Surface Tension
Metallic surfaces possess a specific capacity to accept molten filler alloys during the heating cycle of a connection. Solderability defines the ease with which a molten metal wets the base copper or plated finish of a printed circuit board. Wetting occurs when the adhesive forces between the solder and the substrate exceed the cohesive forces within the molten solder itself.
A contact angle of zero degrees indicates perfect wetting, while angles above ninety degrees signal a total lack of adhesion.
Wetting Dynamics
Chemical cleanliness at the board surface dictates the final result of the interaction. Oxidation layers act as thermal and mechanical barriers that prevent the molten alloy from creating an intermetallic bond with the pad finish. Flux serves to remove these oxides during the thermal ramp of the reflow process, provided the finish has not degraded during shelf storage.
Excess heat applied for too long a duration causes the copper to dissolve into the solder, creating a thick and brittle intermetallic layer that weakens the joint. Controlled atmospheres within the reflow oven prevent oxygen from reforming these oxides before the filler alloy cools.
Acceptance Criteria
Automated optical inspection systems measure the shape and outline of the fillet to verify that the components formed a secure attachment. Standards such as those published by the IPC establish the minimum amount of wetting required for a joint to remain reliable under mechanical stress or vibration. Variations in plating thickness or the presence of organic contamination under the finish cause voids or pinholes that compromise the electrical path.
Consistent inspection protocols identify batch issues before the fabrication step becomes a source of systemic field failure. High quality finishes exhibit superior spreading characteristics over extended periods of storage in humidity controlled environments.
