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Alloys and their elemental allies

A deeper look into lead-free solders reveals pros and cons
Alloys and their elemental allies

Lead-free manufacturing has become highly topical, even if the transition has been put off in Europe until 2006 or 2007, respectively. Now is the time to take a careful look at materials and put together the necessary provisions to be prepared for the inevitable step where reliability, costs and compatibility are very important.

Besides of concerns in the environment (both shop floor and nature), treatment of lead-bearing waste is also an issue. There are many kinds of debris generated from board assembly operations such as toxic fumes/pollutants, solder dross, contaminated wipes, fluids and containers. Some have recycling value, and others are simply disposed of as hazardous waste. Moreover, the recycling system in Western Europe has been widely developed, but in other regions landfill is still the disposing method of choice, with all its significantly negative impact on environment. Both the electronics and component manufacturers are in need of solder alternatives which do not compromise the products. According to the requirements, we can establish a list of criteria for lead-free solders:

• No damage to the environment, now or in the future
• Sufficient quantities of materials
• Melting temperatures similar to Pb63Sn37, preferably below 200°C
• Equal or similar thermal and electrical conductivity
• Adequate joint strength and thermal fatigue resistance (reliability)
• Reworkability as before
• Lowest possible cost
• Compatibility with existing processes
Basics of elements and alloys
Many of the new alloys are rich in tin with a variety of other elements added to enhance different characteristics. The most basic solders are binary alloys that have been used for years in non-electronic applications. The greater use of lead-free solders has led to the discovery of some of their noteworthy characteristics. For example, high joint strength, better fatigue resistance, improved high temperature life and harder solder joints are among the features seen in some of the novel materials. Not all lead-frees are created equally, and each should be thoroughly investigated before implementation into production. Looking at the periodic table of elements (figure 1), the choices of acceptable elements dwindles rapidly (figure 2).
Silver is in adequate supply but has a cost disadvantage. Tin-silver as a 96/4 alloy is common and has a long history in hybrid circuits. Unfortunately, with a spike temperature of 260°C, this melting point is too high for many surface-mount applications.
Bismuth poses a potential supply problem since it is a by-product of lead mining, causing embrittlement problems – and it is a poor conductor, both thermally and electrically. As for bismuth-based lead-free alloys, a lower melting temperature than that of tin-lead is offered together with a cost similar to that of tin. Unfortunately, bismuth in soldering alloys tends to create embrittlement, and if a bismuth-based alloy picks up any lead, the melting temperature will drop again, causing joint embrittlement. At first glance, the bismuth-bearing alloys appear to offer high tensile strength, but in peel strength tests they are prone to failure due to poor fatigue resistance. The same can be said for indium-based lead-free alternatives.
The other element, cadmium, is highly toxic and cannot be used. Copper is available in quantity and soluble in tin. In low percentages, copper works well. There is a lot of experience of tin alloys containing copper. Nevertheless, it should not be forgotten that the heavy metal cop-per is also toxic. Gallium supply and costs are the main reason not to use it, as well as its brit-tleness.
The cost, inadequate supply, poor resistance to corrosion and rapid oxide formation during melting can all hinder the use of the element indium. For the most part, indium is safe if kept in low humidity conditions, or if it is conformally coated.
Antimony has an adequate history and supply to be a viable solder additive. It has been known to stop gray tin transformation at low temperatures, and historically, the military required 0.2 to 0.5% antimony to enhance the thermal cycling properties of tin-lead alloys. However, the issue of the toxicity of antimony arises. As with most metals, salts, oxides and organo-metallic compounds of antimony are typically the most toxic forms. They do not, however, form in standard soldering processes. Antimony has been approved for use in potable water systems as well as in food containers. In pewter tableware, antimony is often found at levels of 7 to 9%, and copper at levels of 1 to 3%. Antimony-doped alloy has been environmentally tested to confirm that it will not leach silver or copper into ground water. The reason is that the two elements bind in the tin, and antimony reduces their solubility.
Tin is the base of solders, toxicity is low and supply is adequate. Zinc is in adequate supply, but has oxidation problems and causes solder to become brittle.
Requirements on lead-frees
Generally, most lead-free solders melt at temperatures higher than those of tin-lead do. The exceptions are alloys containing indium or bismuth, which tend to lower the melting point. The main problem with indium is its cost. With only approximately 200 tons of indium produced globally each year (figure 3), the supply is quite easily depleted and prone to drastic price fluctuation. Typically, choosing an indium-based lead-free alloy makes the most sense with temperature-sensitive components that do not require high joint strength and where the assembly will not be subjected to harsh or high-stress environments.
Ternary tin-copper-silver alloys comprise a family of lead-free alternatives that show high promise. Supply seems sufficient, and the alloys exhibit good wetting, fatigue resistance and good overall joint strength. One form (Castin) is doped with antimony and exhibits the advantage of not growing intermetallics with copper when soaked at 125°C.
Lead-free solders are available in paste, bar and cored-wire form, although the latter’s manufacturability remains an issue for alloys that contain high levels of bismuth or indium, which tend to inhibit their drawing ability. The successful implementation of Pb-free solders involves resolution of several critical issues:
• Physical (thermal and mechanical) needs of joints
• Temperature requirements of the components. Most parts and board materials can take up to a 240°C spike, but some specialized exceptions remain
• The field environment (temperature, acceleration and shear) of the assembly
• The phase-in period for a complete transition to a lead-free regime. (How long will tin-lead components and tin-coated PCBs be used during transition?)
• Repair on in-field products. This may militate against using bismuth or indium-based lead-free alloys since lead significantly affects their melting points. Results suggest that tin/silver/copper-based alternatives offer superior long-term reliability and joint strength for most applications, and that the additive antimony brings good high-temperature soak characteristics.
Short comparison of lead-frees
AIM has developed an alloy called Castin that is a combination of 96.2% tin, 2.5% silver, 0.8% copper and 0.5% antimony. The grain structure is much coarser than with the eutectic 63/37, but the material is not more brittle. The melting range is 210 up to 216°C. Tests at AIM revealed that the alloy was superior to the eutectic 63/37. Thermal stresses showed that the alloy would be more adaptable to a wide range. So, Castin performs almost as a drop-in substitution for 63/37 in most applications. When used in wave soldering, the identical 250 to 260°C pot temperature is used. When applied in reflow systems, the profiles need to be slightly hotter, a spike of 235°C is recommended (figure 4). When soldering on a high-density board, nitrogen is recommended to retard thermal problems with the flux and board materials. When hand soldering, the iron should be set at 400°C. Solder coating on boards with Castin produces flatter pads and a more uniform ability. Not all joints are going to appear bright and shiny, since the alloy has a coarser grain, but it exhibits a whiter, slightly frostier joint, dependent on cooling rates. The faster the cool-down, the shinier the joint. Storage and wetting characteristics are similar to 63/37 HASL coating.
Based on recent market developments, three suitable lead-free alloys for replacement of tin-lead were evaluated. They are the tin-copper eutectic (Sn99.3Cu.7), the tin-silver eutectic Sn96.5Ag3.5, and Castin (Cu.8Sb.5Ag2.5Sn96.2). The first experiment concerned melting points, an important distinguishing characteristic among these alloys. Of the three, Castin offered the lowest liquidus temperature (closest to the Sn63Pb37 alloy), with a melting point of 216°C.
A second area of concern relates to inter-metallic growth rates during tests at 125°C. It is interesting to note that the tin-silver (Sn96) revealed growth rates similar to tin-lead (Sn63) alloy. The tin-copper (Sn99.3) alloy suffered from relatively high copper inter-metallic growth as well. Though Castin has a similar amount of copper to thetin-copper alloy, it enjoyed significantly lower rates of inter-metallic growth than the othertwo. It is suspected that the addition of antimony as a dopant inhibits copper-tin inter-metallic growth. It has also been known for many years that antimony improves the thermal fatigue re-sistance.
Based on the these results, the tin-copper alloy was not regarded as a viable replacementfor Sn63 alloy, due to poorer wetting characteristics and higher temperature requirements than the other two candidates. In a physical com-parison between Castin and Sn96, the alloys looked very similar, nearly identical in fact.However, when the fatigue test results were compared, Sn96 failed on one of the runs, while Castin passed on all three. The passes that were recorded for the Sn96 alloy were marginal at best.
Microstructures were examined in an attempt to better understand the failure, and this is where a condition of serious concern became evident. Two bars of solder (one Castin, the other Sn96) were melted and then subjected to different cooling rates. The Castin showed a consistent, leafy, dendritic structure, regardless of the cooling rate. The Sn96, on the other hand, went through three different phases, depending on the cooling rate. It was this variance in structure passing cooling that was felt to have been the cause of the failure in the fatigue test. This issue raised serious practical concerns; namely, that depending on the size and location of a component on a board, structural weaknesses could actually occur in the solder interconnect, leading to field failure.
The final consideration when comparing Castin to Sn96 is the cost. According to current prices of raw materials, Castin is considerably less expensive than Sn96, the difference is about 20%. This difference represents high savings for wave-soldering and hand-soldering operations, and results in a lower cost for SMT grade solder paste as well. As a final note, Castin has been used successfully in flip-chip attach, showing both good joint strength and low alpha emis-sion. It also has passed thermal cycling of -40/+125 for 1000 to 1500 hours and -40/+85 for 840 cycles.
(This article is based on information provided by AIM, and derives primarily from papers of VP of technology, Karl Seelig) gbw
aimsolder.com
ZUSAMMENFASSUNG
Zum Zeitpunkt der Recherche war noch nicht geklärt, ob die Europäische Union nun den Bleifrei-Prozeß im Jahr 2006 oder 2007 Gesetz werden läßt, dennoch ist eine längere Übergangszeit für die Industrie wichtig. Mit Blick auf elementare Zusammenhänge gehen wir hier der Frage nach der optimalen Zusammensetzung von alternativen Lotlegierungen nach.
RÉSUMÉ
La question de savoir si l’Union Européenne légiférera sur le processus sans plomb en 2006 ou en 2007 était encore sans réponse au moment de la recherche mais l’industrie a besoin d’une période prolongée de transition. La question de la composition optimale des alliages alternatifs est étudiée ici en tenant compte des faits élémentaires.
SOMMARIO
Al momento della ricerca non era ancora chiaro se l’Unione Europea metterà in vigore la norma del processo senza piombo nell’anno 2006 o nel 2007, ciononostante è molto importante un periodo di transizione più lungo per l’industria. Considerando le relazioni più elementari, qui si pone la domanda della combinazione ottimale di leghe di saldatura alternative.
Current Issue
Titelbild EPP EUROPE Electronics Production and Test 11
Issue
11.2023
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