Metallurgy of the Soldering Process - Melting & Solidifying and Intermetallic Phases
By Dr Hans Bell & Gunter Grossman
Oct 03, 2012
This excerpt follows on from Article One on which touches on Atomic Bonding, Composition of Metals and Crystal Structures.
The melting process is closely associated with the state of aggregation of a material, i.e. whether it's in a solid, liquid or gaseous state.
If a constant amount of energy is applied to a crystalline material
in the form of heat, the specimen heats up initially in a linear fashion over time. Warming then comes to a standstill, even if more energy is applied. After a certain amount of time, the molten material once again heats up in a linear fashion until the evaporation temperature is reached. Here, the temperature remains unchanged again until all of the liquid has evaporated. Further energy input results in linear warming of the vapor. So what is the reason for this behavior?
When a material melts (e.g. solder), a change in the state of aggregation takes place. This means that the atoms of a material begin to vibrate so intensely that the crystalline structure disintegrates and the atoms can move more freely. The material melts. However, the disintegration of a well ordered crystalline structure can also be viewed as a greater degree of freedom of movement which must be introduced to the material in the form of energy. These natural laws of physics are described by the discipline of thermodynamics.
All materials contain two types of thermal energy: enthalpy and entropy. We directly experience enthalpy, the kinetic energy of atoms moving around a neutral position, as warmth. If we grasp a hot object with our hands, we discover that we can directly perceive the motion of the atoms via our skin, as you can see below.
Human beings are not equipped with a sense which detects entropy. Entropy represents disorder, or the number of potential possibilities within a system. The greater the disorder, the greater the entropy. A gathering of people can be used as a good illustration for the concept of entropy. A chaotic crowd of people in great disorder makes a much more energetic impression than a static group which is at rest and well ordered. Here's a visual example to reinforce that.
During the melting process, the disintegration of the crystalline bonds leads to increased entropy. Because the atoms have more freedom of movement, it results in an increased level of disorder. This means that the melting process is a function of energy. For this reason, the points at which the state of aggregation changes become apparent due to the fact that temperature remains constant - even though energy is being constantly applied in the form of heat, see the diagram below.
In fact, the temperature of the molten material does not rise until the entire specimen has been melted. The amount of energy which is applied during a change of state of aggregation without an increase in temperature goes entirely into entropy, and is designated melting energy, or vaporization energy at higher temperatures. The terms latent heat and specific melting temperature, as well as specific vaporization temperature, are also used.
If a constant amount of energy is applied to a crystalline material, for example in the form of heat, temperature does not rise while the material is melting or vaporizing.
Depending upon the mass and the type of material, varying amounts of melting energy must be applied in order to change a solid to the liquid state. Melting energy and vaporizing energy are many times greater than heating energy. This means, for example, that 100 times the amount of energy is required per gram of material in order to cause water to melt than is needed to heat water by an amount of 1°C while it's liquid.
All of this is reflected in the energy balance for the soldering process. The energy required to heat the solder joints and melt the solder must be generated by the process. And this process is hindered by nature. The greater the temperature difference between the soldering device (iron or hot air in the soldering system) and the subject to be soldered (PCB), the greater heat transfer becomes. Initially, when the PCB is still cold, heat transfer is best. But the PCB only needs to be preheated, which takes comparatively little energy. In the peak zone where melting energy must be applied, the temperature difference should be kept as small as possible in order to avoid overheating of electronic components, for example. The smaller temperature difference impedes heat transfer.
This observation is very important for manual soldering. A solder joint requires a specific amount of energy in order to melt. If a manual soldering iron with a fine tip is used, heat transfer is minimal and soldering time is increased. The same applies to soldering irons with minimal power. But soldering systems with too much power can introduce too much heat energy, which may result in the destruction of components. High energy input is more difficult to control than slow energy input.
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.
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.