The melting point of any material is defined as the temperature at which the solid becomes a liquid. From an engineering perspective this temperature dictates which materials can be used for given real world applications. In most applications materials are chosen such that they will be used in solid form, without the chance of melting.
Solders are different. The role of solders is to melt, and upon melting, join two or more electrical components together. Solders comprise dozens of alloy compositions, with melting points as low as 90° to as high as 400°C. The selection of any particular solder alloy for an application is based on the melting point of that solder. For example, if the application is such that the device will operate in high temperature environment the solder chosen will need to have a melting point higher than the operating temperature.
In commercial application solders can be characterized into one of two categories:
- Eutectic
- Non-eutectic
The word ‘eutectic’ derives from the Greek “eútēktos”, meaning easily melted. In practice eutectic refers to an alloy that melts at a single temperature, and upon cooling, solidifies at a single given temperature. This capability is important in certain manufacturing processes.
A non-eutectic alloy is therefore an alloy that does not melt at a single temperature. These alloys have what is called a melting range. The alloy begins to melt at a specific temperature, then continues to melt as the temperature is increased, until a final temperature is reached, and the alloy is fully liquid. The difference between the melting start and melting finish temperatures is referred to as a Melting Range. Some alloys have melting ranges as low as 3°C, while others have a melting range as high as 75°C.
Which solder should be used?
The selection of a particular solder alloy is based on several factors, but two of the primary criteria are:
- solder melting point of the process at hand
- any subsequent thermal processes.
Perhaps a high temperature solder is required because the manufacturer will perform subsequent thermal processes and he does not want to reflow the first solder. The high temperature solder allows him the freedom to do more than one thermal process without compromising device integrity. Or, maybe a low temperature solder is needed because the manufacturer has thermally sensitive components and does not want to damage them through exposure to high temperatures.
A manufacturer can solder a component or components using, for example a high Lead (Pb) solder alloy. These alloys melt in the 300° range. He can then do a secondary solder using a Tin Silver solder (SnAg), which melts in the 220° range. Finally, if necessary, he can do a third solder reflow using an Indium-based solder with a melting point in the 150°C range. This step-down soldering technique, starting with a high temperature solder, affords the manufacturer significant process flexibility.
In other situations, a low temperature solder is required. Different electronic packages contain thermally sensitive components, or perhaps organic components with low thermal thresholds. The solder selected will be one where the melting point will be below 150°C, perhaps as low as 100°C. The common thread is that the solder was chosen because of its melting point.
Why would I use a eutectic alloy over a non-eutectic alloy?
The metallurgy of a eutectic alloy allows it melt and freeze at a single temperature. This translates into a fast reflow and cool down. A faster process is a more cost-effective process. When the solder melts and freezes quickly the quality of the solder joint is optimal. Any specialized fixturing to hold components in place during the reflow can generally be less complex. The choice of a eutectic alloy therefore has many benefits and most manufacturers prefer to use a eutectic alloy when they can.
However, the number of commercially available eutectic alloys is limited, while the number of different soldering applications is vast. More often than not, the process dictates that the alloy selected will be non-eutectic. Using non-eutectic alloys should not be considered a detriment; it simply means the manufacturer will need to pay particular emphasis to optimizing his reflow process. Where a eutectic alloy process can be fast, the non-eutectic alloy, during solidification, will go through a period of being partly solid and partly liquid. During this period of being a solid/liquid mix the solder joint is susceptible to a phenomenon called ‘hot-cracking’. Parts shifting during reflow and reflow process interruption are typical root causes of hot cracks; these are difficult to detect during regular QC inspection. With longer process times come increased opportunities for less than optimal solder reflow. Plus, any specialized tooling may be more complex to achieve the same end result.
Why would I use a gold alloy solder over a lead alloy solder?
Soft solder alloys, namely lead-, tin- and/or indium-based alloys, start losing their strength at temperatures above 75% of their melting point. For Pb, melting point of 327°C, the alloy becomes softer above 245°C. For Sn, this calculates to 175°C. The softness of these solders poses a problem when they are used for making components and/or subassemblies that will go through a final assembly process like reflow or wave soldering in the 250-260°C range. Gold solder alloys like AuSn consist of a homogenous mixture of Au atoms and AuSn intermetallics. This mixture makes these alloys very strong so that they have almost no loss of strength close to their melting point. As a result, Au80Sn20 with a melting point of 280°C is the alloy of choice for soldering/closing subassembly packages, which are hermetic sealed and must sustain final assembly processing. The high temperature strength of the Au80Sn20 assures the integrity of the package, even when it is required to endure unwanted final assembly interruptions and associated overheating issues.
Bottom Line
Solders alloys may be manufactured in many forms and shapes. They can be formed into a preform. This is the primary way that solders are used by the semiconductor industry in which the solder preform becomes part of the electronic circuitry. A solder preform used in an automated soldering requires batch-to-batch consistency.
Dozens of solder alloys are available, some eutectic, some not; some with narrow melting ranges, some with wide melting ranges. The alloys can be Lead (Pb) based; Gold (Au) based; Tin (Sn) based or Indium (In) based, among others. They can be used in applications from cell phones to satellite systems. Choosing the correct solder alloy for a particular application is predicated on knowing the environment where it will be used. The first questions a manufacturer will ask are: “What is the melting temperature needed” and “What are the processes the solder will have to endure”?
AMETEK Coining is the world-leading manufacturer of solder preforms. We have a vast array of solder alloys readily available, including lead-free alloys, that comply with EU Directive 2002/95/EC ‘RoHS’ (Reduction of Hazardous Substances), banning the use of Pb (lead)-based solders for most applications. Coining is completely vertically integrated with the capabilities to work and develop new alloys. Coining has sophisticated tooling capabilities and expertise, and a vast tool library with 18,000 different tool sizes available. New tools can also be manufactured to exact customer’s specifications.
Solder melting characteristics are an important criterion while choosing a solder alloy, however there are many other factors that contribute towards the selection of the specific alloy. Our Engineering team is readily available for a consultation and our unique in-house capabilities enables us to develop customized alloys for your specific application.
Please contact one of our offices with any inquiries. AMETEK Coining has sales offices in North America, Asia and Europe.
US Sales Office: +1 201-791-4020
Coining.info@ametek.com
China Sales Office: +86 21-3763-2111 EXT 8894
angela.chen@ametek.com
Europe Sales Office: +381 62 291 143
sladjana.milorad@ametek.com
Malaysia Sales Office: +60 46 43 3062
Alvin.Ng@ametek.com