Select a surface finish for the connection it must support. Soldered pads, wire-bond areas and repeatedly mated contacts can require different properties. Begin with package geometry, assembly operations, storage and the service environment, then confirm a compatible qualified process with the fabricator.
Solder-coated and organic options
HASL leaves a solder coating and requires review of pad flatness for the intended packages. OSP protects copper with an organic coating and needs a compatible handling and thermal process. Compare the actual assembly sequence and storage conditions rather than treating either finish as universally suitable.
Immersion and multilayer metallic options
Immersion tin and immersion silver provide different surface chemistries with their own handling and process considerations. ENIG uses nickel and gold layers; ENEPIG adds palladium in the finish structure. Evaluate the required soldering or bonding process and the supplier’s qualified construction instead of choosing by appearance.
Contact areas may need a separate finish
Hard gold is commonly considered for wear-resistant contact regions such as edge fingers. That requirement differs from the solderability requirement on component pads. Identify selective finish areas explicitly and confirm that the manufacturing sequence can support the combination.
Write a finish decision record
List each candidate against pad geometry, soldering cycles, bonding, contact wear, storage, environmental exposure and cost. Record unknowns and required validation. A finish change on a repeat order should trigger review of the assembly and acceptance assumptions it affects.
Release checklist
- Identify the purpose of each exposed surface.
- Check package flatness and assembly compatibility.
- Specify selective areas and acceptance requirements.
- Approve storage, packaging and substitutions.