These under bump metallization steps produce optimum results on copper and copper alloys:
- Cleaner – Epithas MCL-10 acid soak cleaner removes surface soils and contamination and wets the surface to enhance solution transfer. (Also available: MCL-067 CMOS-compatible,
low-foaming acid soak cleaner.)
- Micro-etch – removes copper surface oxide and roughens the copper, which enhances nickel adhesion.
- Acid rinse – ensures that the copper surface is oxide-free and residual copper has been removed.
- Pre-dip – minimizes acid dilution of the subsequent activator bath.
- Activator – MCT-14 is an extremely stable activator for copper and copper alloys. It is well-suited for activating isolated copper pads for subsequent nickel deposition. The MCT-14 immersion reaction bath displaces copper and seeds it with palladium. The palladium initiates the subsequent autocatalytic nickel plating step, electroless nickel. The electroless nickel is an autocatalytic process. The nickel layer acts as a diffusion barrier that is solderable or aluminum
- Electroless nickel – NPR-18 electroless nickel is mildly acidic plating bath formulated for fine line circuitry encountered in silicon wafer plating. Bath formulation allows deposition of electroless nickel without bridging. Highly stable, easy to use. 6-8% phosphorus content as plated.
- Immersion gold – TDS-20 cyanide-free immersion gold bath was engineered specifically for surface mount applications and wafer plating. Neutral pH; will not corrode the underlying nickel surface. Provides excellent solder joint reliability,
including small solder balls. The thin gold coating maintains long-term solderability and prevents nickel oxidization, and passivation.
- Auto-catalytic gold – TMX-15 neutral pH autolytic gold bath is cyanide-free, and was designed specifically for surface mount applications and wafer plating. Will not corrode the underlying nickel surface, thus it provides excellent solder joint reliability.
We recommend TDS-20 immersion gold as the under-layer, and NPR-18 electroless nickel with TMX-15 electroless gold.
Read Under Bump Metallization: Some Economical Alternativesby Mario Orduz, published in US Tech.