TGA/DSC data show paste can be used in extended reflow profiles with reduced DTs even at fast printing speeds.

The widespread use of SnAgCu-based solder paste drives manufacturing toward reflow profiles considerably longer than those used for Pb-bearing products. This is due to the minimized process window that results from higher reflow temperatures, and also the limited thermal resistance of the current generation of components. The tighter process window requires a significant reduction of DTs between the small and large components. The most common way to minimize DTs is to extend the soak or ramp before the reflow phase.

The thermal breakdown of most materials is impacted more by prolonged dwell times at higher temperature levels than by temperature level itself. The consequence for the solder paste is thermal breakdown of the organic material, loss of the protective flux blanket and inferior reflow performance. Industry sectors that typically have assembly designs with major differences between small and large components have the option to reflow these assemblies in a Pb-free process with nitrogen to protect the solderability of the assembly. However, nitrogen adds cost.

This article describes the development and implementation of SAC-based solder paste for reflow processes with extended temperature profiles and without nitrogen. It explains the need for flux systems in this generation of solder paste that incorporates organic materials of longer molecular chain length. Thermal resistance studies show the advantages of these materials. Also, solutions for enhancing the mobility of these materials are provided, impacting the printing properties of SAC-based paste.

The heat profile for Pb-free SAC-alloys is elevated because the liquidous temperature of these alloys is approximately 217°C. This temperature rise by itself, however, does not jeopardize the reflow performance of many Pb-free solder paste formulations.

The Pb-free production of mobile phones is a good example. This technology may be leading the trend toward fine and ultra-fine pitch technology. Therefore, printing solder paste for that type of board may be a challenge. In the reflow process, however, these assemblies are relatively easy compared to other types of assemblies. The components on a mobile phone board generally have a relatively small and uniform thermal mass throughout the entire collection. Thus, perfect reflow results can be obtained without nitrogen by using relatively short profiles.

The real problem is that many PCBs - such as those for automotive and machine controls - have components with significant variations in thermal mass. The fact that many components cannot survive even a short-term exposure to temperatures beyond 250°C stipulates a smaller process window. Since the higher liquidous of SAC-alloys, and the 10° and 5°C margins, respectively, for diffusion and measurement inaccuracies are a given, the implication is that tighter DT's between the smallest and the largest components are key to creating an adequate process window (Figure 1).¹ The target maximum DT in Pb-free processes is generally 7°C.

The only way to achieve this is by extending the reflow profile, so that the temperatures of the largest components are permitted time to rise well beyond the liquidous temperatures, with a safety margin permitting the diffusion of the metals and for equipment and instrument variations.

Effects on Paste Chemistry

Longer exposure time to the elevated temperatures will cause accelerated melting, driving the active ingredients in the paste-flux away from the solder joint where they are supposed to react with the oxides on the metallic surfaces of the paste, board and components.

Thermal properties of a solder paste have an affect on:

• Separation.

• Printing speed.

• Open time.

• Tack time.

• Smearing.

• Beading.

• Wetting.

• Flux spattering.

• Pin-in-paste.

• Voiding.

• Residues (contact errors; SIR/electromigration; equipment contamination).

Absent a nitrogen environment to prevent materials from further oxidation, the wetting performance in the process will progressively deteriorate as the profile is extended. This will also cause thermal breakdown to 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 help the soldering process.

Nitrogen blanketing. The benefits expected from nitrogen blanketing the reflow process vary. Data from the field show that nitrogen will compensate the smaller thermal window of flux materials to a certain extent, but not entirely. The melt viscosity of the flux materials, impacting the dripping of solder paste in pin-in-paste applications, cannot be controlled by nitrogen blanketing.

Larger molecules. Solder paste is a suspension metal powder in a flux vehicle. The metal percentage as well as the particle size distribution has a significant impact on some of the rheological properties of a solder paste, such as slumping², 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, solvent(s), activator(s), rheological and numerous other property modifying additives (Figure 2).

Generally, larger molecules may offer more thermal bulk and therefore could better withstand demands of an extended profile. However, simplifying the solution to a replacement of the flux constituents, such as ordinary rosin, by materials of a larger structure in many cases appears to be a shortcut to problems in areas such printing. To understand why there is a conflict between the formulation of paste-flux with higher molecular weight materials, generally yielding a higher thermal resistance and a smooth printing performance of the solder paste, we shall first discuss some basics of solder paste composition.

The flux vehicle of a solder paste consists of functional groups such as resins, rheological and other property modifying additives. They affect the mobility of the system, solvent retention properties, long- and short-term dielectric properties and thermal behavior. The entire formulation forms a complex of short-chained linear substances, long-chained linear and even branched molecules. Some of those substances will truly dissolve in the solvent system, while others will swell to form a colloidal structure.

The printing of solder paste, in particular the cutting of the wet deposit by the squeegee, occurs in the high shear rate range. In this context it is important to realize that rheological additives alone do not determine the overall rheology of a paste. All constituents contribute to the flow properties of a product. Besides the load and size of the metal particles, the resin system, solvents and some property modifying additives primarily affect the high-shear rate viscosity of a paste and thus its printing properties. Generally the high-shear rate viscosity will increase as the metal content or molecular weight of the resin system increases. Also, when the particle size of the metal powder or the strength of the solvent system decreases, the high-shear rate viscosity of the paste increases (Figure 3).

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 such cases 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 rapidly restore the structure of the network. 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.

Each rheological network has specific requirements with regard to its processing temperatures to develop its optimal degree of entanglement. The processing temperatures required are related to the solvent system that is used. The rheological network impaýts 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 heat profile in the reflow equipment and the post-reflow properties of the organic residue are determining factors in the selection of the chemistry that builds the flux vehicle for solder pastes.

Stencil life and tack time of 8 hrs. or longer requires 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. That is, because - 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 - regardless of the usual tolerances of physical and chemical properties of the raw materials, and the loss of traces of the solvent during production and application of the paste - the window for solubility power should be adequately large to deal with all aforementioned tolerances and still provide consistent printing properties.

Low processing temperatures during production of the flux usually results in incomplete development of the rheological network. Excessively high processing temperatures during production, storage or transportation may partially dissolve some groups in thœ rheological network and will cause disentanglement and the formation of agglomerates, causing inconsistencies in the visco-elastic behavior of the paste.

Thermal Resistance and Printing Properties

The first step in any development of the thermal properties of a solder material is to gather data on typical thermal histories of soldering processes. This can be done by wiring a test board with thermocouples at precise top- and bottom-side points with varying thermal mass. In one study, using scans from a compact telemetric thermal indicator, it was easy to see from profiles 4 and 5 in the diagram a substantial amount of thermal energy going into the assemblies during preheat/soak that could effectively burn out many organic materials before they can do their job in the reflow process.

The key to maximizing the thermal resistance of the paste-flux materials as well as printing performance lies in a thorough understanding of interactions between these polymers and certain property-modifying additives.

Using thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques, we have characterized the crystallization behavior, sublimation energy and optimum activity range of several organic systems suitable for use as filmformers, rheological additives and flux activators (Figure 4). With this information, it is possible to tailor the key materials such as resins and activators for a specific thermal profile and reflow atmosphere. Properly applied, these techniques - in conjunction with the tuning of ratios of property modifying additives - permit substantive improvements in thermal resistance while maintaining exceptional rheological properties such as resistance to slumping and smearing, and optimum printing performance.

The TGA/DSC techniques readily adapt to different atmospheres such as nitrogen. The formulation of no-clean, Pb-free solder paste by its nature requires defining the optimum balance between two main objectives that initially seem to conflict: thermal stability and printing performance.

Additional requirements such as certain pin-in-paste applications require a specific melt viscosity of material along the temperature time line to prevent paste from dripping from connector pins before reflow has begun.

There is an analogy between a racing car with a strictly controlled weight limit and a no-clean Pb-free reflow process. The way to make performance improvements in the racing car is not to design bigger and bigger engines, but rather to tune an engine of optimum size as carefully and precisely as possible and boost performance by technical skill rather than brute force. With paste flux we cannot just add another shovel of activator to the tank. We must carefully and accurately characterize the behavior of the paste flux as it travels along a known thermal history. We can eliminate chemical dead weight and optimize ratios between the key constituents and - by the same techniques - define the balance between maximized printing performance and the highest thermal resistance to withstand the rigors of the Pb-free and N₂-free reflow processes.

Larger molecules will create a stronger entanglement in the rheological network. Therefore, substantial ratios of this material contribute to a solder paste that may survive virtually any extended reflow profile, but will require a tractor to move it across the stencil during the printing process.

We have seen that some resins and activators are better than others in certain applications, but no single resin nor activator is universally better than all others in all applications and in all soldering systems.

If in the ramp or soak zone we can continuously optimize the cleaning of the metal surface at lower temperatures with organic materials that are relatively more volatile, the surface will become substantially more solderable, whereas, on the other hand, we can incorporate a sufficient ratio of materials with an adequately high melt viscosity, thus preventing the paste flux from early migration away from the solder joint. When the most thermally stable part of the organic materials in the paste flux finally kicks in, the joint formation at the end of the extended high temperature profile can successfully be accomplished, leaving a relatively clean surface that features excellent dielectric properties.

When using TGA/DSC, it is important to study the resolidification behavior of the flux materials which details the temperature, time and place where fumes from these substances will resolidify and deposit 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 horsepower is increased when they enter the more mobile liquid melt phase. Therefore, a rough correlation between melt range and cleaning efficiency exists. While the sharpness of the melt range is an excellent indicator of the purity of the starting material (an essential parameter to reduce product variation in today's solder 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.

TGA/DSC permits tailoring of systems, such as 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 or some additional peaks at a certain temperature level. One can also 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 cleaning of the substrate and provide a larger window for the fluxing reaction. A well-designed system preferably exhibits a single, very narrow and sharply defined peak upon cooling. That implies a single highly ordered (crystalline or amorphous) resolidification. Ideally most of the synergists have been volatilized at the reflow temperatures. This facilitates the design of a flux management system by reflow equipment suppliers.

The more complex mixture survives the Pb-free reflow process by a small but safe margin. According to its no-clean objective, a reduced but effective amount of the material survives the reflow-soldering process to complete joint formation.

The 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. 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 and precise shape to promote the wetting of the metallic surfaces, but it is also important to ensure that the molten solder mass smoothly, reliably and completely parts from the nonmetallic upon reflow.

As with most other functionalities in chemistry, there is no ideal, universal surfactant suitable to meet all requirements and morphologies. Obviously, different surfaces have different morphologies, yielding different surface energy states, and no single type of surfactant will interact with all types of surfaces. Advanced surface chemistry in a no-clean Pb-free solder paste uses a range of chemically different surfactants at carefully tuned, relatively low ppm ratios to interact with many types of different morphologies. Many surfactants perform better and in a more universal way when they can work with other carefully-selected surfactants.

A properly designed surfactant system will assist in repelling the hot liquid solder mass from nonmetallic areas, thereby reducing the occurrence of beading and solder balling, and will also improve paste printing or dispensing performance.

Additions in the low-end ppm range of an advanced, well-designed system of surface chemistry will contribute to higher SIR values. However, high ratios of surfactants would create more cons than pros:

• Air bubbles in the wet paste deposit.

• More residue.

• Reduced SIR values.

• Increased costs.

The surface chemistry in solder paste should improve (and not jeopardize) the dielectric properties of the assembly after reflow. The surface chemistry needs to survive the process to such an extent that it can perform the task it has been formulated for. In addition to printability, chemical and thermal stability are the keywords in this context.

Conclusions

Using the data from TGA/DSC, we adjusted the retention time of our solvent system and modified our resin, activator and synergist system to remove the paste flux constituents that caused the anomaly, replacing them with more effective systems. Further, the developed flux system displays exceptional reflow soldering ability as well as strict compliance with world standards with regard to reliability criteria. With advanced rheometry such as specially defined sweep tests², we were able to predict the printing performance in application testing.

Therefore, solder paste can be designed for use in Pb-free applications using extended reflow profiles to achieve reduced DTs while avoiding the use of nitrogen blanketing, even if it requires substantial printing speeds.

Ed.: This paper was first presented during IPC Apex in February 2006 and is reprinted with permission of the author.

References

1. Günter Grossmann, Grundlagen des Weichloetens.

2. Ineke van Tiggelen-Aarden, "A Simpler Approach to Cost-Effective Solder Paste Testing," IPC Apex Conference Proceedings, February 2003.

 

Eli Westerlaken is president and CEO, Cobar Europe BV (cobar.com); e.westerlaken@cobar.com.

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