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Lead-free: Component Compatibility Takes Center Stage
December 31, 1969 |Estimated reading time: 7 minutes
The role of a solder joint in the quality and reliability of electronics circuits has evolved. However, little attention is given to specific material properties required to create a viable solder joint. when considering materials in lead-free technology, component compatibility is key.
By Lance Larrabee
key issue and goal in lead-free technology is sound metallurgy and long-term joint reliability. This is critical to ensure that future generations of electronics circuits do not fail in the field. Second on the list of concerns and issues is changing process windows of lead-free electronics production. When we look at the materials used in lead-free technology, the compatibility of different components takes center stage.
The doctrine of the solder joint as key to quality and reliability of electronics circuits has evolved over the last four decades. Still, little attention is given to specific properties of materials that are necessary to form a good lead-free solder joint.
In the mainstream of applications with lead-free soldering materials, it is obvious that with the introduction of Sn/Ag/Cu, or SAC alloys, process temperatures will increase. This implies that more heat will be exposed to solder materials and electronic circuits. Materials that contain both metallic and organic components must accommodate this extra heat in different ways.
We could consider early evaporation of solvents, melting, activation range, thermal decomposition and recrystallizing of constituents as some of the chemical and physical changes along the temperature/time line of the flux in any solder material. The implications of heat in the production of solder powder - an essential component in solder paste - has been underestimated. Heat has its own effect on the topography of solder particles during the solidification of the droplet. Parameters such as cooling rate and the atmosphere in which solidification takes place influences topography. In turn, these parameters impact the distribution of alloying elements on the surface, and the formation of passive films such as oxidation.
Figure 1. A mind-map approach to the interactions that should be addressed when designing lead-free solder materials.
Several interactions that impact the performance of lead-free soldering materials are shown in Figure 1. Heat is an independent parameter and has a major impact on wetting of metallic elements. When lead is removed from a solder alloy, even at ambient temperatures, the lead-free material will oxidize more quickly. When the temperature rises, the oxidation processes accelerates. Oxidation impacts the topography of solder particles and solder surface tension. The topography of the particles are a parameter in the rheologic system and thereby, the printing properties of the solder paste. Changes in surface tension affect the wetting of surfaces to be joined, ultimately impacting soldering performance.
The Importance of Printing
Because it is considered common knowledge that the majority of defects in a surface mount assembly process have their roots in the printing process, printing properties of a solder paste are extremely important. When considering the impact of heat on the flux system, three key parameters should be observed: volatilization of both solvent systems and volatile fractions of other materials; melting, melt viscosity and spread rate of solid substances; and decomposition of all organic materials. These parameters directly impact issues such as SIR/electro-migration, IC testing and volatile-fraction condensation on electronic circuits and in reflow equipment. When the organic system breaks down prematurely in the temperature/time line, however, metallic parts in the solder joint that lack a protective blanket may exhibit early and more intense oxidation.
Qualification studies and field experience have uncovered significant issues with the material, including shortened shelf life of several types of lead-free solder paste and significantly different results regarding voiding. Both phenomena have a potentially common root, which is oxidation of solder powder during production.
Figures 2a and 2b. Lead-free solder particle with Sn95.5/Ag4.0/Cu0.5 (a). EDX analysis results in percentage distribution on the surface of Sn94.6/Ag4.4/Cu1.0 (b).
Oxidation appears to be self-propagating. Therefore, when solder paste is manufactured with powder that is relatively oxidized, it will deteriorate further once it is in suspension with specific flux systems. Shelf life may be shortened, evidenced by a solder paste that has become extremely hard. Studies, including techniques such as EDX (Figures 2a; 2b) and Atomic Force Microscopy (Figure 3), are performed to disclose relationships that have been underestimated until now.
Figure 3. 3-D image of surface of 64 µm2 of an Sn95.5/Ag4.0/Cu0.5 solder particle generated by Atomic Force Microscopy.
More evidence is needed to support the assumption that an increased voiding rate in lead-free solder connections bears a relationship to pad oxidation, component metallizations or solder paste powder. The theory is that organic materials in the flux system cannot be the direct reason for this problem. 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 gas bubbles are trapped inside the solder mass, resulting in voids.
Thermal Properties
Thermal properties of organic materials in wave soldering flux, solder paste and solder-wire flux is another area of focus. Assemblers generally would prefer larger molecules to build-in more resistance to heat exposure of solder reflow profiles. Larger molecules, however, are less mobile, affecting solder-paste rheology and resulting in solder paste that will not print well (Figure 4).
Figure 4. A typical search plan for more thermal stability in a flux system implies searching for bigger molecules. The latter may affect system mobility, causing differences in printing properties.
A well-trained and experienced formulation chemist knows how to overcome these hurdles. Oven contamination and flux-management systems were common topics of discussion before lead-free became a buzz-word. When oven contamination is a problem with lead-bearing solder paste, it will be more of an issue with lead-free SAC alloys. Increased process temperatures will decompose the condensed flux. This results in contaminants that are increasingly difficult to remove. In some cases, the consequence is that maintenance will run behind, disturbing the gas flow through clogged filters and causing higher defect rates. One advantage to working with synthetic resins is that their decomposition products significantly are lower in quantity and easier to remove from reflow equipment (Figures 5a; 5b). While a nitrogen blanket on a lead-free solder pot is considered essential to avoid dross formation and reduce solder defects, it may be a consideration for lead-free reflow where longer soak times are required to minimize Delta T (ΔT) between small and large components.
Figures 5a and 5b. The gas filtration system of a reflow oven contaminated with dusty deposit built-up over 33 shifts (a). Users of this paste technology have seen savings of up to 65% in maintenance time and money (b).
Although tombstoning in lead-free technology seems to occur less frequently than with traditional leaded solder pastes, some cases have been observed. In those cases, 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 ΔT of 4ºC between initial and final melting of solder mass on each pad. Therefore, before the solder on one of the pads of a bi-polar component is completely molten, at least 50% of the solder on an adjacent pad is liquid. This restores equilibrium surface-tension forces - keeping the component in place.
Conclusion
Heat is the most important parameter with lead-free solder materials. The challenge is to develop flux systems that provide greater thermal stability, whereby the heat exposure dwell time is more important than absolute temperature levels. Despite the fact that our industry has to switch to lead-free technology, increasing demands for higher quality of final electronics products is the driving force for achieving more consistent material performance. The introduction of lead-free has presented additional challenges to delivering more batch-to-batch consistency of solder paste. While flux system consistency with higher thermal stability has received attention, a number of surface properties of powder impacting the interaction with the flux system have received greater recognition as important parameters. Surface roughness and differences in alloys between the mass of the solder particle and its specific surface area impact both wetting properties of solder paste and rheology; and therefore, its printing properties.
Lance Larrabee, general manager, Cobar Solder Products, Inc., may be contacted at (603) 432-7500; e-mail: info@cobar.com.