The shelf life of a circuit card (cc) assembly prior to manufacturing is about 6 months in normal processing environments. Considering that the world's electronic suppliers have expanded globally to include locations such as China and India, it is likely that a printed wiring board (PWB) may sit in transit for up to 4 months prior to reaching the circuit assembly manufacturer. Since environmental restrictions regarding use of hazardous materials have been implemented in the U.S. and Europe, lead-based paints and lead-based surface finishing of electronic devices have been replaced with lead-free versions. However, lead-free surface finishes contain high levels of tin which form tin whisker protrusions in the presence of corrosion or high levels of oxidation. These metal protrusions can transmit high levels of electrical current when bridging to adjacent circuit traces.
Printed wiring board (PWB) surface finishes require a well-defined environment and/or a long shelf life to allow for reliable solder joints. The surface finish degradation due to airborne contaminants and oxide surface formation are both significant factors in determining PWB soldering shelf life. Electroless nickel over immersion gold (ENIG) has been used in the past for military application. Commercially robust surface finishes such as immersion tin, immersion silver, Organic Solderability Preservative (OSP), and other imidazole coatings are used to form a barrier and extend the shelf life on the surface finishes to be soldered. The degree to which the soldering process is impeded depends on the degradation thickness and surface area of the OSP and the imidazole coatings. As time passes, a “worse than” condition can occur using these standard organic coatings. For OSP-coated PWBs, the plastic can be contaminated with finger oils and other manufacturing support materials that deteriorate the solderability of the coating. For non-coated PWBs, the oxide which forms on the metallized surface raises the melting point of the solder when reflow occurs. If high enough concentrations of oxide permeate the unprotected PWB, a cold solder joint will be formed and will fail prematurely. This becomes critical for fine pitch, high density components in electronic designs.
Accordingly, a protective coating to preserve the solderability of a circuit card during storage is desired to improve the manufacturing of circuit card assemblies and printed wiring boards.