How to Meet Stringent Purity Requirements of Silica Powder for PCB Substrates

Copper clad laminates (CCL) are the core base material of printed circuit boards (PCBs). High-purity spherical and angular silica powder is added into epoxy resin as inorganic filler to lower thermal expansion, improve heat resistance, dimensional stability and insulation performance.

PCB substrates, especially high-frequency high-speed PCB materials for servers, communications equipment, aerospace electronics and automotive radar, impose extremely strict purity limits on silica powder. Trace metal impurities, radioactive elements and ionic contaminants will cause:

• Increased dielectric loss and signal distortion at high frequency
• Ion migration, electrochemical corrosion and PCB short-circuit risk under high temperature and humidity
• Yellowing of substrate resin, reduced long-term reliability
• Radioactive alpha particle emission leading to soft errors in chips

This article systematically introduces the full-process technical route to produce qualified silica powder that satisfies high-end PCB substrate standards.

1. Raw Quartz Ore Selection & Pre-Purification (Source Control)

Purity defects formed in raw ore cannot be fully eliminated by later grinding.

1. Select low-background quartz ore deposit
   Prioritize quartz resources naturally low in iron, aluminum, alkali metals (Na, K, Li), uranium and thorium. Radioactive elements must be strictly controlled for high-end computing PCB substrates (low alpha requirements).
2. Manual sorting + optical sorting
   Remove visible gangue, mica, feldspar and iron-stained quartz lumps before crushing.
3. Multi-stage crushing with anti-contamination design
   Crusher liners adopt ceramic or polyurethane; avoid manganese steel directly contacting quartz.
4. Scrubbing, desliming and acid leaching (wet purification optional)
   Remove surface clay, iron oxide film and soluble ionic impurities. Dilute mineral acid washes eliminate surface adsorbed metal ions, followed by thorough dewatering and drying. Moisture content must be controlled below 1.2% before dry grinding.

2. Multi-Step Magnetic Separation to Remove Magnetic Impurities

Mechanical wear iron and mineral iron are major hazards for PCB-grade silica. A combined magnetic separation sequence is required:

1. Low-intensity permanent magnetic drum separator (primary stage)
   Remove free ferromagnetic iron debris generated by crushing equipment. Protect subsequent high-gradient magnetic equipment from overload.
2. Rare earth roll magnetic separator (dry pre-separation)
   Remove weakly magnetic minerals such as hematite, limonite and iron-bearing silicate impurities for dry production lines.
3. Vertical ring pulsating high gradient magnetic separator (wet deep purification)
   For highest purity requirements, wet HGMS captures micron-level wrapped iron impurities that dry separation cannot remove.

Process rule: Complete all magnetic separation procedures before ultrafine grinding. Once quartz is ground fine, iron inclusions inside particles become extremely difficult to separate.

3. Zero-Pollution Ultrafine Grinding System (Critical Control Point)

Grinding is where most secondary metal contamination is introduced. Any iron pollution will directly disqualify silica for high-frequency PCB substrates.

Mandatory equipment configuration

1. Grinding chamber full ceramic lining
   99% high alumina ceramic lining or zirconia lining; eliminate steel liner abrasion pollution.
2. Non-metallic grinding media

• Closed-circuit ball mill: 99% alumina ceramic balls
• Stirred media mill: yttria-stabilized zirconia beads
  Forbidden: chrome steel, forged steel balls.

3. Optimized operation parameters
   Proper media filling rate, avoid violent impact that causes ceramic cracking. Periodically screen broken media to prevent sharp fragments from wearing liners.
4. Sealed negative pressure grinding environment
   Prevent external workshop dust and foreign impurities entering the circuit.

Recommended process for PCB silica:
High-purity quartz feed → closed-circuit stirred mill + independent vertical ultrafine air classifier
Stirred mill provides lower contamination risk and narrower PSD compared with ball mills for D97 2–15 μm silica used in high-speed CCL.

4. Low-Pollution Precision Classification

Ordinary steel rotor classifiers continuously shed metal wear particles.
Requirements for PCB-grade production:

1. Classifier wheel, volute, elbows and all powder contact surfaces lined with alumina ceramic.
2. Equipped with secondary air dispersion structure to reduce particle agglomeration and achieve sharp classification curve. Narrow PSD improves substrate filling density and lowers resin viscosity.
3. Stable negative pressure airflow control to avoid dust backflow.
4. Prevent oil pollution: oil-free lubrication for high-speed rotating shafts; compressed air for seal protection must be dry and oil-free.

5. Surface Modification Process Control for CCL Epoxy System

Silica used in PCB substrates requires silane modification to improve compatibility with epoxy resin. Poor modification will cause microvoids inside the substrate and reduce insulation reliability.

1. Preferred coupling agent: KH-560 epoxy silane, matching epoxy CCL resin matrix.
2. Adopt closed heating modification mixer; avoid external impurity entry.
3. Preheat powder fully to remove adsorbed water before adding silane. Excess moisture causes poor silane grafting and free silane residue.
4. Control coating rate stably at 0.7–1.5 wt%. Excessive free silane will degrade thermal resistance and dielectric properties of the PCB substrate.
5. After modification, cool and sieve to remove soft agglomerates.

6. Conveying & Intermediate Storage Anti-Contamination Measures

Many manufacturers ignore conveying links, causing secondary pollution:

• All pneumatic conveying pipelines, elbows adopt ceramic lining.
• Avoid open screw conveyors; use fully sealed conveying equipment.
• Intermediate finished silos equipped with nitrogen blanketing (optional for high-end grades) to isolate moisture and workshop contaminants.
• Regular cleaning schedule to prevent material cross-contamination between different product grades.

7. Strict Quality Testing Standards & Batch Traceability

Establish complete laboratory testing items before delivery to PCB material manufacturers:

1. Element impurity analysis
   ICP test: Fe, Al, Ca, Mg, Na, K, transition heavy metal content
2. Ionic impurity test
   Water-soluble ion (Cl⁻, Na⁺) extraction test; ionic impurities easily cause PCB electrochemical migration failure.
3. Radioactive test
   U, Th content and alpha particle emission test for high-end server PCB substrates.
4. Particle size distribution
   Laser PSD detection (D50, D97, span value)
5. Moisture content test
6. Modification effect verification
   Contact angle test, organic carbon content test
7. Composite trial test
   Mix silica with customer’s CCL epoxy formula to test thermal expansion, dielectric constant and thermal resistance of cured sample.

All test records, production parameters, raw ore batch numbers should be archived for full traceability, which is a basic requirement for entering formal PCB material supply chains.

8. Typical Process Flow for PCB-Grade Silica Powder

High-purity quartz ore → crushing & washing → acid leaching purification → filtering and drying → multi-stage magnetic separation → sealed ceramic-lined stirred mill → all-ceramic vertical dynamic air classifier → powder collection → optional continuous dry silane modification → screening → finished silo → sampling inspection → packaging

Packaging bags must adopt low-ion inner film bags to prevent contamination during transportation.

9. Common Pitfalls That Lead to Purity Failure

1. Using steel grinding media or unlined steel mill body → sharp rise in iron content
2. Magnetic separation arranged after grinding; internal mineral iron cannot be removed
3. Oil-containing compressed air entering classifier or modification equipment → organic contamination
4. Mixing different silica grades in silos and conveying lines → cross contamination
5. Insufficient drying before modification → poor silane grafting, high free ion content
6. Lack of online particle size control → unstable PSD, inconsistent substrate performance batch by batch

Meeting strict purity specifications for silica filler used in PCB substrates relies on whole-chain contamination prevention: starting from high-quality quartz ore pre-purification, multi-stage magnetic impurity removal, fully ceramic anti-wear grinding and classification equipment, controlled surface modification, sealed conveying systems and complete laboratory quality control.

High-frequency high-speed communication PCBs, automotive electronic substrates and aerospace PCB materials represent high-value markets with high entry barriers. Quartz processing plants equipped with full low-contamination production lines and complete testing capabilities can form stable long-term cooperation with CCL manufacturers.