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Eutectic Solder Alloys – A Guide

Wednesday, February 17, 2021

Picture for just a moment the hundreds of planes currently flying above you - or the ships pulling out of the nearest harbor en route to the sea, the satellites high above Earth's atmosphere, the banks of heavy-duty servers supporting cloud-computing infrastructure for global communications and the screen upon which you're currently reading this article. What exactly unites them? Most obviously it's the electronics, of course. But more specifically, it's the fact that the incredibly strong bond of soldering is keeping so many of these machines' and devices' vital components together - often at a granular and sometimes near-microscopic level.

Along with welding and brazing, soldering represents a key method of joining oft-tiny electronic components together. The most basic distinguishing factor differentiating these three processes is temperature, as soldering easily involves the lowest temperatures during the course of the bonding process. In a considerable number of situations, soldering will be much more preferable than either of its counterparts for that precise reason, as well as the lower strength of the completed bond: When dealing with extremely delicate electronic and microelectronic parts, the bond doesn't need to be ironclad (figuratively speaking) because the high heat that must be applied to create a stronger bond could damage the components beyond repair.

Ultimately the assured integrity of the parts involved in high-stress microelectronic contexts - everything from the sonar providing guidance to a naval defense submarine to the circuits powering a cutting-edge communications satellite - is much more important than the specific strength of an individual bond. But things get a bit more complicated when you delve into factors such as the different types of welds, brazes and solders, particularly within the latter of the three. Eutectic solder alloys - and the essential tenets of eutectic soldering, in and of itself - are among the most critical concerns to examine when making the choice of alloys to employ in various electronic manufacturing processes, and Coining has you covered with a comprehensive breakdown:

A quick refresher on soldering standards

The basic definition of a soldered bond, of course, is that the filler metal being used to join the two work pieces (the parts that must be bonded) has a melting point at or below 842 degrees Fahrenheit (450 degrees Celsius), according to the Copper Development Association's citation of standards codified by the American Welding Society. CDA notes that many metallurgists and manufacturers round down to 840 Fahrenheit, as its Celsius counterpart - 448.889 - has no real difference to 450 in theory or in practice. Anything above those temperatures, meanwhile, is considered brazing, and as for welding, the electric arc of a common weld makes those temperatures look like child's play: The Physics Factbook notes that such an arc's temperature has a minimum heat level of 3,000 degrees Celsius and can go up to 20,000 degrees.

solder and braze alloy

The singular nature of eutectic alloys

Discussing all of those near-unimaginably hot temperature levels makes it even more interesting when you consider that eutectic soldering and the metals it involves go entirely in the opposite direction: According to FCT Solder, alloys that meet the criteria for eutectic soldering can melt and freeze (or, more accurately, solidify) at the exact same temperature, while non-eutectic metals or alloys have wildly different solidus and liquidus temperatures. Such eutectoid metals also, of course, melt at a lower temperature than either of the two work pieces in a soldering operation, which creates a de facto adhesive that's ideal for maintaining the necessary bond. (Curiously, the Merriam-Webster Dictionary definition of "eutectic" does not mention the identical melting and freezing temperatures of the alloy, merely stating that it's an adjective to describe "an alloy or solution having the lowest melting point possible." That said, this would hardly be the first time the dictionary wasn't fully in sync with modern science.)

Blends of tin and lead are among the most common eutectic solder alloys, and for quite a while, tin-lead was the dominant alloy for this purpose. Each of these two metals has a respectable complement of notable qualities on its own, but when they are mixed at the perfect proportions, they become perfect for eutectic soldering. For example, in an alloy made of 63% tin and 37% lead, the melting and freezing points will both be 183 degrees Celsius! In their pure forms, tin and lead have respective melting points of 232 and 327 degrees. Rounded total shares of the two elements, such as 60% tin and 40% lead, are more common than the specific figure cited earlier in this paragraph - which could be described as the purely eutectic tin-lead alloy - in part because purity costs more.

It's also worth noting that tin content in a solder alloy, eutectic or otherwise, should rarely if ever exceed 63%. According to Multicore Solders, any share of tin greater than that will not only be more expensive but also make it so the resulting alloy has a higher melting point, thus potentially negating the chance of useful eutectoid properties. In general, while pure metals can have some of the positive qualities of eutectic alloys, it simply isn't all that cost-effective to go with any single metal. There are occasional exceptions, like silver soldering, but more often than not the pure precious metal will be combined with copper, tin, cadmium or zinc.

Why eutectic vs. non-eutectic for soldering?

If someone is trying to convince you to choose eutectic alloys over non-eutectic metal blends, they likely won't start by pointing out the former has a much shinier appearance than the latter (although that does happen to be a true detail). There are uses for both of these alloy types in soldering. However, we'd be remiss not to stress that are many more applications for eutectic solder alloys than their non-eutectic counterparts.

The main difference between the two types is that non-eutectic alloys will end up in a state between solid and liquid - what Keith Sweatman and Tetsuro Nishimura of Nihon Superior Co. in Osaka, Japan referred to as a "mushy" stage in their 2006 white paper on solder alloys - that prevents the substance from flowing easily. As a result, the heated filler metal won't be as useful in bonding the two work pieces together because it can easily be damaged or otherwise altered before it solidifies entirely. This significantly diminishes the likelihood of any electrical connection that's been forged in such a non-eutectic fashion having reliable input and output.

The resulting bond simply won't conduct electricity to its fullest potential if it's compromised at some point during the soldering process. And in any sensitive electronics application like those we mentioned earlier on, the potential for interference or failure that such an imperfect connection would facilitate simply isn't an option for any of the engineers, technicians or other operational personnel involved in these processes. You'll most frequently find non-eutectic soldering in various plumbing and pipe-fitting operations, in which the "mushy" aspect is a strength rather than a weakness.


Reviewing common eutectic solder alloys and their properties

As FCT Solder noted, until very recently, the majority of the soldering conducted for the operations of electronic devices and their countless internal components involved the eutectic tin-lead alloy described above - either its pure 63/37 blend or the more common 60/40. (You'll still find tin-lead alloys and other eutectic blends featuring lead for sale in various global markets, but it's not as frequently seen as it once was for reasons we'll touch on in the next and final section of this piece.

The tin-lead alloy became popular for a number of reasons. For starters, both elements of the alloy have solid conductivity and are notably corrosion-resistant. As for the overall strength of the bond between common metals used in electronics manufacturing, well, to quote Deutsche Bank chief securities economist Torsten Slok about the recent U.S. jobs report, "Goldilocks is the best description of this." In other words, the eutectic tin-lead is just right in terms of its tensile strength.

Some of the other common eutectic alloys used in soldering for electronic components are as follows:

  • ●Sn-Pb-Ag (62% tin, 36% lead, 2% silver; melts and solidifies at 179 degrees Celsius/354.2 Fahrenheit)
  • ●Sn-Ag (96.5% tin, 3.5% silver; becomes eutectic at 221 Celsius/429.8 Fahrenheit)
  • ●Sn-Cu-Ni-Ge (99.3% tin, 0.7% copper, 0.6% nickel, 0.005% germanium; 227 Celsius/440.6 Fahrenheit)
The emergence (and rising popularity) of lead-free eutectic solder alloys

As the potential health hazards associated with considerable exposure to lead started entering the popular consciousness, there developed a resulting search for an alternative. While a difficult transition for manufacturers in some respects, there was no doubt that it was necessary: The correlation between constantly being around lead and the development of various ailments was far too obvious to write off. According to the U.S. National Library of Medicine, these include but are not limited to high blood pressure, nerve damage, neurological disorders, muscle and joint pain, infertility and - in children - serious gastric disorders, severe brain damage or death. With children exposed to so many more electronic devices than they were just a decade or so ago, the urgency only grew more intense.

Lead is still used in some eutectic soldering purposes because of its thermal fatigue resistance and its relatively low cost, particularly as solder wire (a product to which the average child is highly unlikely to be exposed). But regulators all over the world began taking note of lead-related health risks, and some started phasing it out of their domestic manufacturing of electronic packaging and related components. According to a March 2019 piece in the Science and Technology of Advanced Materials academic journal written by Meng Zhao and Liang Zhang, lead is prohibited from use in electronics manufacturing in Japan and all member nations of the European Union, among others.

A few exceptions allow for limited application of lead, but its status as a raw material is highly restricted. It likely won't be particularly long before lead-free eutectic solder alloys are the norm rather than the aberration. The most popular alternatives use tin in lead's place as the primary metal, and then buttress it with silver, bismuth, copper, indium, antimony and zinc, to name just a few. Tin-copper eutectic solder alloys seem to be emerging as one of the particularly prominent lead-free solder alloys, according to Zhao and Zhang's piece, though the authors did take the time to address copper's higher-than-usual melting point and state that further metallurgical development might be necessary to ensure it retained viable eutectoid properties. Some of the operations that could bolster tin-copper eutectic alloy performance include particle strengthening, alloying with additional metals like nickel and bismuth and developing appropriate fluxes.

Coining carries a wide range of metal and alloy options to meet all microelectronic soldering needs, including gold-germanium, gold-tin and tin-silver. Contact us today to review our alloy options and learn more!


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