Andrew

Soldering is the process by which two pieces of metal may be joined using solder in order to allow for conductivity of electricity. In our every day lives solder is crucial to the function of almost all pieces of technology; however, as the creation of electronics moves to a more and more automated world, soldering slowly becomes more of a creative tool. You can make whatever you want as long as you have the components, a soldering iron, and some solder. The personal computing revolution of the 1980s was made possible by at home soldering. Companies like Apple that started in a garage, started off as just two guys, some computer parts and an affinity to put it all together with some solder. I conducted this research and experiment in an attempt to determine what has gone wrong in my own soldering. I have noticed that some of the time joints seem to fall apart much easier than others, so I set out to determine what makes a good joint in soldering.

Typical solder, and the solder explored in this experiment is around 60% tin and 40% lead by mass. Typically the components which solder joins are copper, meaning most soldered joints or connections are held together by copper, lead, and tin. Yet, there are two ways a joint can be held together with these same metals. The wetting effect in soldering, is a term used to describe the effect by which metals can form an alloy allowing for increased joint strength and durability. As shown in Figure 1, when the wetting effect is occurring the solder, shown in gray, flattens out across the work* shown in green. The flattening of this metal occurs as a function of adhesion between the solder and the work (green plate in this scenario) and the cohesion between the solder and itself pulling the metal out. Externally this process is clear, however, the real importance of the wetting effect can not be seen from the outside. As shown in figure 2, as the contact angle (theta) decreases, the wetting effect shows more adhesion between the two different substances. Additionally the pad is crucial to the wetting effect in soldering because it is where an alloy of the three metals is formed, allowing for --after the cooling of the work-- a seamless joint. This joint will both maintain conductivity and strength under vibration and tension.

For the wetting effect to occur in soldering two main conditions must be met. First there must be direct metal to metal contact, and second the work must reach a temperature which allows for the bonding of the two compounds. Flux helps with both of these conditions by both deoxidizing the works surface, and aiding heat transfer from the solder tip to the work. Metals oxidize as they are exposed to oxygen, making a thin sheet of oxidized metal upon the surface of the metal. Oxygen forms this bond due to oxygen's high electronegativity. Tin foil for example, despite the oxymoronic name, is actually made of aluminum with an oxidized layer upon the surface of the foil. This oxidized layer in tin foil is useful due to the fact that oxidized metals are far less reactive. The structure of oxides is what creates its lack of reactivity. As shown in Figure 3, in the example of Copper (I) Oxide, the copper atoms are contained within the structure of the oxide preventing direct contact between copper and other elements.

The deoxidization of the work by flux occurs via the process of breaking the Cu-O bonds. This process requires heat, first and foremost to excite the electrons; however, often fluxes include metals found high on the activity series and halides. These substances create alternate bonds with the oxygen causing the oxidized layer of copper to become deoxidized. Heat now plays a crucial role in removing these new compounds containing oxygen, as the flux is boiled off to leave a clean, deoxidized surface of the work. With a deoxidized surface, the work must be heated even more enough to excite the copper electrons enough to form a bond with the solder allowing for final formation of alloy between the work and the solder and the wetting effect to take hold.

*The work in soldering is defined as the metal being joined by solder