OSP and Selective Electroless Nickel for Mixed-Metal Finish PWBs and BGA Substrates

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The documentation relating to the requirements for alternative surface finishes have been well publicized at many industry forums. Pressure to eliminate lead in electronics assemblies is forcing fabricators and OEMs to reevaluate their surface finish and joint attachment procedures. Many of the requirements, in fact, are obvious. Regardless, the PWB fabrication industry needs to work closely with contract manufacturers and end users to fully appreciate the true impact of technology trends. These trends are significant and include:

• Surface mount continues to increase at the expense of through-hole with finer and finer features.
• The use of non-tin/lead coatings and surface finishes will increase. Surface finishes such as electroless nickel/immersion gold, OSP and immersion tin will be utilized.
• With political movements toward banning lead in all electronic assemblies gaining significant  momentum, lead-free solder pastes and wave solder materials will be adopted.
• The need for more robust solderable finishes under extreme environmental conditions including corrosive atmospheres.
• The alternative surface finishes must perform through multiple thermal cycles with less active pastes and fluxes.
• The use of multiple metal finishes on the same bare board will place new emphasis on the compatibility of coating with each other, and the actual assembly module.
• Increased I/O demand and reduced lead pitch will require much tighter controls over the processes used to fabricate the bare board.

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Editor’s Note: This article originally appeared in the February 2015 issue of The PCB Magazine