Evaluating Crystallographic Quality of Blind Via Bottom Copper Layers via Metallographic Cross-Section Analysis

In Hdi Pcb manufacturing, the crystallographIC quality of blind via bottom copper layers critically impacts interconnection reliability. Coarse grains or abnormal textures accelerate electromigration and reduce mechanical strength. Metallographic cross-section analysis combined with microscopy enables quantitative evaluation of grain size, orientation, and defects. 

1. Sample Preparation

1.1 Sectioning and Mounting

• Precision cutting: Diamond wire saw aligned perpendicular to via axis (≤5μm error, Figure 1);

• Cold mounting: Low-shrinkage epoxy (e.g., EpoFix™) under vacuum, cured at 25℃×24h.

1.2 Grinding and Polishing

• Coarse grinding: SiC paper (#320→#1200) with water cooling;

• Fine polishing:

  • Initial: 3μm diamond suspension, 150rpm×5min;

  • Final: 0.05μm silica colloid, 100rpm×2min (Ra<0.01μm).

2. Microstructure Visualization

2.1 Chemical Etching

• Etchant: Fe(NO₃)₃ (10g) + ethanol (100ml) + H₂O (50ml);

• Conditions: 15-30s immersion at 25℃ (gray-level contrast >30%, Figure 2).

2.2 EBSD Analysis

• Sample preparation: Ion polishing (5kV×2h) to remove damage;

• Scanning: 0.1μm step size, 20kV, IPF-Z mapping.

3. Crystallographic Metrics

3.1 Grain Size Measurement

• Line-intercept method: Draw 5 random lines (total 500μm), calculate average grain size:

  ​

  where L=total length, N=intersections, M=magnification.

3.2 Texture Coefficient

• ODF analysis: Calculate pole densities for (111)/(200) planes:

  ​

  TC>3 indicates strong anisotropy.

3.3 Defect Quantification

• Porosity: ImageJ analysis of dark area ratio (<0.5% required);

• Twin density: Annealing twins counted per mm² (normal range:50-200/mm²).

4. Case Study

Server motherboard blind via analysis:

• Grain size: 8.2μm (plated at 2ASD, additive A=5ml/L);

• Texture: TC(111)=2.1, TC(200)=1.3 (weak texture);

• Porosity: 0.12%;

• Reliability: <2% resistance change after 1000 thermal cycles (-55℃~125℃).

Conclusion

Metallographic cross-section analysis with quantitative imaging provides precise evaluation of copper layer crystallography, guiding plating process optimization to ensure long-term reliability in high-density interconnects.