ENIG

/ˈiː nɪdʒ/

n. "Electroless nickel immersion gold PCB surface finish protecting copper pads via Ni barrier + thin Au unlike HASL solder."

ENIG, short for Electroless Nickel Immersion Gold, plates 3-6μm mid-phosphorus nickel barrier over exposed copper followed by 0.05-0.2μm immersion gold—prevents Sn diffusion creating brittle Cu3Sn while ensuring wire bondability and solder wettability for BGA/LGA fine-pitch. Contrasts HASL's uneven tin-lead by providing flat coplanar surface ideal for SerDes 224Gbps transceivers where black pad (Ni-P hypercorrosion) risks assembly yield; Pd activator catalyzes electroless Ni without current.

Key Characteristics: Ni Barrier Layer 3-7μm 6-9% phosphorus prevents Cu-Sn diffusion, blocks oxidation. - Immersion Au Cap 0.05-0.23μm self-limits via Ni displacement, protects Ni 12+ months. - Black Pad Risk hypercorrosion at Ni/Au interface from excess P; mitigated by pH control. - IPC-4552 Spec Ni 120-240μin, Au 2.5-5μin; wire bond >30g force guaranteed. - Lead-Free Compliant RoHS alternative to HASL for BGA/ENIG fine-pitch 0.3mm.

-- ENIG process control parameters for PCB fab
-- Typical production recipe for SerDes HDI boards

library ieee;
use ieee.std_logic_1164.all;

package enig_params is
    constant NI_THICKNESS_MIN : integer := 120;  -- μin IPC-4552A
    constant NI_THICKNESS_MAX : integer := 240;
    constant AU_THICKNESS_MIN : integer := 3;    -- μin
    constant AU_THICKNESS_MAX : integer := 8;
    
    constant NI_P_CONTENT     : real := 0.07;     -- 7% mid-P for ductility
    constant BATH_PH_MIN      : real := 4.5;      -- Hypercorrosion control
    constant BATH_PH_MAX      : real := 5.2;
    
    type pcb_zone_array is array (0 to 3) of integer;
    constant ZONE_THICKNESS : pcb_zone_array := (150, 140, 155, 145);
    
    function check_enig_ok (ni_um, au_um : integer; p_percent : real) 
        return boolean is
    begin
        return (ni_um >= NI_THICKNESS_MIN and ni_um <= NI_THICKNESS_MAX) and
               (au_um >= AU_THICKNESS_MIN and au_um <= AU_THICKNESS_MAX) and
               (p_percent > 0.06 and p_percent < 0.09);
    end function;
end package;

Conceptually, ENIG sandwiches Cu-Ni-Au sandwich protecting SerDes QSFP-DD 400G transceivers from Sn whisker shorts while enabling flip-chip C4 bumps—Ni barrier blocks Cu electromigration into solder joints while Au ensures zero-oxide wetting during reflow. Hypercorrosion avoided via hypophosphite reduction balance; contrasts OSP organics by surviving 3x reflow cycles for PAM4 PHYs backhauling Bluetooth 240GHz aggregators on ENIG-clad 2.5D interposers.