Soldering is a process that is used to bond similar or dissimilar materials by melting a filler metal or alloy that is placed between the components being joined. Soldering has many applications, including plumbing, jewelry making and repair, and the making of electrical connections.
In the manufacture of printed circuit boards, soldering is used to make electrical connections to and between printed circuits. Specifically, soldering is carried out by coating the through hole walls and other conductive surfaces of a printed wiring board with hot, molten solder to make electrical connections by wetting and filling the spaces between the conductive through hole surfaces and the leads of electrical components which have been inserted through the through holes. If the solder adheres inconsistently to the conductive surfaces, or forms too weak a bond with the conductive surfaces, the circuit board may fail or malfunction.
Soldering inconsistencies are often the result of difficulties in keeping the conductive surfaces of the printed circuit board clean and free of tarnishing (oxidation) during the soldering process. A number of treatments have been developed to preserve conductive surfaces (in particular, copper surfaces) in order to improve solderability. For example, Hot Air Solder Leveling (HASL) techniques apply a thin layer of solder to preserve the conductive surfaces and improve solderability in subsequent soldering steps. Other techniques which have been used to prevent surface oxidation and improve solderability include Electroless Nickel/Immersion Gold (ENIG), Organic Solder Preservative (OSP), immersion tin and immersion silver techniques. One immersion silver technique is described in U.S. Pat. No. 6,200,451 (Redline et al.), which proposes the use of a silver plating solution with certain additives to enhance the solderability of a surface. Another immersion silver technique is described in U.S. Pat. No. 6,395,329 (Soutar et al.), which proposes the incorporation of tarnish inhibitors into an immersion plating bath.
While many of the above techniques have been used commercially to manufacture printed circuit boards, there exists a need for improved techniques for treating metal surfaces to enhance solderability, such as techniques which reduce the cost of the treatment or improve the effectiveness of treatment, while maintaining or improving the aesthetic appeal of the resulting surface. For example, the immersion silver technique described in U.S. Pat. No. 6,200,451 (Redline et al.) suffers from the disadvantage that the additives used in the solution are unstable, i.e. the additives deteriorate if the solution sits idle. As a result, the immersion silver solution must be frequently analyzed to determine whether the additives are present in the appropriate concentrations and, if not, the solution must be replenished with additional additives. The additional analysis steps and need to replenish the additives increases the cost and overall complexity of using this immersion silver technique. This is complicated by the fact that the additives used in the solution are particularly difficult to detect through standard analysis techniques. As is described below, the inventors have developed an improved solution and process for treating a surface in order to improve the solderability of the surface which avoids the aforementioned disadvantages and provides other improvements over the prior art techniques.