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:
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:
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
Günter Grossmann, Grundlagen des Weichloetens.
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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