What Are The Best Materials For High-Temperature Soldering

Soldering is an undeniably delicate operation and surprisingly complex at its essence. Success depends on understanding and controlling certain thermodynamic processes, including surface oxidation, intermetallic formation and surface free energy.  The process can quickly go wrong if not guided by the hand of an experienced operator or professional.  Moreover, after the completion of a soldering process, the circumstances under which soldered joints must operate can frequently be considered extreme situations - environments characterized by notably high or low temperatures, intense pressures in the upper atmosphere or the ocean's depths, and electrical voltages in the upper kilovolt range.

As the expert metallurgists at AMETEK ECP's metalworking Coining division know well from decades of experience: High heat can make for one of the most dangerous environments in which solder-bonded electrical components or microelectronic packages are employed. Because of this risk, choosing the best pure alloy will reap dividends for industrial customers requiring solder preforms intended for operation in temperatures up to 300 degrees Celsius.  Today, we'll take a closer look at some of these materials and examine what makes them ideal for the hottest environments.

Breaking with historical precedent

For much of the history of modern soldering, lead served as the primary metal, for both preform bonds and solder wire. In fact, AMETEK ECP still uses lead for the manufacturing of its soldering and bonding wire. Typically, Coining metallurgists create this wire by using at least 85 percent lead, and then filling in any incomplete spots with silver and tin, to ensure the sturdiest possible component.

A notable reason for this is lead's high melting point. For example, Coining alloy No. 1000, which is entirely lead (save the infinitesimal presence of impurities), can withstand temperatures up to 326 degrees Celsius (just under 620 degrees Fahrenheit) before it melts.

However, as the general public became well aware of how toxic lead can be in certain contexts - due to the fumes exuded when it's being smelted or during other steps of the production process, to name just two examples - lead's usage has been minimized or eliminated in many trades, aside from situations in which it's absolutely necessary. Stipulations enforced by the Environmental Protection Agency (and, to a lesser degree, the Occupational Safety and Health Administration) strictly control the element's application.

In reaction to such evolving regulations, AMETEK ECP and Coining had to think outside the box. This demand led to the development of the 200-plus unique alloys that currently make up the Coining product catalog of solder preforms. A number of these materials contain lead, but many do not. As we'll touch on in just a moment, other metals are better for extreme high-temperature environments.

Alloy Choices

Reviewing the list of AMETEK ECP solder preform alloy options, it becomes clear that available options are either Gold-based or Lead-based alloys. The gold-based alloys are useful in high-reliability applications, eg aerospace applications where cost is a lesser consideration to quality and reliability.  The lead-based alloys are useful in industrial applications where cost is a major consideration.

In spite of the environmental and health hazards posed by its use, lead is still used in many industrial applications where a non-lead replacement option does not yet exist.  RoHS has an exemption – Exemption 7a – for lead used in high temperature solders where the lead content is > 85% by weight.  

Silver- and Copper-based Alloys

With all of that said, bulk purchases of pure gold or gold alloy preforms may not be ideal or realistic for every company's budget. Coining has thus taken the initiative to devise alloys derived from other strong metals to provide an option offering similar performance for AMETEK ECP's customers at a more modest cost.

Culturally, silver often finds itself as the runner-up to gold, in metrics including its dollar value on global commodity scales and figurative value in the eyes of lay people and jewelry appraisers alike. And by the standard of melting points, it also occupies second place, with its purest form turning from solid to liquid at 961 degrees Celsius (approximately 1,762 degrees Fahrenheit). But for many applications of solder preforms, 961 degrees is a perfectly acceptable threshold. For example, most applications of silver solder preforms involving hermetic seals, like radio-frequency transmitters and receivers or even aerospace technologies, won't find themselves exposed to temperatures higher than 900 degrees Celsius.

Most high-temperature solder preforms made with silver use alloys containing about 45 to 90 percent silver and the remainder composed of copper, zinc, tin and cadmium. They can withstand heat ranging from about 700 to 950 degrees Celsius. Alloys made largely of copper - such as Coining alloy No. 40489, a copper-phosphorus blend - have similar strength, with that particular metal boasting a melting point of 905 degrees.

Customization for special purposes

With more than 200 established alloy blends produced by Coining, AMETEK ECP offers customers in need of high-temperature soldering preforms and parts a wide spectrum of choices. But on the off-chance a client's need does not precisely conform to one of the existing options, AMETEK ECP and Coining personnel are always ready to create brand-new metal blends.

Even when customers require precise specifications to withstand the high-temperature environments of niche applications, metallurgists will work with client contacts to craft the perfect proprietary alloy. This simultaneous commitment to on-the-fly innovation and high-level customer service is a core factor behind the decades of success AMETEK and its satellite companies have achieved in the field of electronic components and packaging.


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