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					Fluxes, Materials in Lead-free Must be Able to ‘Take the Heat’


By Lance Larrabee, Cobar Solder Products


When soldering with lead-free materials (e.g., flux and solder pastes), heat is the most
important parameter to consider, because it affects everything from component packages to
board materials to the flux. It’s fairly well understood that along with the generally higher
temperatures requisite of lead-free alloys, the risk of thermally induced damage to
components and board material is also higher. Another concern, though, is the effect of higher
temperatures on the stability and activity of the flux, and the potential for oven maintenance
issues associated with burn-off of solvents and other flux components. It’s not just about peak
temperature, though. The challenge for the flux chemist is to develop flux systems that
provide greater thermal stability during heat exposure dwell time. Establishing this stability is
more important than focusing on absolute temperature levels.


The process engineer implementing (or converting to) a lead-free process has, as a first
concern, establishing sound metallurgy and long-term joint reliability for all soldered
connections on a product or products. The achievement of such ensures long-time product
reliability and reduces the potential for failure in the field. Second on the list of concerns and
issues is the changing process window associated with the production of lead-free electronics.
When we begin looking at the materials used in lead-free soldering, the compatibility of the
different components becomes critical.


Conversion to lead-free alloys (such as Sn-Ag-Cu, or SAC-alloys) means higher process
temperatures, and thus more heat affecting every part of an assembly. The materials that
contain both metallic and organic components will accommodate this additional thermal load
in different ways.


Some of the effects on solder paste and flux associated with these higher temperatures include
early evaporation of solvents, different melting characteristics, activation range, thermal
decomposition and re-crystallization of certain constituents. These are some of the various
chemical and physical changes in the materials on their journey along the temperature/time
line of the process. One often-overlooked issue, however, are the implications of heat in the
production of solder powder - an essential component in solder paste. Heat has its own effect
on the topography of the solder particle during the solidification of the droplet. The
topography is influenced by parameters such as the cooling rate and the atmosphere in which
solidification takes place. In turn, these impact the distribution of the alloying elements on the
surface and the formation of passive films such as oxidation.


When lead is removed from a solder alloy, even at ambient temperatures, the lead-free
material will oxidize significantly faster. When the temperature rises, oxidation accelerates. In
its turn, oxidation impacts both the topography of the solder particles as well as the surface
tension of the solder. The topography of the particles are a parameter in the rheologic system
and therefore affect the printing properties of the solder paste. The changes in surface tension
affect the wetting of the surfaces to be joined, ultimately impacting soldering performance.
It is generally considered common knowledge that most defects in a surface mount assembly
process have their origins in the printing process; consequently, the printing properties of a
solder paste are of critical importance in maximizing yields.


Since heat affects flux systems; thus, three key parameters should be observed; first,
volatilization of both the solvent systems as well the volatile fractions of other materials;
second, melting, melt viscosity, and spread rate of the solid substances; and thirdly, the
decomposition of all organic materials. These parameters directly impact issues such as
SIR/electro-migration, IC testing, and condensation of volatile fractions on the electronic
circuit and in the reflow equipment. However, when the organic system breaks down
prematurely in the temperature/time line, the metallic parts in the solder joint, lacking their
protective blanket, may exhibit early and more intense oxidation.


Qualification studies and field experience by major end users of lead-free solder paste have
uncovered significant issues with the material, such as a disappointingly short shelf life of
several types of lead-free solder paste and significantly different results regarding voiding.
The chemists and metallurgists at Cobar Solder Products (www.cobar.com) have concluded
that both problems have a potentially common root, i.e., oxidation of the solder powder
during production. It is commonly understood that oxidation appears to be self-propagating;
thus, when solder paste is manufactured with powder that is partially oxidized, it will further
deteriorate once it is in suspension with specific flux systems. A QC technician opens a jar of
solder paste only to discover that the material has acquired the hardness and consistency of
concrete!


During manufacture of the powder, the cooling rate during the transformation of the droplet
into a particle, as well as the atmosphere in which this solidification takes place, have an
effect on the oxidation and the distribution of the alloying elements on the surface of the
particle. Generally, a rougher particle surface may be an indication of differences in oxide
levels, but it may also result in less particle mobility, which impacts the paste’s printing
qualities. When it appears that particle characteristics and other properties change from batch
to batch, it’s not a good sign!


Cobar’s chemists also maintain that increased rate voiding in lead-free solder connections is
related to oxidation of the pads, the component metallizations, and/or the powder in the solder
paste, rather than t he organic material in the flux system. Why? Because the flux becomes
extremely mobile when the paste still is in the preheat and soak zone. Consequently, it flows
to the boundaries of the solder joint. Metal oxides break down at higher temperatures,
releasing gaseous decomposition products when the joint is exposed to peak zone
temperatures. This happens so close to the cool-down of the exterior of the solder joint that
some of these gas-bubbles are entrapped inside the solder mass, creating voids.


Oven contamination and flux management schemes were common discussion topics well
before the advent of lead-free. Studies have shown that when oven contamination is a problem
with lead-bearing solder paste, it will be even more of a problem with lead-free SAC alloys.
Higher process temperatures will decompose condensed flux even more, making residues
more difficult to remove. If maintenance schedules are not rigorously adhered to, clogged
filters may disrupt gas flow and cause problems such as higher defect rates. A key advantage
associated with the use of synthetic resins in flux is that their decomposition products are not
only significantly lower in volume, but are also much easier to remove from reflow oven
interiors and filters.


A nitrogen blanket on lead-free solder pot is considered an essential precaution to avoid dross
formation and to reduce solder defects. The same approach can be used in lead-free reflow in
cases where longer soak times are required to minimize Delta-Ts between large and small
components.
Tombstoning appears to occur less frequently in lead-free soldering than with traditional lead-
containing solder paste, but it still occurs. In such cases, Cobar minimizes the problem with
the use of a special powder system consisting of 50% Sn95.5/Ag4.0/Cu0.5 with a eutectic
temperature of 217oC and the balance of the powder Sn96.5/Ag3.5 with a eutectic temperature
of 221oC provides a T of 4 o C. between the initial and final melting of the solder mass on
each pad. So, before the solder on of the pads of a bi-polar component is completely molten,
at least 50% of the solder on the adjacent pad is liquid as well, thereby restoring equilibrium
surface tension forces keeping the component in place.


Ever-increasing demands for higher quality electronic assemblies is the driving force toward
more consistent material performance. The introduction of lead-free technology has presented
additional challenges to deliver more batch-to-batch consistency of solder paste. It is not only
the consistency of the flux system with a higher thermal stability that is critical to this end, but
also the surface properties of the powder, impacting the interaction with the flux system.
Surface roughness and differences in the alloy between the mass of the solder particle and its
specific surface area not only impact the wetting properties of a solder paste but also its
rheology, and thereby its printing properties.


For more information, visit www.cobar.com, or contact Lance Larrabee at Cobar Solder Products, 53
Wentworth Avenue, Londonderry, New Hampshire 03053 USA, Tel. (603) 432-7500, Fax (603) 432-7519.

				
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posted:5/22/2011
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