Basic Metallurgy and Wavesoldering Trends

Estimated reading time: 11 minutes

For wave soldering to be fully accepted in the electronics industry, metallurgists, industry and governing agencies must work together and extensive research must be conducted.

By Jason M. Smith

Since its inception, wave soldering has continuously evolved. The basic metallurgy principles involved in soldering have been overlooked by many nonmetallurgists to find suitable materials that meet today's demand and drive for a friendlier environment. Research is necessary to determine and understand the kinetics of "drop-in" replacements to the widely accepted solder of choice in wavesoldering processes. It is necessary to review basic metallurgy principles to develop and understand future proposed alternatives.

Wavesoldering EvolutionFrom the 1920s to the 1940s, interconnections were made using a wiring method with soldering irons. The development of the printed circuit board (PCB) necessitated a more economical and robust method of creating soldered connections. The first mass soldering concept was dip soldering in England. In the 1980s, the concept known as wave soldering was developed. This method still is widely used today, but machine and operator controls have been changed for the better. The basics of soldering remain the same. Solder forms have only changed to meet equipment demands; however, the compositions and theoretical kinetics are still basic and simple. Attachment methods basically only need flux, heat and solder to create a metallurgical connection.

Figure 1. Eutectic composition of Sn/Pb.

Flux is used to clean the oxidized surfaces to be soldered. Heat is added to remove flux carriers and reduce thermal shock, adding increased heat to the dissimilar materials that make up the circuit assembly. The materials found on an assembly can host plastics, ceramics, metals, coatings, chemicals and their vastly different compositions. Mass wave soldering use presents a concern for component solderability because of the need to make the connections right the first time and not performing rework on the assembly, which is not as tolerant on today's products as it was on less complicated assemblies. Reliable joints need to be made correctly the first time to withstand the environments that PCBs are exposed to. Each situation must be analyzed in accordance with thermal and mechanical strains induced while ensuring proper signal transfers, crosstalk elimination and unacceptable vertical-standing wave ratio.1

Figure 2. Non-eutectic composition of Sn/Pb.

The first dipping method had problems: it was very difficult to reproduce desired yields; laying the board in molten solder trapped gases underneath, interfering with heat transfer and solder contact; solder can only wet to metallic surfaces; and dross (a composition of oxidation and burned flux) had to be skimmed off, continuously hindering production.2 This set of problems led to the introduction of wave soldering. This method uses a wave of molten solder raised from a pot or bulk surface to meet the PCB, which then is transferred across the wave. Wave soldering shortened contact time by more than one half. The transferring conveyor system usually is on an angle so the board will not trap anything under the PCB as it goes across the wave. This incline also allows molten solder to peel off into the pot, reducing bridging between neighboring solder joints. Because the molten metal is pumped from below the molten bath surface, only clean, oxidation-free metal is introduced to the assembly.

Soldering KineticsThe reaction that takes place when creating a soldered joint is basic in principle. The solder alloy is heated well into its liquidus region to promote solder joint wetting. Oxidation is removed from the metal surface to ensure a clean contact between the joint and the molten solder with flux. The flux is then preheated to remove the flux solvent (usually water or alcohol) from the PCB. The increased heat is needed to overcome the temperature difference between the PCB and the molten solder bath. The PCB is heated to compensate for the thermal difference, not to cause any damage to the components. The PCB is run across the wave with the necessary metal areas exposed. Solder wets to the metals at proper bonding and wetting angles. The surface energy and contact angle depict the molten solder adhesion to the exposed metals. The liquid wets and runs if the surface energy of the solid is high relative to the sum of the liquid and solid/liquid interface surface energies. Capillary action takes solder to the topside of round plated holes in the PCB.

In some systems, nitrogen-inerted soldering atmospheres are used to promote the wetting/capillary action. These holes usually are interconnecting layers in the assembly, indicating:

1. The height to which a liquid rises in a capillary space increases as the separation of the surfaces is reduced.

2. The flow rate into the joint decreases as the separation of the surfaces is reduced.

Metallurgical factors have an important and often dominating effect on the soldered connection.3 The molten solder creates an intermetallic layer between the solder lead and the bulk of the molten solder added to make the connection. After cool-down, a solder joint remains.

Intermetallic Formation and GrowthUntil the connection is cooled for handling, the intermetallic layer continuously grows. The growth rate is linear with the square root of time at a specific temperature and exponential with temperature. This suggests that growth is controlled by the diffusion of the interacting atoms to the interface. This intermetallic layer generally is 1 µm of Cu6Sn5. The Cu comes from the PCB interconnect and the Sn comes from the solder alloy.

Intermetallics have properties that arise from a mixture of metallic and covalent bonds. These bonds are strong with a high modulus. The self-diffusion coefficients and, hence, greater stability of diffusion-controlled properties are results of the strong bonding and ordered structure.4 This bonding is good for connections until their growth governs the property of the joint; it is then deemed detrimental to the assembly.

Soldering MaterialsThe preferred alloy for wavesoldering processes today is eutectic 63Sn/37Pb because of its cost and availability. Sn provides the interconnect characteristics and Pb is used as a filler material. The demand for increased production calls for materials that solidify quickly and can make hundreds of joints in a few seconds. The common name given to eutectic solder is misleading. The designated composition is not the true eutectic composition. The eutectic composition by weight percent is 61.9 percent Sn, as seen in Figure 1. The discrepancy arises from earlier miscalculations of the eutectic composition. Compositions of higher Sn content cannot justify the increased cost vs. the improved performance in electronics assembly. Only when an assembly is used in a corrosive environment is the cost justified. Metallurgically, solders can be viewed as simple mixtures of pure metals constituting a binary alloy. The alloy diagram is a typical diagram for a binary alloy system and basic metallurgy principles do apply. As expected, the properties and characteristics of various alloys differ when swaying from the eutectic. The liquidus temperature increases, density increases, hardness decreases, the coefficient of thermal expansion (CTE) increases, and thermal and electrical conductivities decrease as the Sn content is decreased in the alloy.1

Figure 3. Sn/Cu binary diagram.

Non-eutectic CompositionsWhen considering a non-eutectic alloy, it is assumed to consist of a plus eutectic, where the lever law from Figure 2 governs the fraction. Demonstrations prove that it is possible to solidify so that no dendrites are present and the entire microstructure matches the eutectic. The composition is an average composition. How are eutectic structures attained at non-eutectic compositions? The rate of cooling to solidify is faster than the solid transformation kinetics. The average composition of the alpha phase is cored when compositions past the solid solubility limit are cooled at room temperature. The transformation kinetics to convert the solid phases are far surpassed by the solid transformation. When ambient air cools the solid solution, the remaining liquid may undergo the eutectic reaction giving a eutectic microstructure at room temperature at non-eutectic compositions. When a pure binary liquid of eutectic composition freezes, the average composition of the solid is identical to the liquid when it forms. Reportedly, there is no solute buildup and no constitutional buildup in front of the a plates and a solute depletion in front of the b plates. These solute profiles can produce constitutional supercooling even though this phenomenon is not a sufficient condition for planar instability.5 In microstructures, it is sometimes convenient to use the term microconstituent, i.e., a microstructure element having an identifiable and characteristic structure. In Figure 2, the grains of the primary microconstituent are cored and the fraction of eutectic microconstituent formed is greater than the solder alloy equilibrium situation.

As solder pots in wavesoldering machines are run for long periods of time, the solder exposed to all metals can have effects different than originally attained. Oxidation and intermetallic formation change the solder pot composition over time and when the composition changes, the properties also change. Set points must be altered and monitored to control defects that could arise from the metallurgical changes in pot alloy composition.

Lead-free Wave SolderingApproximately 60,000 metric tons of solder are used annually worldwide. While electronics assembly is not the major user, there is an increased worldwide focus to reduce lead use because of its toxicity and mishandling by recyclers.6 The switch to lead-free is not widely accepted by the industry. The main reason to omit lead in electronics assembly is machine operator exposure, which is strictly scrutinized by the Occupational Safety and Health Administration's (OSHA) PEL limits. Dross byproduct disposal can have serious environmental impact if not handled, transported, recycled and disposed of properly. Ingestion of lead fumes and direct contact when hand soldering also have a major impact when appropriate hygiene practices are not followed.

For a lead-free alternative to be accepted, it must offer the following:

• Availability in adequate quantities
• Compatibility with existing processes
• Adequate melting temperatures
• Good joint strength
• Thermal and electrical conductivity similar to Sn/Pb
• Easily repairable
• Nontoxic
• Low cost.

Many companies are developing suitable replacement alloys as "drop-in" replacements from mandates in Europe and Japan. These mandates propose lead reduction in assembly processes by 2002 and elimination by 2004. Waste from Electrical and Electronic Equipment (WEEE) is the directive ban from the European Union's proposal.7

North American-based National Electronics Manufacturing Initiative's (NEMI) goal is to equip North America with the capability to produce a lead-free alternative by 2001. The organization is aiming to develop standards for its manufacturability in conjunction with other agencies whose directives focus on selecting an alternative, composing worldwide databases and collecting material property data.

Processing ConcernsThe International Tin Research Institute (ITRI) launched SOLDERTEC, a lead-free soldering technology center to disseminate leading-edge information and narrow the available choices. Table 1 lists the alloys and several considerations with one through 10 designating good to bad, respectively.

The alloys all had to take into account the following considerations prior to their acceptance:8

• Materials used in manufacturing the product
• Material consumption when using the product in the operation
• Energy used in manufacturing the product and process
• Recyclability and reusability at end of product life
• Emissions through life cycle including material extraction, manufacturing and disposal/recycle
• Recyclability in manufacturing waste streams.

The Sn/0.7Cu alloy is the material of choice for wavesoldering process applications primarily for its low metal cost and availability. The phase diagram for this proposed alternative is seen in Figure 3. To the extreme Sn-rich region, there is an eutectic point at 0.7 weight percent Cu that makes this alloy compatible with existing materials used in assembly practices today. The solidification is somewhat similar to that seen in Sn/Pb eutectic alloy systems. Both Cu and Sn are highly available and the binary system minimizes the chance for low melting phases that appear when using ternary alloy compositions.

SpecificationsMetallurgists wonder what happens when various metals begin accumulating in the molten solder pot and how the bulk material properties then will be affected. Contamination at high levels can be considered third elements. Groups have begun making recommendations for contamination limits that still provide acceptable soldering results without full understanding of what is going on at the microstructural level.

The impurity metals found to cause the most adverse effect are those that either form intermetallic compounds with the Sn in the alloy or change the alloy composition in such a way that properties change. Impurities can have the same effect as adding a third element, which has been found to decrease solder wetting characteristics.

ConclusionThere is much work to be done on the metallurgical study of proposed solder alternative alloys. Studies of the industry-standard eutectic solder for soldering applications have been conducted for some time and mostly are accepted. However, this seemingly does not hold true for the new age of soldering alloys, which appears to be inevitable. Returning to metallurgy basics is the method for predicting systems in the near future as the drive to find more environmentally friendly materials are sought and dictated by government regulations. Metallurgist, industry and the governing agencies must all work hand-in-hand to find more robust solutions.

REFERENCES

1. ASM International, Electronic Materials Handbook, Vol. 1, Packaging, 1989, p. 633.
2. H.H. Manko, Soldering Handbook for Printed Circuits and Surface Mounting, 2nd Ed., Van Nostrand Reinhold, 1995, p. 126.
3. J.F. Lancaster, The Metallurgy of Welding, Brazing and Soldering, Institution of Metallurgists, 1970, p. 122.
4. R. Bishop and R. Smallman, Modern Physical Metallurgy & Materials Engineering, 6th Ed., Butterworth Heinmann, 1999, p. 312.
5. J. Verhoeven, Fundamentals of Physical Metallurgy, Wiley, 1975, p. 277.
6. P. Biocca, "Global Update on Lead-free Solders," SMT, June 1999.
7. S. Crum, "Targeting Lead-free Solutions," Electronic Packaging & Production, June 1999.
8. Muncie et al., "Environmental Issues in Electronics Assembly," Journal of SMT, January 2000.

JASON M. SMITH may be contacted at Lexmark Electronics, 740 New Circle Road Northwest, Dept. H15L, Lexington, KY 40550; (859) 323-7667; Fax: (859) 5696; E-mail: jasmith@lexmark.com.