Materials:Solder materials for lead-free technology

A 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 electronic circuits do not fail in the field and result in catastrophe, especially, for example, electronics guidance systems in aerospace applications.

Second on the list of concerns and issues is the changing process window of the production of lead-free electronics.
When we start looking at the materials used in lead-free technology, the compatibility of different components takes
center stage.

Whereas the doctrine of the solder-joint being the key to quality and reliability of electronic circuits has evolved over the last four decades, little attention has been given to the specific properties of materials that are necessary for the formation of good lead-free solder joints.

In mainstream applications of lead-free soldering materials, it is obvious that with the introduction of Sn-Ag-Cu, or SAC-alloys, process temperatures will increase, exposing solder materials and parts of the electronic circuit to more heat.

Materials that contain both metallic and organic components must accommodate the extra heat in different ways. We could consider early evaporation of solvents, the melting, activation range, thermal decomposition and re-crystallization of the
constituents concerned as being among the various chemical and physical changes on its journey along the temperature/time line of the flux in any of the solder materials as listed in Table 1. The implications of heat in the production of solder powder––an essential component in solder paste––have, however, been thus far underestimated.



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 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.



In Figure 1, a number of interactions that impact the performance of lead-free soldering materials are shown. Heat is the independent parameter, having among other things, a major impact on the wetting of the metallic elements. When one takes the lead out of a solder alloy, even at ambient temperatures, the lead-free material will oxidize significantly quicker. As temperature goes up, the oxidation process accelerates. This in turn impacts both the topography of the solder particles as well as the surface tension of the solder. The topography of the particles is a parameter in the rheologic system and thereby affects the printing properties of the solder paste. Changes in surface tension affect the wetting of the surfaces to be joined, ultimately impacting soldering performance.

Solder paste properties
Since it is generally considered common knowledge that the majority of defects in a surface mount assembly process have their roots in the printing process, the printing properties of a solder paste are of paramount importance.

When considering the impact of heat on the flux system, three key parameters should be observed: volatilization of both the solvent systems as well the volatile fractions of other materials; melting rate, melt viscosity, and spread rate of 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, 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 have uncovered significant issues with lead-free solder paste.

These include the surprisingly short shelf life of several types of lead-free solder paste and significantly different results regarding voiding. Both phenomena have a potentially common root, and that is oxidation of the solder powder during production. It is common knowledge that oxidation appears to be self-propagating. So, when solder paste is manufactured with powder that is relatively oxidized, it will further deteriorate once it is in suspension with specific flux systems. Thus, shelf life may become surprisingly short, evidenced by a solder paste that, for example, has become hard as concrete.

The cooling rate during the transformation of the droplet into a particle, and also the atmosphere in which this solidification takes place, have an effect on the oxidation and distribution of the alloying elements on the surface of the particle. Studies, including techniques such as Atomic Force Microscopy and EDX, are performed to disclose relationships that have been underestimated until now (See Figures 2 and 3).

The general idea is that a rougher surface may be an indication of differences in oxide-levels, but also a cause of less particle mobility, which impacts the printing properties of the solder paste. The situation worsens when it appears that these properties change from batch to batch, resulting in performance differences.

More evidence will need to be presented to support the assumption that the increased rate voiding in lead-free solder connections bears a relationship to oxidation of the pads, the component metallization, or the powder in the solder paste.

The theory is that the organic material in the flux system cannot be the direct cause of the problem. The flux becomes extremely mobile when the paste is still in the pre-heat 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, showing up as voids.

Achieving thermal stability



Thermal properties of the organic materials in the wave soldering flux, solder paste, and solder wire flux is another area of focus in lead-free soldering materials. Generally, one would opt for bigger molecules in order to build in more resistance to the heat exposure of the solder reflow profile. Bigger molecules, however, are in general less mobile, affecting issues such as the rheology of a solder paste, ultimately resulting in a solder paste that will not print well (see Figure 4).

However, a well-trained and experienced formulation chemist knows how to overcome such hurdles. Oven contamination and flux management systems were common topics of discussion before lead-free became a buzzword. It is clear that when oven contamination is a problem with lead-bearing solder paste, it will be even more an issue with lead-free SAC-alloys. Increased process temperatures will decompose the condensed flux even more. This will result in increasingly difficult to remove
contaminants. In some cases, the consequence may be that maintenance will run behind, disturbing the gas flow through clogged filters, ultimately resulting in higher defect rates. One advantage to working with some of the synthetic resins is that their decomposition products are not only significantly lower in quantity, but are also much easier to remove from the reflow equipment (Figure 5).



Whereas a nitrogen blanket on a lead-free solder pot is considered an essential precaution to avoid dross formation and to reduce solder defects, it may be a consideration for lead-reflow in those cases where longer soak times are required to minimize Delta-Ts between small and large components (Figure 6).



Thus far, tombstoning in lead-free technology seems to occur less frequently than with traditional lead-containing solder paste. In cases where this occurs, 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 DT of 4oC between the initial and final melting of the solder mass on each pad. So, before the solder on 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.

Conclusion
With lead-free soldering materials, heat is the most important parameter. The challenge is to develop flux systems that provide greater thermal stability, where heat exposure dwell time is more critical than absolute temperature levels.

The introduction of lead-free technology has presented 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 has received more attention but also the surface properties of the powder, impacting its interaction with the flux system that have recently received greater recognition as important factors. Surface roughness and the 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.