How to produce electronic-grade silica for PCB substrates

Electronic-grade fused silica is the primary inorganic filler for copper-clad PCB laminates. It lowers thermal expansion (CTE), improves dimensional stability, maintains low dielectric constant, and enhances mechanical strength. PCB-grade silica demands strict control of trace metals, radioactive impurities (U, Th), particle size distribution, maximum oversized particles, and low moisture. Spherical fused silica is preferred over crystalline quartz for high-frequency PCB substrates.

1. Raw Ore Selection & Pre-Purification

Select high-quality vein quartz with low inherent alkali and radioactive impurities. Reject ore with high U, Th, Fe, Na, K, and heavy mineral inclusions.

• Scrubbing and desliming: Remove clay slimes and surface iron oxide coatings.
• Multi-stage high-gradient magnetic separation: Eliminate weakly magnetic iron-bearing minerals.
• Reverse flotation: Remove feldspar, mica and aluminosilicate impurities that raise Al and alkali metals.
• Acid leaching + ultra-pure DI water washing: Reduce Fe, Ca, Mg and soluble metal ions.
• ICP testing is performed after purification to confirm impurity levels before melting.

2. High-Temperature Melting & Spheroidization (Key Step)

Purified quartz powder is fed into a high-temperature flame furnace (1700–1900 °C). Quartz melts and forms liquid droplets, then quenches rapidly to form amorphous spherical fused silica.

• Amorphous structure reduces thermal expansion compared to crystalline quartz.
• Spherical morphology improves epoxy resin filling, powder flowability and lowers slurry viscosity.
• Control furnace atmosphere to avoid carbon and metal contamination. Use quartz or ceramic lining for all contact parts.

3. Ultrafine Grinding & Closed-Circuit Air Classification

Crush the fused silica lumps first, then carry out fine grinding with zero-contamination equipment.

• Two options:
  1. Vertical stirred bead mill with zirconia beads and ceramic lining: high throughput for medium-fine grades.
  2. Jet mill: media-free grinding, best for ultra-fine high-frequency PCB grades with minimal impurity pickup.
• Closed-loop air classification separates finished powder and returns coarse particles back to the mill. Strictly limit Dmax, because oversized particles cause short circuit and substrate defects.
• Typical PSD range for PCB substrates: D50 1–10 μm, customized for different high-speed / high-frequency board requirements.

4. Multi-Stage Purification & Radioactive Screening

• Further remove trace heavy metals via secondary acid washing and deionized water rinsing.
• U and Th screening: critical for high-density PCB and chip packaging substrates. Alpha particles from radioactive impurities cause soft errors. U+Th must be controlled below ppb level.
• Dry thoroughly, moisture content kept below 0.2%.

5. Surface Modification (Silane Coupling Treatment)

This step is mandatory for PCB application.

• Use silane coupling agents to modify silica particle surface.
• Convert surface Si-OH groups into organophilic groups. This improves compatibility between silica and epoxy resin, reduces viscosity, prevents voids and enhances substrate mechanical and dielectric performance.
• Control coating thickness and dosage to avoid excess free silane, which harms laminate curing.

6. Cleanroom Handling, Packaging & Quality Inspection

• All powder transfer, screening and packaging are operated in low-dust clean environment to avoid airborne metal contamination.
• Use nitrogen-sealed aluminum barrier bags for final packaging.
• Batch quality tests: ICP-MS for trace metal, laser particle size analysis, BET specific surface area, sphericity, moisture, dielectric property, U/Th radioactivity and residual silane content.

Key Quality Control Specifications

• SiO₂ ≥99.9%, high-frequency PCB grade ≥99.95%
• Low alkali metals (Na, K) to prevent ion migration and insulation failure
• Low iron and transition metals to avoid dielectric loss
• Low U+Th for advanced high-density PCB
• Spherical shape, narrow PSD, controlled Dmax
• Low moisture, proper silane surface coating

Common Production Challenges

1. Contamination from furnace refractory or grinding media, which increases metal impurities.
2. Poor sphericity: irregular particles raise epoxy viscosity and reduce packing density.
3. Incomplete surface modification: poor resin bonding leads to voids and delamination in PCB laminates.
4. Oversized particles: even a tiny number of coarse grains can cause electrical failure.

The full workflow: high-purity vein quartz pre-purification → high-temperature flame melting & spheroidization → non-ferrous ultrafine grinding + precision air classification → trace impurity removal and radioactive screening → silane surface modification → cleanroom packaging and full-property testing. Amorphous spherical fused silica produced by this route meets CTE, dielectric and reliability requirements for PCB substrates.