
The Thermal Profile
Field experience in recent years has shown that the thermal profile for silver–/lead–free* wave solder processes is similar to those based on SAC alloys, with a topside circuit–assembly temperature prior to entering the solder wave at 110–120°C and a wave solder temperature of 260°C.
To generate a fully wicked–up solder joint, thermodynamics and related parameters are critical. The higher the melting point, the more critical the ΔT between the bottom– and the topside in the barrel. Solder wicking begins as the assembly enters the wave. Because of the ΔT between the bottom– and the topside in the barrel, the solder’s temperature decrease is accelerated as it rises up in the barrel. This affects melt viscosity, surface tension, wetting, and diffusion performance. To develop a flux with substantially more thermal bulk will conflict with no–clean requirements. We must carefully and accurately characterize the behavior of the fluxing system as it travels along a known thermal history.
The first step is to investigate the thermal history during the soldering processes. For this purpose, we use benchmark profiles that have been projected from production–floor wave solder equipment operating with the silver–/lead–free solder* and programmed in our TGA/DSC (differential scanning calorimeter) instrument. With differential thermal analysis (DTA), the test sample and an inert reference sample are heated (or cooled) under identical conditions, while operators record any temperature difference between test sample and reference sample. The instrument offers a choice between a time or temperature plot of the differential temperature. Changes in the test sample — either exothermic or endothermic — can be detected relative to the inert reference sample. Thus, a DTA curve provides data on the transformations that occur, such as glass transition, melting, boiling, sublimation, and recrystallization. Before formulating prototypes, we characterize the individual raw materials in each functional group of a wave solder flux with the benchmark profile in the TGA/DSC instrument.
Film–formers
For the purpose of providing a thermally insulating blanket, film–formers are an important functional group in a wave solder flux. Their thermal properties include support to wetting action, thermal bulk, and melt viscosity. Other functions include contribution to the surface tension and potential to neutralize post–solder ionic residues.
Figure 1. Film formers compared in TGA curve, simulating a benchmark wave solder profile for tin–based lead–free solder.*
In Figure 1, Material A is a hydrogenated, esterified resin with a certain melt viscosity and hydroxyl content. Its stability at elevated temperatures is the lowest of the three materials in this group. Material B is of a different chemical family with a molecular structure 100× larger than material A. It is a material of relatively limited mobility, yet it has some specific film–forming features.
Plasticizers
Plasticizers can play an important role in the uniform distribution of the other functional materials in the wave solder flux, in particular the film–formers. To guarantee that they continue to perform toward the end of the soldering process, they must survive the heat excursion of lead–free processing. In general, health concerns have resulted in a negative reputation for many plasticizers.
Activators
We have characterized many common flux activator systems. Adipic acid is a typical no–clean flux activator. Organic acids, unlike halide salts, are only weakly ionic in solvent solution. Their metal–cleaning “horsepower” is increased significantly when they enter the more mobile liquid melt phase. There is, therefore, a rough correlation between melt range and cleaning efficiency. By experience and observation, we have seen that some activators are better than others in certain applications, but there is not a single activator that is universally better than all others in all applications. A valid statement is that the cleaner and the more solderable a surface one has, the less activator is necessary to solder it. Using the data from our differential scanning calorimeter, we were able to modify our activator and synergist system to remove the constituents that caused the anomaly and replace them with other more effective systems. Further, the ultimate system displays exceptional soldering ability as well as strict compliance to SIR and electromigration criteria of most common international standards.
Surface Chemistry
Successful wave soldering of assemblies requires not only perfect wetting of metallic surfaces and wicking up the barrel of thru–holes, but perfect de–wetting of non–metallic surfaces as well. The difficulty that the industry has experienced with solder balling on resist surfaces is graphic proof that de–wetting of the resist surface is not a trivial consideration.
It is necessary to deliver the activator systems smoothly and reliably to the metallic surfaces, but equally important to ensure that the molten solder mass smoothly, reliably, and completely separates from the non–metallic when the board exits the wave. This is done using a specific blend of surface–active materials whose characteristics have been optimized for the acidic environment of the solder flux.
Surface tension considerations alone are not enough to give us a complete understanding of this phenomenon. Surface energies also play a part. For our monomolecular film to be stable, it must be adsorbed on the surface in a stable configuration during the exceptionally high temperature excursion through the solder wave. This implies a strong charge binding mechanism, at least during that phase of the soldering process. Quite often, a combination of several surfactants is required to control both the surface tension and the interfacial tension at the solid/liquid interface.
The objective of a properly designed system of surface chemistry as an essential part of a wave solder flux is as follows: improved wetting of the flux on the non–metallic areas; low surface tension in conjunction with low interfacial surface tension, yielding instant spreading of the flux over the various surface morphologies; and better penetration of the flux via absorption into the subsurface of the solder resist. The force and time that are required to penetrate into small pores can be reduced significantly.
A properly designed surfactant system will not only assist the wicking of the solder flux up the barrel of a thru–hole, it also helps in the repellence of the hot liquid solder mass from the non–metallic areas. Surfactants will control the droplet size of the flux upon spraying.
High ratios of surfactants would create more cons than pros:
- more residue;
- uncontrolled foaming properties;
- uncontrolled spray applications;
- reduced SIR values;
- increased costs.
Melt Viscosity
The wash–off effect of the solid materials by the wave and by wicking is influenced by the melt viscosity of the solid materials in the solder flux. Fluxes including solid materials with a distinct melt viscosity have shown better performance in residue spread and cosmetic as well as ionic cleanliness of the post–solder assembly surface. Improved performance also is observed in topside wicking.
Conclusion
Due to the impact of the bulk solvent system for performing wave solder fluxes with silver–/lead–free alloys in different categories, none of the individual analysis techniques such as TGA/DSC or surface tension and viscosity, nor a combination of these techniques, is capable of characterizing the performance products compatible with the alloy’s wave solder process. Only analyses of the individual raw materials and blends of the solids can provide information to the formulator developing better–performing solder fluxes.
Figure 2. A simplified model mapping the parameters in a solder flux that determine its wicking performance in lead–/silver–free solder processes.
Figure 2 illustrates a simplified map of parameters of the individual raw materials, finally defining the wicking performance of solder fluxes in alloy’s processes. The flux formulator indeed can develop performing products; however, solid knowledge of the relevant thermal properties of the raw materials is a prerequisite.
*Silver– and lead–free solder alloy information in this article is based on the SN100C alloy from Nihon Superior.
Eli Westerlaken, Gerjan Diepstraten, Bob Silveri, and Harry Trip, Cobar Solder Products, may be reached at www.cobar.com.