What is Kirkendall Hollow

Technical Analysis of Kirkendall Cavities and Ni Oxidation in ENIG Pad Welding

I. The formation mechanism and effect of Kirkendall Void (Kirkendall Void)

Different atomic diffusion and interfacial reaction rate is different

Au dissolution and IMC formation: when soldering, ENIG pads in the Au layer quickly dissolved into the tin-lead solder, and Sn reaction to generate AuSn₄ intermetallic compounds (IMC).At the same time, Sn in the solder reacts with the Ni layer to form Ni3Sn4 IMC.

Accompanying of P-rich layer: due to the doping of phosphorus (P) in the Ni layer, P is crowded out near the interface during the growth of Ni3Sn4, forming an amorphous P-rich layer (Ni-P+ layer).

Reasons for the formation of voids

Ni diffusion and lattice mismatch: Ni atoms diffuse into the solder faster than the reverse diffusion of Sn atoms into the Ni layer, resulting in the formation of an atomic flux imbalance at the interface.This imbalance triggers tiny voids between Ni3Sn4 and the P-rich layer, i.e. Kirkendall voids.(Figure 1-17).

Figure 1-17 Kirkendall Hollow

Temperature dependence: P-rich layers crystallise below their self-crystallisation temperature (e.g. reflow soldering 210°C), exacerbating the formation of voids.

Impact on solder joint reliability

Risk of P-rich layer thickening: the thicker the P-rich layer, the greater the number of voids, leading to a decrease in the mechanical strength of the weld and even causing cracking.

Control strategy:

Optimise the phosphorus content of the Ni layer (typically 3-7 wt%) to balance corrosion resistance with IMC growth rate.

Control the welding temperature profile (e.g. peak temperature, heating rate) to reduce excessive Ni diffusion.

Add trace elements to form specific metal particles to inhibit atomic diffusion.

II. Ni Oxidation Problem and Welding Failure

Oxidation mechanism

Au layer defects: If the Au layer in the ENIG process is too thin (<0.05 µm) or the storage time is too long, the Ni surface is exposed to the environment and oxidation occurs to form NiO.

Loss of wettability: The oxidised Ni layer cannot be wetted by the solder, resulting in a discontinuous formation of IMC and only Au-Sn agglomerated IMC (e.g. AuSn₄) remains.

Failure performance

Interface morphology: the solder interface shows an "island-like" IMC distribution, lacking a continuous Ni3Sn4 layer (Figure 1-18).

Figure 1-18 Clustered IMC and solder joint embrittlement due to Ni oxidation

Mechanical Properties: The shear strength of welded joints is significantly reduced, and they are susceptible to failure under thermal cycling or vibration conditions.

Preventive measures

Process control:

Ensure Au layer thickness ≥ 0.05 µm to avoid pinhole defects.

Shorten PCB storage time (<6 months recommended) and control humidity (<30% RH).

Pre-soldering treatment:

Perform plasma cleaning or micro-etching on PCBs stored for long periods of time to remove surface oxides.

Use solder paste containing high wetting power to enhance wettability.

III. Comprehensive optimisation recommendations

Material Selection:

Use low phosphorus content Ni layer (3-5 wt%) to balance corrosion resistance with IMC growth rate.

Select lead-free solder (e.g. SAC305) to reduce the catalytic effect of Sn-Pb eutectic on IMC growth.

Process optimisation:

Reflow soldering temperature profile:

Control the peak temperature at 245-255°C to avoid excessive Ni diffusion due to long time high temperature.

Extend the 200-220℃ insulation section to promote the uniform growth of IMC.

Nitrogen protection: Reduce oxygen content to <50 ppm to reduce the risk of Ni oxidation.

Inspection and monitoring:

SEM/EDS analysis: Periodic inspection of weld interface IMC thickness (ideally 1-3 µm) vs. voiding (<5%).

Reliability testing: Thermal cycling (-55°C to 125°C, 1000 cycles) and vibration testing to verify solder joint life.

Through the above measures, Kirkendall voids and Ni oxidation can be effectively suppressed, improving the reliability of ENIG pads under complex working conditions.