To understand all aspects of the problem, solderability, the use of bronze as a terminal basis material nickel plating, tinning with solder, and the Meniscus Interaction Hypothesis must be discussed.
Solderability is a characteristic that indicates the ease in which molten solder will wet to a material. A material that exhibits good solderability will cause a strong meniscus force to occur between the terminal and the solder. This meniscus force can be measured with a wetting balance analyzer or meniscograph. The output from these devices typically provides a force versus time plot so the phenomenon of both the buoyancy and wetting forces can be seen.
The Use of Bronze as a Terminal Basis Material
Bronze is an alloy of copper and tin. Bronze tends to oxidize, which reduces its solderability and can vary in the percentages of tin and copper. The solderability of most bronze alloys are marginal, and may require the use of stronger fluxes to attain a reasonable level of solderability. Stronger fluxes are not recommended because of their corrosive nature. However, plating or coating the bronze material with a more solderable metal is preferable.
Nickel has low solderability, but does not oxidize as fast. There are two basic functions of nickel in this application. The first function is as a barrier metal between the copper alloy and the tin in the solder alloy. Over time and temperature the tin and copper will metallurgically react and generate an intermetallic compound. From this reaction, the tin will be depleted from the solder making the solder lead rich. This lead-rich solder, in addition to the intermetallic formed, will cause wetting problems.
The second function of nickel plating is to prevent corrosion. This metal plating reduces the oxidation of the terminal basis metal and provides a mechanism for maintaining solderability after storage. One problem with electroplatings and storage, however, is the porosity of the electroplated material. The porosity tends to allow oxides to form on the basis metal which in turn reduces the solderability of the terminal. Therefore, the storage life of electroplated terminals is generally not as long as hot solder dipped or fused terminals.
Tinning with Solder
The purpose of solder coatings is to restore solderability and prevent corrosion. Through the use of fluxes and hot solder baths, a small oxide layer can be removed. By removing the oxide layer on the terminal, the meniscus interaction between the terminal and the solder will increase, and thus improve the wetting time during the soldering operation. If stronger fluxes are used, such as RA or OA fluxes, a thicker oxide or more corroded terminal can be restored to an acceptable level of solderability. The thicker oxides can be removed because of the increase in the chemical reaction rate between the activator in the flux and the oxide on the terminal.
Solder coatings prevent corrosion. This is the process of inhibiting the build-up of oxides on the terminal. The characteristic of solder coating that accomplishes this is the small amount of porosity. It is the porosity of metal that allows the diffusion of gases in the environment, causing oxidation and corrosion of the terminal basis material, or in this case, bronze.
Meniscus Interaction Hypothesis
Meniscus interaction can be used to explain the phenomenon of solder wetting to a base metal. This hypothesis involves surface energies. Surface atoms of a molten solder or liquid are not bonded to the maximum number of nearest neighbors and therefore, are at a higher energy level than the interior atoms. Surface atoms whose bonds are not satisfied result in a surface energy. To minimize this surface energy, liquids minimize their total surface area. When a liquid comes in contact with a solid having a higher surface energy, the liquid spreads, reducing the total surface energy of both.
This is not the case when an intermetallic is formed. The intermetallic compound at the liquid-solid interface produces a molten layer whose energy may be lower than the solder's, causing the liquid solder to minimize its surface area. Thus, a wetting phenomenon occurs. It is the compositional and chemical nature of a spreading liquid front that controls the wetting. The ability of a liquid to wet a flat solid surface is characterized by the contact angle formed at the solid-liquid-vapor intersection.
If the angle of the liquid is small, better wetting
). This means that a liquid's surface has a higher proportion of one or more of its components than the plain internal bulk material. This surface enrichment alters the surface energy of the liquid and is the driving force behind FSF.
The solid surface has an absorbed layer of one or more components of the liquid. This absorbed layer, not the solid's surface, is the important factor determining the energy at the solid-flux interface. This energy is the driving force behind FBF.
The FBS force is determined by the compound formation at the solid-liquid interface. This compound formation for tin based solder and copper is the tincopper intermetallic. These chemical and physical changes affect the surface energy of the materials used and, therefore, affect the wetting action.
Other factors affecting surface energy are flux, plating material diffusion, time, and temperature. By calculating the surface energies, taking into account the affects of the physical and chemical changes, it is possible to predict, in a semiquantitative way, the wettability of a solid by a liquid.