For making sound interconnections, the characteristics and properties of the circuit board surface finish are as important as the component leads and termination. Hot air solder leveled SnPb (HASL) has been used successfully as the surface finish for surface mount and mixed PCBs.
As the need of a flat surface with uniform thickness becomes increasingly important to forming consistent and reliable fine-pitch solder joints, the HASL process often falls short.
Alternatives to HASL include immersion Sn, electroplated SnPb (reflowed or nonreflowed), electroplated Au/Ni, electroless Au/electroless Ni, immersion Au/ electroless Ni, immersion Pd, immersion Pb/electroless Ni, electroplated Sn- Ni alloy, and organic coating.
When selecting an alternative surface finish for a PCB assembly, the key parameters to consider are solderability, ambient stability, high-temperature stability, suitability of the use as contact/switch surface, solder-joint integrity, wire bondability of assemblies that involve wire bonding, and cost.
Whatever its deficiencies may be, HASL provides the most solderable surface. However, comparing a metallic system with HASL, the latter subjects PCBs to higher temperatures (above 200°C), producing inevitable thermal stress in PCBs.
Furthermore, HASL is not suitable for wire bonding. To choose a valid replacement for HASL, many variables need to be assessed. Understanding the fundamentals behind each variable, in conjunction with setting proper priority of importance among these variables for a specific application, is the way to reach the best balanced solution.
When selecting an alternative surface finish for PCB assembly, the key parameters to consider are solderability, ambient stability, high temperature stability, suitable for use as a contact/switch surface, solder-joint integrity, and wire bondability for those assemblies that involve wire bonding, and cost.
Table 5.14 summarizes the relative performance of PCB surface finish systems.
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