What Limits Sealing Board CTI in 690 V AC Variable-Frequency Drive DC Links?

In 690 V AC variable-frequency drives, the snap-in aluminum electrolytic capacitor bank sits on a DC bus that can swing between 900 V and 1100 V DC under braking transients. The sealing board is not merely a mechanical closure: it maintains separation between the anode and cathode terminals while resisting carbonization from residual electrolyte creep and surface contamination. Batch records from multi-cavity compression molding lines show that cavity-to-cavity thickness variation remains 1.2–1.8% when fill mass tolerance is held within ±0.5 g and the tool temperature is maintained above 165°C. The safety envelope is governed by IEC 61800-5-1:2007, IEC 60384-4:2017, UL 94 V-0 at 2.0 mm, IEC 60112:2003 CTI ≥ 600 V, and RoHS 2011/65/EU as amended by (EU) 2015/863. The material specification additionally requires water absorption below 0.3% after 24 h at 23°C per ISO 62:2008 and dielectric strength above 12 kV/mm at 2.0 mm per IEC 60243-1:2013.

For VFD DC-link service, the molding compound is metered to a mineral filler plus fiber addition of 160 ± 8 phr relative to 100 phr ortho-novolac resin. A representative production batch consists of 120 ± 10 phr alumina trihydrate with d50 8–12 μm, 40 ± 5 phr chopped E-glass fiber 3 mm, 16 ± 2 phr hexamethylenetetramine, 2.5 ± 0.3 phr calcium stearate, 1.5 ± 0.2 phr fumed silica, and 0.8 ± 0.2 phr carbon black. The carbon black loading is a process-critical variable: below 0.6 phr, static discharge during terminal insertion produces board surface defects, while above 1.0 phr, comparative tracking index drops below the 600 V threshold in third-party audits. The glass fiber length must not exceed 3 mm because longer fibers orient along the gate edge in compression tools and create anisotropic shrinkage that opens a leakage path after terminal swaging. Free-amine additives are excluded from the release system because residual amine accelerates novolac cure during preheating and causes non-fill at the terminal-hole ring.

Processing proceeds on a 250–400 t hydraulic compression press with vacuum venting at −0.095 MPa. The preheated compound is loaded at 85–95°C into a tool held at 173 ± 3°C. Full cavity pressure of 18–22 MPa is applied for 90–120 s before ejection at 150–160°C. Immediate post-cure at 160°C for 4 h reduces residual hexamine-derived ammonia below the level that would otherwise corrode tinned copper terminals. After deburring and hole punching, capacitor assembly inserts tinned copper terminals, cold-stakes the terminal shanks, applies the sealing board to the aluminum case, and crimps at 8–12 kN. Vacuum impregnation of the electrolyte follows, with aging at 85°C for 2 h at 90% of rated voltage. Terminal products are 400–450 V class snap-in capacitors, commonly 470 μF 450 V or 680 μF 400 V, installed in DC-link capacitor banks of 0.75–250 kW drives.

The daily thermal transient in a 1500 V photovoltaic string inverter imposes a different sealing-board demand profile than fixed-speed industrial drives. Before sunrise the DC link can sit at −40°C in cold-climate sites, rise to 105°C by mid-afternoon, and then drop sharply when grid dispatch curtailment interrupts power flow. This cycling occurs while the capacitor remains at 1100 V DC, so dimensional change in the sealing board must not permit the terminal-to-case clearance to fall below the values required by IEC 62109-1:2010 clause 7.3.7.1 and IEC 60664-1:2020 for pollution degree 2 environments. Qualification for photovoltaic service includes IEC 60384-4:2017 damp heat steady-state testing at 85°C/85% RH for 1000 h, UL 94 V-0 at 2.0 mm, and CTI classification of Material Group I under IEC 60112:2003.

Production batches for this segment are adjusted to a mineral filler plus fiber addition of 195 ± 10 phr relative to 100 phr novolac phenolic resin. A typical formulation is 150 ± 10 phr fused silica with median particle size 5–10 μm, 45 ± 5 phr chopped E-glass fiber 3 mm, 14 ± 1.5 phr hexamethylenetetramine, 2.0 ± 0.3 phr amino-silane coupling agent, 0.8 ± 0.2 phr carbon black, and 1.5 ± 0.3 phr calcium stearate. The high fused-silica loading reduces the linear thermal expansion coefficient to 18–22 ppm/°C below the glass transition, which keeps the board inside the aluminum case crimp groove during deep thermal cycles. Silane coupling agent is required at the stated level because fused silica wetting by novolac is less complete than alumina trihydrate wetting, and without surface treatment the moisture uptake under 85°C/85% RH exceeds 0.4% by mass and initiates electrolytic leakage paths. Production holds the material for 2 h at 80°C when ambient relative humidity exceeds 60% before molding.

Molding uses vacuum-assisted compression presses with pellet preheating to 85–95°C and mold temperature of 175 ± 3°C. Cure time extends to 110–130 s because the higher filler volume fraction increases melt viscosity and delays flow to the terminal-hole ring. After ejection at 140–150°C, boards are post-baked at 150°C for 6 h under nitrogen to strip residual moisture below 0.1 wt%. Laser flatness scanning on the production line rejects boards exceeding 0.20 mm bow over 100 mm span. The assembled snap-in capacitors for this segment are typically 500–560 μF 1100 V DC units mounted directly across the inverter MPPT DC bus.

In 800 V on-board charger DC-link stages for electric vehicles, the sealing board operates at continuous internal temperatures approaching 125°C while exposed to electrolyte vapor and high dv/dt transients from silicon carbide MOSFET switching. The board also functions as a terminal-locating component during automated case crimping and must retain pull-out strength after 3000 thermal cycles from −40°C to 125°C. Qualification references LV 123, IEC 60384-4:2017 clause 4.14 damp heat, UL 94 V-0 at 0.75 mm, IEC 60112:2003 CTI ≥ 600 V, and REACH EC 1907/2006 substance restrictions. For 800 V systems, creepage and clearance requirements under IEC 60664-1:2020 push terminal pitch and hole-to-edge dimensions beyond the standard 10.0 mm snap-in pattern, which increases the sealing-board bending stress during stake insertion.

The compound formulation for this thermal class uses a mineral filler plus fiber addition of 135 ± 10 phr relative to 100 phr high-ortho novolac resin. A representative batch consists of 70 ± 8 phr fused silica, 35 ± 5 phr wollastonite, 30 ± 4 phr chopped E-glass fiber 3 mm, 16 ± 2 phr hexamethylenetetramine, 2.0 ± 0.3 phr amino-silane, and 1.5 ± 0.3 phr internal release agent. The wollastonite splits the difference between CTI retention and crack resistance after thermal shock. The post-cured glass transition temperature is controlled at 155–165°C by differential scanning calorimetry, and the molded density is held at 1.78–1.83 g/cm³. Published data for the exact high-temperature compound formula used by competing board suppliers is limited; production qualification therefore relies on thermal cycling and terminal pull-out coupons rather than resin-level disclosure.

Transfer molding is used in this segment instead of bulk compression molding because wall thickness at the terminal-hole ring can drop to 0.70 mm. A 150–200 t transfer press with pot temperature 70–80°C and mold temperature 178 ± 2°C injects the compound at 60–80 MPa. Cure time is 100–130 s. Post-cure uses a ramped nitrogen cycle: 25°C/h to 170°C, hold 4 h, then cool at 15°C/h to avoid microcracks. In capacitor assembly, the board is inserted after terminal cold-staking, the aluminum case is crimped at 10–14 kN, and the assembly is subjected to 100% helium leak testing before electrolyte filling. Terminal products are snap-in aluminum electrolytic capacitors rated 450 V 100 μF to 500 V 82 μF, installed as DC-link capacitors in 7.2 kW and 11 kW on-board chargers and 400 V/800 V DC-DC converters.

Thermal Ageing of 63 V Sealing Boards in Telecom Hot-Swap Rectifiers

In 48 V bus hot-swap rectifier modules for telecom and server power shelves, the snap-in capacitor sealing board is selected for bulk capacitance retention in 65°C ambient air rather than high-voltage tracking resistance. The aluminum electrolytic capacitors operate at 63 V DC and are mounted in parallel arrays with forced-air cooling across the capacitor tops, which creates a 5–10°C surface temperature gradient between the center and edge of each capacitor on the rack. The dominant sealing-board failure mode observed on production returns is not thermal decomposition but relaxation of terminal fit after long-term exposure to 55°C/95% RH, which increases interfacial impedance between the terminal and the board. The relevant safety standard is IEC 62368-1:2020, supplemented by IEC 60384-4:2017, UL 94 V-0 at 2.0 mm, and RoHS 2011/65/EU.

Formulation for this volume segment uses a mineral filler plus fiber addition of 190 ± 10 phr relative to 100 phr novolac phenolic resin. A typical batch is 120 ± 10 phr aluminum hydroxide, 35 ± 5 phr calcium carbonate, 35 ± 5 phr chopped E-glass fiber 3 mm, 15 ± 2 phr hexamethylenetetramine, and 1.5 ± 0.3 phr calcium stearate. The aluminum hydroxide provides flame retardancy, but its dehydration onset near 180°C means mold temperature must remain below 175°C to avoid blistering. Calcium carbonate above 12 phr relative to resin is avoided because it increases water absorption beyond 0.25% and promotes terminal corrosion in high-humidity telecom central offices.

Multi-cavity screw-preplasticized compression molding with 12–24 cavities is the standard processing route. Melt temperature at the screw tip is 85–95°C, mold temperature 170 ± 3°C, cure time 80–100 s, and demolding at 130–140°C. In-line 3D laser thickness inspection rejects boards outside 2.10–2.30 mm before terminal insertion. Terminal products include 4700 μF 63 V snap-in capacitors used in 3 kW hot-swap rectifier modules and system-level output filtering.

If Ripple Current Exceeds 2.5 A at 120 Hz, CTE Mismatch Governs Sealing Board Life

Uninterruptible power supply and energy-storage power conversion system DC buses combine the highest ripple-current and thermal-cycling stress among major snap-in capacitor markets. In a 500 kVA UPS, a single DC-link capacitor bank may carry 2.5–5.0 A at 120 Hz per capacitor continuously, while battery charging cycles raise the internal hot-spot temperature above 105°C for 8–10 h each day. The aluminum case expands at roughly 23–24 ppm/°C, while a poorly formulated phenolic sealing board expands at 28–35 ppm/°C below its glass transition. This mismatch produces tensile stress at the case rim after thermal equilibrium, eventually cracking the board around the crimp groove. Life testing per IEC 62040-1:2017 and UL 1778 is therefore supplemented by CTE measurements and crimp-zone thermal shock cycling. Electrical safety and material compliance are verified under IEC 60384-4:2017, IEC 60112:2003 CTI ≥ 600 V, and UL 94 V-0 at 2.0 mm.

The critical formulation variable in this segment is the ratio of wollastonite to fused silica, not the total filler burden. A production formulation achieving the required 18–22 ppm/°C CTE below Tg contains a mineral filler plus fiber addition of 175 ± 8 phr relative to 100 phr novolac resin. The addition ratio is 90 ± 5 phr fused silica, 50 ± 5 phr wollastonite, 35 ± 3 phr chopped E-glass fiber 3 mm, 15 ± 2 phr hexamethylenetetramine, 2.0 ± 0.3 phr silane coupling agent, and 1.2 ± 0.2 phr release agent. The wollastonite-to-fused-silica ratio is maintained between 0.55 and 0.65. Above 0.65, CTI drops below 600 V in mold-flow weld lines; below 0.55, the board becomes too brittle for the 10–14 kN case-crimp operation.

Processing uses a 300–500 t hydraulic compression press with vacuum chamber and heated platen uniformity of ±2°C. Mold temperature is 180 ± 2°C, cure time 100–140 s, and pressure 20–24 MPa. The board is cured to 95% conversion, then post-cured at 170°C for 4 h to achieve stable CTE. Board thickness is 3.0 mm, and flatness after post-cure is held below 0.15 mm to prevent uneven crimp force. During capacitor assembly, the sealing board is pressed into the case with a force of 8–10 kN before the final 10–14 kN crimp. Terminal products are 2200 μF 450 V and 2700 μF 400 V snap-in capacitors used in UPS DC bus and energy-storage power conversion systems.

The property gradient across three production variants is given in the table below.

The sealing board in inverter-driven air-conditioner and heat-pump compressor drives is a mature, cost-sensitive application where the capacitor must survive −20°C suction-line starts and 50°C condenser-side operation. Moisture absorption and terminal pull-out strength after thermal shock are the primary control points, and the board is qualified against IEC 60335-1:2020, UL 94 V-0 at 2.0 mm, and RoHS 2011/65/EU. The formulation uses 100 phr novolac resin, 120 phr aluminum hydroxide, 40 phr chopped E-glass fiber 3 mm, 15 phr hexamethylenetetramine, and 2.5 phr release agent. Board production is standard compression molding at 170–175°C and 18–20 MPa, followed by automated terminal staking and case crimping. Terminal products are 450 V 330 μF snap-in capacitors for 1–3 kW compressor drives.