Electronics potting compounds and encapsulants demand precise rheology — low enough viscosity to flow around components, high enough thixotropy to prevent…
1–5% Typical Loading in Potting Resin
Thixotropic Index Achievable
Total Ionic Contamination
Electronics potting compounds and encapsulants demand precise rheology — low enough viscosity to flow around components, high enough thixotropy to prevent settling before cure. SEMISIL fumed silica gives epoxy, polyurethane, and silicone systems the structured flow needed for reliable PCB protection, LED encapsulation, and module potting — without introducing ionic contamination.
Potting compounds and encapsulants protect PCBs and electronic components from moisture, vibration, thermal shock, and…
Potting compounds and encapsulants protect PCBs and electronic components from moisture, vibration, thermal shock, and chemical exposure. Most systems are based on epoxy (bisphenol A/F), polyurethane, or silicone resins — all requiring specific viscosity profiles for automated dispensing on production lines. Too thin and the compound drips or runs off vertical board surfaces; too thick and it traps air voids around fine-pitch components and connector pins.
Fumed silica functions as a rheology modifier by forming a thixotropic network within the liquid resin system. Under high shear — such as during mixing or pump dispensing — the particle network breaks down, reducing viscosity and allowing the compound to flow freely. When shear is removed, the network reforms within seconds, preventing dripping and sagging before the system gels and cures. At dosages of 1–5% by weight, SEMISIL fumed silica achieves a Thixotropic Index (TI) of 3–8, measured as the viscosity ratio at 0.5 rpm versus 5 rpm on a Brookfield viscometer.
Low ionic purity is a critical parameter for electronics-grade fumed silica. Ionic contaminants — principally Na+, Cl−, and K+ — present in the encapsulant can migrate under DC bias conditions, causing electromigration, dendritic growth, and leakage current on fine-pitch PCB traces and semiconductor bond pads. SEMISIL electronics grades are tested by ion chromatography and provide less than 50 ppm total ionic extractables, making them suitable for direct contact with sensitive circuitry.
Liquid compound flows off vertical boards and over component edges before the system gels, causing uneven coverage and exposed areas.
Viscosity too high leads to voids around fine-pitch components and connector contacts — air pockets that compromise moisture and vibration protection.
Na+ and Cl− ions from poorly controlled fillers cause corrosion and leakage current on PCB traces under bias voltage.
Filler loading affects the coefficient of thermal expansion (CTE) of the cured compound, introducing mechanical stress on component solder joints during thermal cycling.
Thixotropic additives must not accelerate gelation of two-part epoxy or PU systems — premature viscosity rise wastes material and clogs dispensing equipment.
Rheology must be uniform across production temperatures (20–50°C). Viscosity shifts with temperature affect shot weight and coverage — critical for automated dispensing robots.
1. Thixotropic Index 3–8 at 1–5% Loading Sag and drip eliminated on vertical boards without excessive zero-shear…
Sag and drip eliminated on vertical boards without excessive zero-shear viscosity. Application viscosity under pump pressure remains low enough for fine-pitch dispensing.
Less than 50 ppm total ionic extractables (Na+, Cl−, K+) measured by ion chromatography. Suitable for direct contact with PCB traces, bond pads, and semiconductor packages.
SEMISIL R202 (PDMS-treated) and R272 (DDS-treated) provide moisture-barrier performance for silicone potting and hybrid epoxy/PU systems that require low water uptake.
Compatible with epoxy (bisphenol A/F, novolac), polyurethane, and silicone resin systems. No interaction with amine or anhydride hardeners; does not interfere with platinum or tin catalysts in silicone RTV.
Thermally stable above 300°C. No out-gassing during cure or thermal cycling. Suitable for high-reliability automotive electronics and industrial power modules.
| Grade | BET Area | Surface | Best Resin System | Use Level | Key Property |
|---|---|---|---|---|---|
| SEMISIL 200 | 200 m²/g | Hydrophilic | Epoxy (bisphenol A/F) | 1–4% | Efficient thixotropic network at low dosage |
| SEMISIL 300 | 300 m²/g | Hydrophilic | Waterborne epoxy | 1–3% | High thixotropy efficiency, lower dosage needed |
| SEMISIL R202 | 110 m²/g | PDMS hydrophobic | Silicone potting | 2–5% | Moisture barrier, fully silicone-compatible |
| SEMISIL R272 | 130 m²/g | DDS hydrophobic | PU potting, hybrid epoxy | 2–5% | Low polarity surface, organic resin compatible |
Two-Component Epoxy Systems: Add fumed silica to Part A (resin) only. Adding to Part B (hardener) may cause unexpected viscosity change and pot life shortening. Pre-mix with a high-shear disperser at 1000–3000 rpm until fully wetted before combining with Part B.
1. Pre-Dry Fumed Silica If the production environment exceeds 60% relative humidity, dry fumed silica at 105°C for 2…
If the production environment exceeds 60% relative humidity, dry fumed silica at 105°C for 2 hours before use. Moisture adsorption on the surface of hydrophilic grades reduces dispersion efficiency and can cause agglomerate formation.
Add fumed silica gradually to resin Part A under 1000–3000 rpm mixer speed. Avoid applying vacuum until the powder is fully wetted — premature vacuum can lift undispersed powder and create surface contamination.
Target TI of 3–8, defined as the viscosity ratio at 0.5 rpm divided by 5 rpm on a Brookfield RV or DV viscometer. Adjust fumed silica loading in 0.5% increments and re-measure after 30 minutes at rest to allow network recovery.
After blending Part A and Part B, degas the mixed compound at less than 1 mbar for 10–20 minutes before dispensing. This removes entrapped air introduced during mixing — critical for eliminating voids around fine-pitch components.
Warming the compound to 35–45°C can reduce application viscosity by 30–50% for fine-pitch dispensing, without significantly compromising sag resistance at rest after dispensing. Validate TI at the target dispensing temperature.
Recommended Dispersion Equipment: Cowles dissolvers, bead mills, or three-roll mills provide sufficient shear for full dispersion. Planetary mixers operating above 1000 rpm are acceptable for lab-scale batches. Avoid low-shear stirring — fumed silica aggregates will not break under gentle mixing, resulting in a grainy, under-structured compound.
Request technical samples and ionic purity data sheets for your PCB or LED encapsulation application.
Typically 1–5% by weight in the resin component (Part A). At 2%, most epoxy systems reach a TI of approximately 4–5 (Brookfield 0.5/5 rpm ratio). Higher loadings give more structure but increase zero-shear viscosity and make dispensing more difficult. Optimize by measuring both application viscosity under shear and sag resistance at rest on a vertical surface.
Ionic contaminants — Na+, Cl−, K+ — present in the encapsulant can migrate under DC bias voltage, causing electromigration, dendritic growth, and leakage current on fine-pitch PCB traces. Electronics-grade fumed silica is tested by ion chromatography; SEMISIL grades deliver less than 50 ppm total ionic extractables, meeting the requirements for high-reliability PCB and semiconductor assembly applications.
Yes, but use hydrophobic grades (SEMISIL R272, R620) to minimize moisture competition with the photoinitiator. Loading should be kept at or below 1.5% to maintain sufficient UV transmission. Higher loadings increase light scattering, which can result in undercured zones in deep sections of the encapsulant, particularly in thick dams or glob-top applications.
At typical loading (1–5%), fumed silica has minimal impact on tensile strength and elongation of cured epoxy or polyurethane systems. It may slightly increase the coefficient of thermal expansion (CTE) and marginally reduce brittleness due to energy absorption at the filler-matrix interface. For critical thermal-mechanical specifications — particularly in automotive power modules — verify with dynamic mechanical analysis (DMA) on cured specimens.
Fumed silica creates a true thixotropic network through reversible hydrogen bonding between surface silanol groups — viscosity drops under shear and reforms at rest. Talc and mica provide a permanent, non-thixotropic viscosity increase, add density, and may introduce ionic contamination. Fumed silica is preferred for high-reliability electronics due to its lower effective dosage, controllable thixotropy, measurable ionic purity, and negligible density contribution to the compound.
Yes. Hydrophobic SEMISIL R202 (PDMS-treated surface) is specifically designed for silicone systems. The polydimethylsiloxane surface treatment ensures compatibility with polydimethylsiloxane matrices, preventing phase separation and maintaining long-term stability in platinum-catalyzed silicone RTV potting compounds. Hydrophilic grades may show reduced compatibility and uneven dispersion in 100% silicone systems.
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