Proper flux and cleaning solvents selection can control cleanliness.

Despite advances in soldering technologies and equipment, hand soldering remains an important element in electronics manufacturing. The multitude of available solder alloys has expanded in recent years mainly because of the influx of Pb-free assemblies for many commercial applications.

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

1. The hand-soldering operation still greatly depends on the technician’s discretion to ensure the underlying procedures and standards for a specific material set are rigorously followed 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 from soldering can cause corrosion and leakage currents, which will manifest 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 will often have designators that describe the type of flux core used, giving an indication of the cleaning process for that solder. The equation (below) is a guideline for flux removal in rosin-type solders.

M x V = T

T/A = µ NaCl Eq./cm²

where:

M = Molarity of NaCL

T = µg equivalents

V = Volume (liters)

A = Area

The upper limit for a clean assembly is about 30 µg/cm².

3. Rosin fluxes are generally accepted for military applications because of the non-ionic nature of the flux. In the event a combination of a Pb-free solder and a non-wettable surface is encountered or specified, additional flux may be required for proper wetting. An ionic, water-soluble flux will assist in the soldering process, but will also require different cleaning agents for 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 inline technique for testing cleanliness, which can be employed by the soldering technician, uses a conductivity probe and a meter. The assembly can be quickly hand-washed with DI water, and the effluent measured for resistance. Following the IPC-TM-650 test method, the concentration of NaCl equivalents can be calculated using standards to form a calibration curve for solids concentration based on resistance measurement.

The probes can be connected to a handheld resistance or conductivity meter, which can accurately measure levels of ionic activity. This method is an expedient way to gauge a cleaning technique’s effectiveness, and is fairly accurate, especially for smaller assemblies.

5. Although no-clean fluxes are intended to negate cleaning, there are applications in which a no-clean flux must be cleaned to meet a specific class requirement. In cases 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 assembly cleanliness. In response, flux and cleaning vendors have formulated specific semiaqueous solvents that can be used to rinse no-clean fluxes, similar to ones used for rosin systems.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.