Metallurgy of the Soldering Process - Covering Phase diagrams and Intermetallic Phases
By Dr Hans Bell & Gunter Grossman
Oct 11, 2012
This excerpt follows on from Article Two on which touches on melting and Solidifying.
The processes involved when a material melts and vaporizes are repeated in reverse order when energy is withdrawn. Vapor cools down to the vaporization temperature, which is called the condensation temperature in this case. The vapor then turns into a liquid.
This results in greater orderliness and entropy is thus reduced, because the atoms have less freedom of movement in the liquid than in the gaseous state. This leads to a release of enthalpy, i.e. heat. The temperature remains unchanged, although heat energy escapes from the system. The same process occurs when the solidification temperature (melting temperature) is reached.
Mixing pure metals with alloying elements plays an important role in soldering technology, for instance, when lead is added to pure tin. If alloys of various compositions cool while their temperatures are continuously measured, we observe the same behavior as above, except that the temperature does not remain entirely constant. Instead, the speed at which the material cools shows a clear-cut change within a given temperature range.
At first, molten material cools down at a uniform rate. Break points appear at certain temperatures, at which the rate of cooling is significantly changed.
By repeating this experiment with various compositions, the break points S and L in the characteristic cooling curve occur at different temperatures for each alloy. Plotting these break points to the temperature-alloy level results in a polyline. Polyline S indicates the temperature at which the material begins to melt for each composition. This line is called the Solidus line because everything underneath it is solid. Polyline L indicates the temperature above which each alloy is liquid, and is known as the Liquidus line. Everything above this line is liquid. In the intermediate zone, i.e. in the solidification gap, the material consists of a mixture of liquid and solidified crystals.
If the intersections from the characteristic cooling curve are projected to the level which results from temperature and composition, the break points are located on closed curve traces. Tin-lead is a very clear example of this type of phase diagram as shown below.
A few points to note: Tin concentration is represented by the horizontal line, and temperature by the vertical line. At the left side is 0 % tin and 100 % lead which melts at a temperature of 327.5°C. Pure tin, to the right, melts at a temperature of 232°C. Maximum solubility of tin in lead (18.3 %) and maximum solubility of lead in tin (2.2 %) are also shown.
However, the liquidus line is the most important piece of information, especially when the diagram is viewed from the tin side. If tin, which melts at a temperature of 232°C, is mixed with lead with a melting temperature of 327°C, the melting temperature of the resulting alloy drops. This holds true up to a mixture of 38 % lead and 62 % tin, where a melting temperature of 183°C is reached. The liquidus line begins to climb again thereafter. This appears paradoxical. Why should the liquidus temperature of tin drop when it's alloyed with a metal which has an even higher melting point?
The answer is to do with entropy. The unit cell of a pure metal contains atoms of the same type at all positions. This represents a state of minimal entropy. If one of the atoms is replaced with a foreign atom, disorder is introduced to the system and entropy increases.
Each change to the state of aggregation of a material takes place at a certain level of energy, which is the sum of entropy and enthalpy. Increasing entropy by mixing two materials results in reduced enthalpy at the point where the desired effect occurs, i.e. solidification. The more disorder prevails, the lower the solidification temperature becomes. A pure metal (A) has only minimal entropy. For this reason, it also exhibits a maximum melting temperature. Entropy is increased when a second metal (B) is added. The more that's added, the greater entropy becomes. Because the metal melts at a given level of energy content, melting enthalpy (which is the melting temperature) must decline as entropy is increased.
So, if material A is added to material B, or vice-versa, the liquidus temperature drops because entropy is increased. The two curves meet at the alloy which has the lowest melting temperature, which is called the eutectic alloy. Due to the fact that enthalpy is necessarily at a minimum with this alloy, entropy, or chaos, is at its highest possible level.
There's only room for a limited number of atoms in a unit cell. The blue atoms are bigger than the green atoms in this illustration.
The more blue atoms are mixed with the green ones, the more disorder (entropy) prevails and the lower the melting temperature of the alloy. The melting temperature is lowest at the point of greatest entropy. The limited number of blue atoms which fit into the green lattice represents maximum solubility of blue into green. If more blue needs to be added to the alloy, a mixture occurs which includes blue unit cells with the maximum number of intercalated green atoms, and green unit cells with the maximum number of blue atoms.
Intermetallic Phases (IMP)
In principal, an intermetallic phase is a homogenous chemical compound consisting of two or more metals. In contrast to alloys, they demonstrate lattice structures which differ from those of the neighboring metals in the phase diagram. This is due to the mixture of various types of chemical bonds (metallic, ionic and covalent bonds). The composition of an intermetallic phase is either fixed in accordance with a set mixing ratio (intermetallic bond in the narrower sense with stoichiometric, i.e. atomically determined, fixed composition), or it varies within a broad range of homogeneity around the stoichiometric composition. The phase width, also known as the homogeneity range, specifies the range within which the volume ratio of the various metals can vary within the intermetallic phase.
Intermetallic phases sit between metallic alloys and ceramics, and are highly organized. Each atom has its own precisely determined place within the unit cell. This contrasts with metals where it is generally irrelevant which corner the foreign atom occupies within the mixed crystal. Thermodynamically, this results in drastic limitation of the atoms' freedom of movement, and thus reduces entropy. In turn, this means that the liquidus temperature of an intermetallic phase is higher than that of the neighboring alloys, as a rule.
Tin forms intermetallic phases with many different metals. In the case of nickel, Ni3Sn, Ni3Sn2 and Ni3Sn4 are formed. Cu3Sn and Cu6Sn5 phases occur with copper during the soldering process. Which phases occur depends upon soldering temperature and position. Copper-rich Cu3Sn is formed at the boundary to the copper; and tin-rich Cu6Sn5 at the boundary to the solder. See the image below.
Understanding Melting & Solidifying and Intermetallic Phases is the next step in the foundation of applying knowledge to deliver successful and repeatable soldering processes. Chapter One of Dr Bell & Herr Grossman's book continues from its explanations of these critical factors into subjects that include Wetting, Diffusion and the Soldering Process itself. Chapter Two addresses Solderable Surfaces.
Look out for more edited excerpts from the Fundamentals of Reflow Technology book or download the PDF now to get more detail, illustrations and the full solder process story.
Based on snippets from Dr Hans Bell & Gunter Grossman's book: Fundamentals of Reflow Technology
Most of us in the electronics assembly industry recognise that there's more to the many processes involved than meets the eye. And that's certainly the case with Reflow Soldering. It's pretty obvious that soldering is a very specific science that must encompass a number of disciplines and demand considerable expertise to master. In its ultimate analysis, the process of soldering is all about the interaction of materials at a molecular, or even atomic, level. Arguably, to understand this fully, you need to be a bit of a scientist.
Here at Rehm, Soldering is what we do. Fortunately for us, and for our customers worldwide, we have several experts in this demanding field. One is called Dr Hans Bell who has been Head of the Development and Technology department at Rehm Thermal Systems since January 2000. Dr Hans is quite bright. To borrow from Douglas Adam's Hitchhikers Guide to the Galaxy: "He has a brain the size of a planet!" But he's no android... No, he's a consummate scientist who understands more about the materials interactions behind the processes that are at the heart of Rehm's business than we could ever fit into a technical article. That's why, along with Gunter Grossman of the Swiss Federal Institute for Materials Testing and Research (EMPA), Dr Hans has troubled to write a comprehensive book entitled the Fundamentals of Reflow Technology.
This brief article is one of a series of short edited excerpts from Hans & Gunter's book designed to be enjoyed and absorbed in the time it takes you to drink your morning coffee. Our hope is that it will not only illustrate the exceptional depth of knowledge on tap, for free, here at Rehm, but will inform you about intriguing aspects of the science behind soldering, and encourage you to want to read more.