Metallurgy of the Soldering Process
By Dr Hans Bell & Gunter Grossman
Sep 27, 2012
Touching on Atomic Bonding, Composition of Metals and Crystal Structures
Dr Hans points out that the soldering process differs fundamentally from the welding process. During welding, materials are fused together by applying heat, pressure, or both, with or without the use of filler materials. The parent metals are fused together while in the
plastic or liquid state within the weld zone. In contrast, soldering is only possible with the help of a filler material, namely the solder. The solder is injected in the liquid state between the two parent metals, which themselves are not melted. Positive bonding is the result of the process of diffusion.
￼In welding, the parent materials and any filler materials are melted during welding, and the liquid phases are mixed, whereas during soldering the parent materials retain their solid state - only the solder is liquid.
In order for atoms to combine into a solid body, forces come into play to hold the atoms together. These bonding forces are generated in the outer electron shells. The three most important types of bonding are:
Metallic Bonding - where atoms release a portion of their electrons to the 'community'. These conduction electrons move 'freely' between the atoms. Because the atoms become positively charged after releasing electrons, the free electrons essentially function like glue to hold the atoms together.
Covalent Bonding - where electrons from neighbouring atoms share a single orbital, so it's no longer possible to say that the participating electrons belong to one atom of the other.
Ionic Bonding - where some atoms are electrically charged because they either have too many electrons, or a given number of electrons are missing. This state is designated electron negativity. Atoms are drawn to each other due to differences in electron negativity.
Electron negativity describes the tendency of atoms to release or gain electrons from their outer shells. Atoms with nearly empty shells tend to release electrons. Atoms with nearly full shells demonstrate a strong tendency to fill the shell up. In reality, however, covalent and ionic bonding always occur together in a mixed fashion. To what extent bonding is covalent depends upon the difference in electron negativity: the greater the difference in electron negativity between two materials, the larger the ionic portion of the bond becomes.
Composition of Metals and Crystal Structure
As a rule, metals congeal into a crystal lattice. This means that the atoms in the metal order themselves into a regular spatial sequence (a lattice), which resembles a stack of eggs. If the surface of a metal is polished and viewed with weak magnification, large areas with differing colours, i.e. the phases, can be observed. Here a phase is defined as a uniform constituent of a system with identical macroscopic composition, which is volume in the case of metal.
The fact that the phases consist of crystals cannot be determined until they are examined microscopically. The dark lead phases and light‐coloured tin phases can be seen below very well.
Needle‐shaped structures - so‐called intermetallic phases (IMP) - are embedded in the tin and are clearly visible in the enlargement.
If the specimens are prepared suitably, variously dark colouring makes it visibly apparent that the tin phase is composed of grains. These grains are actually crystals, and are comprised of atoms which are stratified on top of each other in a uniform fashion. The difference between one crystal and another lies in the fact that crystal planes are arranged at different spatial angles. Crystal lattices are seldom free of imperfections. They usually include stacking defects (offsetting) or extraneous atoms.
The areas where the grains butt up against one another are known as grain boundaries and are metallurgically quite interesting, because a sort of anarchy prevails within them: The lattice positions of the atoms are not precisely defined, and atoms can belong to one as well as to another grain, gaps occur, and so forth. For this reason, processes readily occur at the grain boundaries which would necessitate greater amounts of energy within the well‐ordered interior of a crystal. The crystal lattices can be described as a regularly reoccurring pattern of atomic configurations, namely the unit cell.
Here's an intriguing way to visualise crystalline structures, borrowed from the plant world. Sweetcorn on its cob provides an excellent model. The arrangement of its grains (corns) demonstrates many of the characteristics of a crystal lattice.
All unit cells which occur in crystalline substances can be described by means of a system which is based upon 7 geometric shapes* which are namely: cubic, tetragonal, orthorhombic, rhomboedric, monoclinic, triclinic and hexagonal. In addition to the basic types with one atom in each corner, other unit cells also occur which have additional atoms on the outside faces (face‐centred) or in space (space‐centred). This results in a system with 32 unit cells.
* See the full text in Dr Hans Bell & Gunter Grossman's book for diagrams of the seven shapes along with mathematical formulae for their geometry.
The shape of the unit cell determines many of the attributes of an element. For example, metals with simple unit cells (e.g. cubic) are very easy to deform. Metals with unit cells which occur in other metals in a slightly modified form can be easily alloyed with numerous metals. For example, tetragonal tin is readily deformable and can be easily alloyed with a great number of metals, for example lead (Pb), copper (Cu), silver (Ag), bismuth (Bi), etc.
Understanding atomic bond behaviours, metallic compositions and crystal structures is a key first 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 Melting & Solidifying complete with phase diagrams, Intermetallic Phases, 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 boffin.
Here at Rehm, Soldering is what we do. Fortunately for us, and for our customers worldwide, we have several boffins. 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.