Cosmetic blemishes and corrosion (Fig. 1) can hinder the application of other coatings afterward, whether metallic or organic. Environmental compatibility is particularly problematic, since it can be difficult to accurately determine corrosion rates under varying service conditions or to take into account the effects of other applied mechanical, thermal, or electrical loads on lifetime predictions. Because of these potential contributions to premature solder joint failure, the conservative approach is to remove all flux residues and related undesirable surface contaminants from the workpiece after soldering prior to next assembly, storage, or service.

When hand soldering, the resulting flux residues can be based on organic or inorganic chemistries. These residues are often hygroscopic, discolor with time, and cause fit-up problems for closely toleranced or moving parts.

Loose solder particles can cause several performance problems. If these features break off or interfere during subsequent processing or service, the function of the soldered assembly can be compromised by jamming of critical moving parts, blocked passages, or electrical shorts. The consequence is costly rework or catastrophic system failures.

Fig. 1 - Ionic flux residues left on soldered assemblies can cause visual blemishes and potential sites for corrosion: A - white flux residue on soldered surface; B - residue corrosion product on soldered electronic assembly.

Solder stop-off coatings, which are intended to restrict solder wetting and flow over selected areas of the base metal, are usually easily removed. The powder slurry vehicle of the stop-off is formulated to vaporize before or during soldering. The remaining inorganic particles in the stop-off restrict solder flow and the remaining powder material can be removed by light burnishing or rinsing in a cleaning solution recommended by the stop-off supplier.

There are many approved commercially available cleaning solutions and associated equipment for postcleaning of soldered assemblies. They range from simple rags soaked in solvent, spray bottles or wire brushes to vapor degreasers and ultrasonic cleaners, to more automated batch-type and in-line cleaning equipment. In some cases, dry-cleaning processes such as RF plasma cleaners can be used after solvent or aqueous cleaning to rid the last atomistic layer of organic contamination. The selected technique depends on the type of flux residues or other surface contaminant(s) that must be removed. Materials compatibility between cleaning chemicals and the product materials (particularly with thermoplastic materials), accessibility of the cleaning equipment to the soldered assembly, and compliance with environmental, safety, and health regulations are other issues that require consideration when selecting a cleaning procedure. Once the cleaning method has been chosen, it is important to remember that the sooner the cleaning operation is performed, the easier it is to remove the targeted surface residues and contaminants.

Postprocess Cleaning

Postprocess cleaning has three stages. The first step uses a cleaning agent to release the desired contaminants from the selected workpiece surfaces. This includes the use of abrasive media. The second stage is removal or rinsing of the cleaning agent and residues from the workpiece. The final stage is drying of the cleaned assembly to remove any remaining rinsing agents. During these steps, care must be taken to not modify surface conditions or other properties that could affect product performance.

Common cleaning agents for organic residues are based on organic solvents, semi-aqueous cleaners, and water- containing saponifiers. Ionic residues require more polar solvents to effectively remove the contaminants. Generally speaking, "like dissolves like." With increasing international, federal, state, and local regulations on chemicals that harm the environment, newly formulated cleaners have been developed to comply with these more restrictive regulations. Environmentally responsible alternatives, which have been approved by the Environmental Protection Agency (EPA), can be found on the Significant New Alternatives Policy (SNAP) program list. The approved cleaners may not have the same effectiveness as the older formulations or may pose other safety concerns, such as low flash points and a potential fire hazard. Material safety data sheets should be consulted before using these alternative cleaners to ensure proper use. More detailed general information on the different organic, semi-aqueous, and aqueous release and rinsing agents can be found in the Handbook for Critical Cleaning (edited by B. and E. Kanegsberg, CRC Press, 2001). Although the reference is intended for the electronics industry, the information is pertinent to all postsolder cleaning operations.

Similar information and planning should be used when selecting the rinsing agent. A common rinsing practice is to first clean with an organic solvent, such as trichloroethylene, and then rinse with a polar solvent, such as ethyl alcohol or isopropyl alcohol, to effectively displace all nonpolar and polar contaminants, respectively. If an aqueous cleaning method is preferred, the simplest polar rinsing agent that can be used is deionized or distilled water. Deionized water is preferred, since it is less likely to recontaminate the cleaned surfaces. Tap water is not recommended since it can leave organic and inorganic contaminants behind that usually require an alcohol rinse to remove them. Agitation or heating during the rinse cycle can further assist the process. There are also commercially available rinsing solutions that contain surfactants, which lower the solution surface tension and are more effective when rinsing restricted and blind passages.

The final cleaning step is the drying stage. It can occur by natural evaporation, heat lamps, forced displacement by a dry gas, or forced evaporation by heating in a controlled environment (e.g., drying chamber). Most hand soldering operations use one of the first three drying options. Baking and/or vacuum-assisted drying may be necessary to completely remove absorbed or entrapped residual rinsing materials from blind or tight passages and porous materials. Care must be taken also in the type of drying gas selected. A common choice is electronic-grade dry nitrogen or an approved environmentally safe duster propellant. Compressed air should be avoided since it can contain oil particulate from the compressor unit and condensed water from the gas line.

Once the workpiece has been properly postcleaned, caution is still necessary to prevent recontamination of the cleaned surfaces, which could affect processing at the next assembly level. Proper handling and storage procedures should be developed to satisfy these requirements.

Finally, the cleaned parts should be inspected to confirm the effectiveness of the cleaning process. For hand soldering, this typically involves a visual inspection either at 1x or 10x magnification. Portable ultraviolet (UV) or black light units can also be used to fluoresce for dust particles and flux residues. If the cleaning operation requires quantification, the area of interest can be rinsed with a known volume of liquid, extracted, and then analyzed by ion chromatography and liquid chromatography for residue concentration per unit of rinsed area. Ionic residues are especially problematic in corrosive environments. As such, the Ionograph or Omega Meter® method is used to determine the ionic concentration of the rinsed solution by its electrical conductivity. The conductivity measurement is calibrated against a known ionic species concentration, such as sodium chloride. Other analytical techniques, such as surface insulation resistance (SIR) and Fourier-transform infrared spectroscopy (FT-IR), have been successfully used to determine the identity and quantity of flux residues.

The product application should dictate what the required level of postprocess cleaning pass-fail inspection criteria will be. As the reliability of the application increases, more rigorous cleaning criterion is necessary. For most hand soldering operations, qualitative, visual inspection is adequate. The AWS Soldering Handbook (3rd edition) should be consulted for more detailed information on these soldering and related cleaning processes.u

F. M. Hosking and E. P. Lopez are with Sandia National Laboratories, Albuquerque, N.Mex.