The typical method for connecting capacitors into electronic circuits is by some soldering process. The commonly used metals in the circuits and component leads often acquire an oxide coating, a non-metallic film, which will form an effective electrical barrier and also inhibit the joining of these metals by the soldering process.
This oxide film is normally removed prior to soldering by pre-cleaning and/or the use of a soldering flux. The flux promotes the inter-metallic bonding of the solder and keeps the oxides and other foreign material suspended in the flux and out of the solder joint. These fluxes and any contaminants must be cleaned from the circuit boards after the soldering to prevent any degradation of the circuit board.
In general, fluxes can be divided into three groups:
- Chloride or inorganic acid
- Rosin or resin
- Organic (water soluble)
Acid being the most active, its residue after soldering is apt to be the most corrosive and the ref ore is seldom used in electronic circuits. Over the years, rigid specifications have been established for the rosin type fluxes and the flux cleaning agents so that virtually neutral or non-corrosive residues are left. With the extensive use of the rosin type flux, the component manufacturers and users have developed soldering processes and systems which are not harmful to the components or the circuit assemblies.
In recent years the increased cost of processing and handling the flux cleaning agents, particularly to meet environmental pollution requirements, has resulted in the extensive use of organic fluxes with which water can be used as an inexpensive and plentiful solvent.
Unfortunately, the use of water as a solvent makes many of the systems and processes previously used actually harmful to the circuit components. Unless a hermetically sealed case is used, capacitors are particularly subject to degradation by water and/or the chemical agents in the flux or the cleaning solutions. The amount of degradation is a function of the type of capacitor construction, the type of dielectric used, the length of time in the flux or cleaning solution, and temperature, pressure and the strength of the solutions.
The short term effect of the absorption of the water vapor is a rise in the measured capacity and a decrease in the insulation resistance. If only a very small amount of water vapor has been absorbed, a drying cycle at some temperature above ambient will drive the moisture out and the capacitor will normally return to its original value. Any water absorption beyond a minute amount can result in a long term degradation. Metallized film capacitors will lose a significant amount of capacity due to the reaction of the water molecules with the aluminum metallization.
In addition to the loss of capacity, any chemicals or contaminants carried with the water vapor into the capacitor can reduce the insulation resistance of the unit or the chemicals can attack the dielectric film itself. A good example of this is the severe failure of polycarbonate films in the presence of amines which are found in solder fluxes activated by amine hydrochloride.
The sheer number and complexity of the fluxes, cleaning agents and processes available preclude a comprehensive discussion of them in this bulletin. However, certain generalities may be stated:
- Avoid the use of highly activated fluxes such as those containing hydrogen bromide and/or amine hydrochloride.
- Use heat or self-neutralizing fluxes whenever possible.
- Use a minimum of surfactant additives to reduce moisture wicking on component leads.
- Use deionized water.
- Do not use cleaning systems such as "dishwashers" which flood the components with cleaners.
- Use the minimum cleaning time possible.
- Hot air drying is preferred over oven-drying.
- Select components which are minimally, or not at all, affected by the flux and cleaning process.
Although some types of non-hermetic capacitor constructions are less susceptible to moisture vapor penetration to the dielectric, they will all have some microscopic openings in the outer surface, particularly where the wire leads exit the capacitor body. No truly 100% effective method has been developed to economically manufacture nonhermetic plastic film capacitors. However, techniques and materials have been developed to minimize the problem of water soluble fluxes and cleaners. If trouble is experienced in circuits and/or components after water cleaning is initiated, it is suggested that the component manufacturers be advised of the problem and the materials and processes used.
If the problem cannot be solved by changing the materials or processes, Electrocube has developed a proprietary construction method which greatly minimizes the susceptibility of plastic film capacitors to water vapor and certain other solvents. This method has been successfully adapted to most of the Electrocube wrap-and-fill or plastic cased capacitors and is available by special order.