IPC International Community magazine an association member publication
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24 I-CONNECT007 MAGAZINE I MAY 2026 The Trend Toward Mixed-metal Finishes To achieve high-density surface mounting on printed wiring boards (PWBs), the number of termi- nals on circuit components has been increasing, and the pitch has been significantly reduced. With the trend toward increased packaging density has come the use of chip-on-board (COB), flip chip, and tape automated bonding (TAB). In many instances, the surface mounting of such components may be required on PWBs with copper pads and other features plated with gold, silver, tin, or solder. These mixed-metal finish boards are becoming increasingly common, and the surface treatment of such circuits is becom- ing more important. The demand was such that a water-soluble surface-treating agent capable of protecting bare copper from oxidation without leaving a film on other metals needed to be devel- oped and implemented. In other words, the need for an OSP that selectively bonds to the copper without adversely affecting other metals, such as gold or solder, was established (Figure 1). Conventional OSP processes, based on long- chain alkylimidazole compounds and substituted benzimidazole compounds, have functioned ade- quately to protect the bare copper. However, these materials also deposited a significant film on other metals such as gold, tin, and solder. This additional film interfered with subsequent operations, such as wire bonding and surface mounting of quad flat packs on solder surfaces. In addition, the contact resistance on the gold in- creased to unacceptable levels. Thus, when PWBs are fabricated with multiple metal finishes, the metals (such as gold or solder) must be masked to prevent OSP film formation on their surfaces. In some instances, the coating must be removed with alcohol, adding additional labor and cost to the fabrication process. One factor in promoting this film formation on the metal surfaces is the copper present in many organic solderability formulations. The copper ions form a complex with the active azole ingredient in the OSP chemistry and actually help to promote film growth. When a copper-solder mixed-metal board is processed through such a process, the OSP forms on the solder and discolors it, making long- term solderability virtually impossible to achieve. It has also been determined that the copper ions in the OSP protective film contribute to ionic con- tamination, a situation that is constantly scrutinized by assembly houses and end users. It is desired to keep ionic residues as low as possible. It has been demonstrated that the copper contributes to the staining/darkening of the solder and causes undue build-up of residue on the gold. The Solution The optimal solution to prevent OSP deposition on gold and improve solderability under lead-free assembly conditions is based on a unique organic compound: an imidazole synthesized as the active ingredient in the OSP (US Patent 5,795,409). This unique compound is solubilized in water and a nominal amount of acetic acid. Acetic acid helps maintain a buffered pH in the OSP process. Figure 1: An advanced package with wire bonding (top side) and OSP processed copper on bottom side. 1 T RO U B L E I N YO U R TA N K