Despite the advances in soldering technologies and equipment, hand soldering still remains an important element in the manufacturing of electronic assemblies. The multitude of solder alloys available for use has expanded in recent years due mainly to the advent of Lead-Free assemblies being required for most commercial applications.

The additional influx of solder types, fluxes, and board finishes can create a confusing and convoluted situation, making it more difficult for hand soldering operators to maintain process efficiency. Regardless of the type of solder that is being used, the same principles of solder applications will apply. There will be some minor modifications in the soldering techniques and equipment specific to the solder and flux, but the core competencies of hand-soldering remain the same.

1) The hand soldering operation still greatly depends on the discretion of the technician to make sure that the underlying procedures and standards for a specific material set are followed as rigorously as possible to avoid latent, or even overt reliability failures. One area of concern is to ensure an adequate cleaning process once the assembly has been soldered. Residual contamination of the fluxes left behind from soldering can cause corrosion and leakage currents which will manifest themselves after exposure to high humidity testing.

2) R, RMA, and RA fluxes are generally used for hand soldering operations, and require different levels of cleaning. The solder itself will often have designators that are descriptive of the type of flux core used, giving an indication of the cleaning process to be used for that solder. The table in Figure 8-1 is a guideline for the removal of flux in rosin type solders.

3) Rosin fluxes are generally accepted for military applications because of the non-ionic nature of the flux. In a case where a combination of a Lead-Free solder and a non-wettable surface is encountered or written into specification, additional flux may be required for proper wetting. The use of an ionic, water soluble flux will assist in the soldering process, but will also require different cleaning agents for their removal. Since these cleaners are water based, care must be taken to ensure proper drying of the assembly, and appropriate test measures should be employed to guarantee an ion free surface.

4) A quick in-line technique for testing cleanliness, which can be employed by the soldering technician, utilizes a conductivity probe and a meter. The assembly can be quickly hand washed with DI water, and the effluent can be measured for resistance. Following the method employed by IPC-TM-650, the concentration of NaCl equivalents can be calculated using standards to form a calibration curve for solids concentration based upon resistance measurement.

The probes can be connected to a hand held resistance or conductivity meter, which can measure very accurate levels of ionic activity. This method can be employed as an expedient way to gauge the effectiveness of a cleaning technique, and is fairly accurate, especially for smaller assemblies.

5) Although no clean fluxes are intended to avoid the cleaning process, there are applications where a no-clean flux must be cleaned to meet a specific class requirement. In the case where cleaning is necessary, a no-clean flux should not be cleaned with water or alcohol. The flux residue will absorb moisture and re-crystallize on the joint surface, leaving an undesirable white residue. Though the residue is non-ionic in nature, its appearance is unseemly and will foster doubts about the cleanliness of the assembly. Recognizing the need to address this issue, flux and cleaning vendors have formulated specific semi-aqueous solvents that can be used to rinse no-clean fluxes, similar to the ones utilized for rosin systems.

Cleanliness is an important aspect of hand soldering that can be controlled by the proper selection of fluxes and cleaning solvents. The training center offers extensive training in the area of hand soldering, covering various aspects and techniques for assuring a reliable hand soldered assembly. For more information please contact our helpline at 610-362-1320.