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The Silver Debate: Sn/Cu-based Solder Explained
December 31, 1969 |Estimated reading time: 13 minutes
By Gerjan Diepstraten; Harry Trip; Ineke van Tiggelen-Aarden; and Di Wu, Ph.D., Cobar BV
Lead-free solder alloys with micro-additions like nickel and germanium are finding increased acceptance globally, entering the mainstream for high-reliability PCBAs. Thoroughly understanding solder paste and flux materials is key to maximizing print and reflow performance.
Like virtually all lead-free solders, tin/copper/nickel (Sn/Cu/Ni)-based alloys melt at a higher temperature than lead-based solders, driving the industry toward thermal profiles that are considerably more demanding to all the materials on the circuit assembly, including the solder paste.
Also, similarly with SAC-based solder paste, Sn/Cu-based solders’ reflow profiles are considerably longer than those used for lead-bearing products. This is due to the smaller process window, a result of higher reflow temperatures and the limited thermal resistance of the current generation of electronic components. The smaller process window requires a significant ΔT reduction between small and large components. The most common way to minimize ΔT is to extend the soak or ramp before reflow. Additionally, a flat head curve in the peak zone has become a trend for lead-free profiles.
The thermal breakdown of most materials is affected more by prolonged dwell times at higher temperatures than by the temperature level itself. The consequence for the solder paste is thermal breakdown of its organic material, losing the protective flux blanket and creating inferior reflow performance. Assemblers using PCBA designs with major differences between small and large components can add nitrogen to the lead-free process to protect the assembly’s solderability. Since electronics manufacturing is a cost-driven industry, however, adding nitrogen is an undesirable cost factor.
Solder Powder
There are compelling reasons to study the relationship between the higher liquidus of the Sn/Cu-based alloy and the topography of its solder powder, as early developments with paste revealed that some of these materials not only showed a lower wetting potential, but also an inferior mobility when compared to traditional solder paste.
Considering the large surface-to-volume ratio of the lead-free Sn/Cu-based alloy in its powder form, it may not be a surprise that the solder paste formulation may require special actions relative to the higher liquidus.
The impact of oxidation and metallic micro-additions on the topography of solder particles has thus far been underestimated. This factor was of negligible concern in lead’s era, since lead in the alloy made life a lot easier for the solder paste formulator. Its presence ensured minimum oxidation of the particles when compared to lead-free equivalents. Micro-additions were not in demand, nor required.
In previous studies,1 we have observed significant correlations between the cooling rate of the powder and its topography. Because of the difference in thermal mass, the cooling rate variations have a larger impact on smaller particles. The industry’s current trend toward class 4 powder and finer will gain importance for this phenomenon.
Topography has a marked impact on the wetting performance and printing properties of any given solder paste. A majority of defects in a surface mount assembly process have their roots in the printing process; therefore, the printing properties of a solder paste are of paramount importance.
Need for Extended Reflow Profiles
Reflow profiles for lead-free alloys are elevated to a higher level as a natural function of liquidus temperature. This temperature rise by itself does not jeopardize the reflow performance of many lead-free solder paste formulations.
The real problem lies in the fact that many circuit assemblies – automotive, machine control boards, etc. – have components with significant variations in thermal mass. Many electronic components cannot survive even a short-term exposure above 250°C, shrinking the available process window. Since the higher liquidus of lead-free alloys, and the 10° and 5°C margins for diffusion and measurement inaccuracies, respectively, are given facts, the implication is that smaller ΔTs between the smallest and the largest components are key to creating an adequate process window.2 The target maximum ΔT in lead-free processes generally is set at 7°C.
Figure 1. Optimal wetting angle, toe and heel fillet, and wicking properties on a poorly finished termination.
The only way to achieve this is by extending the reflow profile, so that the largest components are allowed time to heat well beyond the liquidus temperatures, adding a safety margin for metals diffusion and equipment/instrument variations.
The key to maximizing the thermal resistance of paste/flux materials, as well as printing performance, lies in a thorough understanding of interactions between the polymers and certain property-modifying additives.
Effect of the Extended Profile on Solder Paste Chemistry
Longer exposure to elevated temperatures will cause accelerated melting, driving the paste/flux away from the solder joints where they are supposed to react with the oxides on the metallic surfaces of the paste, the circuit board, and the components. When there is no nitrogen environment to prevent the materials from further oxidation, wetting performance in-process will deteriorate progressively as the profile is extended. This also will cause thermal breakdown in the organic system of the paste/flux. Depending on their molecular structure, the flux materials will sublimate or decompose to an extent that they can no longer facilitate the soldering process.
Need for Larger Molecules
Solder paste is a suspension metal powder in a flux vehicle. The metal percentage as well as the particle size distribution have a significant impact on some of the rheological properties of a solder paste, such as slumping,3 print definition, smearing, and shorts. Paste/flux is a complex composition of multiple polymer species ranging from relatively simple, slightly modified wood rosins to larger-molecular-weight resin systems, solvents, activators, rheological, and numerous other property-modifying additives.
The flux vehicle of a solder paste consists of functional groups such as resins, rheological additions, and other property-modifying additives. They affect system mobility, solvent retention properties, long- and short-term dielectric properties, and thermal behavior. The entire mixture forms a complex collaboration of short-chained linear substances, long-chained linear, and even branched molecules. Some of those substances will dissolve in the solvent system, while others will swell to form a colloidal structure.
Using thermo gravimetric analysis (TGA)/differential scanning calorimetry (DSC) techniques, we can characterize the crystallization behavior, sublimation energy, and optimum activity range of several organic systems suitable for use as film formers, rheological additives, and flux activators. With this information, it is possible to tailor key materials like resins and activators for a specific thermal profile and reflow atmosphere. Properly applied, these techniques – in conjunction with tuning ratios of property-modifying additives – allow substantive improvements in thermal resistance while still maintaining exceptional rheological properties such as resistance to slumping and smearing and optimum printing performance.
The formulation of no-clean, lead-free solder paste by its nature requires an optimum balance between two main objectives that initially seem to conflict: thermal stability and print performance.
Larger molecules such as those of synthetic polymer will create a stronger entanglement in the rheological network. Therefore, substantial ratios of this material contribute to a solder paste that, on one hand, may survive virtually any reflow profile, but, conversely, may require a tractor to move across the stencil during printing.
By experience and observation, we find that some resins and activators are better than others for certain applications, but there is not a single resin or activator that is universally better than all others in all applications and soldering systems.
If, in the ramp and/or soak zone of the reflow process, we can continuously optimize cleaning of the metal surface at lower temperatures with organic materials that are relatively more volatile, and incorporate a sufficient ratio of materials with an adequately high melt viscosity, we prevent the paste/flux from early migration away from the solder joint. The surface will become substantially more solderable. When the most thermally stable part of the organic materials in the paste/flux finally kicks in, joint formation is accomplished successfully at the end of the extended high-temperature profile, leaving a relatively clean surface with excellent dielectric properties.
Figure 2. Process window of a Sn/Cu + metallic additives solder paste (time/temperature profile). FAT = flux activation time, TAL = time above liquidus.
Studying the resolidification behavior of the flux materials will detail the temperature and, by implication, the time and place where fumes from these substances will resolidify and possibly redeposit on the board in the reflow oven. Resins and many organic acids, unlike halide salts, are only weakly ionic in solvent solution. Their metal-cleaning power is increased substantially when they enter the more mobile liquid melt phase. Therefore, there is a rough correlation between melt range and cleaning efficiency. While the sharpness of the melt range is a strong indicator of the purity of the starting material (an essential parameter to reduce product variation in today’s soldering materials), the position of the peak is important because it gives a quasi-empirical indication of the temperature range at which the activator kicks in.
Figure 3. The SN100C paste’s excellent thermal stability outperforms a typical lead-free solder paste in a simulated 5-minute heating profile in a TGA. The blue line shows more activity at higher temperatures.
By using techniques such as DSC/TGA, one can tailor systems – solvents, resins, activators, and surfactants – enhanced by the addition of several synergistic non-acid materials to improve solvent retention times and to broaden the melt peak substantially, as well as select additional peaks, at a certain temperature level. One also can manipulate the melt temperature of resins and the bulk activator, indicating an earlier availability of the soldering power of these major constituents. The modified temperature peaks assist in initial substrate cleaning and provide a larger window for the fluxing reaction. A well-designed system preferably exhibits a single, very narrow, sharply defined peak upon cooling. That implies a single highly ordered (crystalline or amorphous) resolidification. Ideally, most of the synergists have been volatilized at reflow temperatures. This facilitates the design of a flux management system by reflow equipment suppliers.
Stencil Life and Thermal Stability
The stencil life of a solder paste is defined by the evaporation rate of the solvents and its thixotropy. TGA can help determine the flux evaporation rate. The technique shows how much weight loss occurs in time at a given temperature level. The more solvent evaporates, the more the high-shear-rate viscosity will increase and make printing more difficult. Another factor that affects the high-shear-rate viscosity of the paste is temperature. At 250 Pa*s, Sn/Cu/Ni paste reaches optimal printing performance. The recommended ambient conditions for paste by printing or dispensing are 22° to 28°C and 30?70% relative humidity (RH).
The Rheological Network, Driving Force of the Printing Process
Printing solder paste, in particular when a squeegee cuts the wet deposit, occurs in the high-shear-rate range. In this context, rheological additives alone do not determine the overall rheology of a solder paste. All constituents make their contribution to the flow properties of a product. The load and size of metal particles, the resin system, solvents, and some property-modifying additives primarily affect the high-shear-rate viscosity of a solder paste and thus its printing properties. Generally, high-shear-rate viscosity will increase if the metal content or molecular weight of the resin system increases. Also, when the particle size of the metal powder or solvent system strength decreases, the high-shear-rate viscosity of the solder paste will increase.
The short-chained fractions in the network are only physically and relatively weakly entangled. This in particular is the case with solder paste with a distinct yield point. In that instance, the rheological network has a more physical nature characterized by dipole forces, hydrogen bridges, electrostatic, and/or Van Der Waals forces. The bonds are easy to break and will restore the network’s structure rapidly. A matrix of surfactants can boost the instant restructuring of the rheological network. Upon full development of the mix of substances in the solvent system, the molecules will entangle and form the rheological network.
Figure 4. At 0.4-mm pitch, perfect wicking of Sn/Cu-based solder up the termination, although the component’s finish shows exposed copper.
Each rheological network has specific requirements for processing temperatures to develop its optimal degree of entanglement. Processing temperatures are related to the solvent system. The rheological network impacts the required printing properties. The solvent system is predominantly a function of both the required stencil life and tack time of the solder paste, as well as the solubility power required with regard to the substances selected to form the rheological network. Furthermore, the reflow thermal profile and post-reflow properties of the organic residue are determining factors for the chemistry that builds the solder pastes’ flux vehicle.
The stencil life and tack time, which can be eight hours and longer, require a solvent system with extremely low volatility at ambient temperatures. Even without this prerequisite, the solder paste formulator has to tackle many problems to select the right solvent system. As with most other functionalities in chemistry, there is no such thing as the ideal universal solvent when it comes to solubility power for the considerable number of different types of organic materials that may form the flux vehicle.
To ensure maximum consistency and efficiency, the window for solubility power should be adequately large to deal with usual tolerances of physical and chemical properties of the raw materials, and lost traces of the solvent during production and paste application.
Importance of Surfactants
Solder paste is a complex suspension with a substantial number of different liquid/liquid and solid/liquid interfaces in terms of surface chemistry. The true role of our understanding of interfacial chemistry lies in instantly restoring the rheological network after shear rates have been removed and in perfect wetting of materials to be soldered. Not only is it necessary to deposit material in a precisely defined area, in a precisely defined shape, but also we must ensure that the molten solder mass smoothly, reliably, and completely separates from the nonmetallic upon reflow.
As with most other functionalities in chemistry, there is no such thing as the ideal universal surfactant suitable to meet all requirements and compatible with all morphologies. Obviously, different surfaces have different morphologies yielding different surface energy states and it cannot be expected that a single type of surfactant will interact with all types of surfaces. Advanced surface chemistry in a no-clean lead-free solder paste uses a range of chemically different surfactants at tuned, relatively low parts per million (ppm) ratios to interact with many morphology types. Many surfactants perform better and in a more universal way when they can work synergistically with other carefully selected surfactants.
A properly designed surfactant system will assist in repellence of the hot liquid solder mass from the nonmetallic areas – reducing the occurrence of beading and solder balling – and will improve the paste’s printing/dispensing performance.
Conclusion
Using the data from TGA, we are able to adjust the retention time of our solvent system and modify our resin, activator, and synergist system to remove the paste/flux constituents that cause negative anomalies and replace them with other, more effective systems. In modern lead-free Sn/Cu-based solder, the developed flux system displays exceptional reflow soldering ability as well as strict compliance with global reliability criteria. With advanced rheology, concurrent development of high-speed properties in ultra-fine-pitch applications and long stencil life are a reality.SMT
REFERENCES
- SN100C alloy patented by Nihon Superior Ltd.
- Ineke van Tiggelen-Aarden, Eli Westerlaken, “Oxidation and Topography of Powder in Pb-free Solder Paste,” Proceedings, APEX 2005.
- Ineke van Tiggelen-Aarden, “A Fast, Precise, and Reproducible QC-Rheometry Routine for Solder Paste,” Proceedings, APEX 2004.
Gerjan Diepstraten, process support manager; Harry Trip, product applications manager; Ineke van Tiggelen-Aarden, technical director; and Di Wu, Ph.D., R&D scientist, Cobar BV, may be contacted at Cobar Europe BV, PO Box 3251, 4800 DG Breda, Holland. +31 76 544 55 66; g.diepstraten@cobar.com.