Two-component Silicone Potting Adhesive RH-AS1100

In a liquid-cooled 6.6 kW on-board charger, the power tray is potted after wave-soldered magnetics and ceramic capacitor banks are mechanically fixed to an aluminium baseplate. For RH-AS1100, the manufacturer's batch certificate is required to confirm the mix ratio; the process sequence below is configured for a nominal 1:1 by weight addition-cure system and must be verified against the lot-specific certificate. The mixed material is degassed at 2.0 kPa for 3 min before being introduced into the cavity under 5.0 kPa vacuum. The dispense program begins at the lowest cavity corner and advances at a controlled flow rate of 0.5–1.5 mL/s to prevent air entrapment over large MLCCs and around ferrite core air gaps. The cavity depth ranges from 2 mm beneath the substrate to 18 mm in the inductor wells, a condition that requires a two-step cure: 60°C for 20 min, followed by 80°C for 30 min to stabilize shore hardness without generating internal stress. The cured material is evaluated according to ASTM D5470 for thermal conductivity, ASTM D149 for dielectric strength, and IEC 60068-2-14 thermal cycling from -40°C to 125°C; power cycling of the charger at 85°C ambient indicates that void-free potting reduces hot-spot temperature rise by 8–12 K relative to air-cooled open-frame designs of similar construction when the encapsulant thermal conductivity is in the 0.2–0.4 W/m·K range. Process failures on manufacturing lines typically appear as bubbles trapped under low-standoff ceramic capacitors and as incomplete wetting on solder-flux deposits, so pre-cleaning with an isopropanol-based flux remover and plasma treatment at 50–100 W on the aluminium carrier are mandatory where cavity complexity exceeds the flow-front capability of the mixed adhesive.

Pot life in the static mixer is the main process conflict in this cavity configuration. At a workshop temperature of 25°C, the addition-cure reaction of the mixed mass is already underway; viscosity rises from approximately 1.5 Pa·s to 4.0 Pa·s within 40 min, increasing the pressure drop across an 18-element static mixer from 0.3 MPa to over 1.2 MPa at a flow rate of 1.0 mL/s. If the potting program exceeds this window, the dispense controller may alarm on pressure and reject the cavity before full fill is achieved. The addition-cure platinum catalyst is also susceptible to inhibition by amine-bearing solder resists, sulfur-containing o-rings, and tin-catalyzed RTV residues; therefore separation of work-in-process boards from condensation-cure adhesives is enforced on the potting line. Electrical clearance and creepage requirements are verified according to IEC 60664-1 for pollution degree 2, and the cured material is subjected to dielectric withstand at 2.5 kV AC for 60 s after thermal cycling.

Representative process windows recorded for two cavity geometries in a pilot-run on-board charger are given below. Values are lot-dependent and must be confirmed against the RH-AS1100 certificate.

Outdoor LED Driver Thermal Management and the 2.5 mm Clearance Problem

A 150 W outdoor LED driver for roadway fixtures is potted directly into an aluminium IP66 housing. The two components are metered at the 1:1 by weight ratio on a gear-pump dispense unit and mixed through a 12-element static mixer before entering the housing. The addition-cure silicone is used because it maintains a stable Shore A hardness of approximately 25–35 at 25°C and does not embrittle after 1,000 h exposure in an ASTM G154 cycle of 340 nm UVA and condensation. The housing is preheated to 50°C so that the mixed viscosity falls below 1.5 Pa·s, allowing the compound to fill the 2.5 mm clearance above the main switcher IC without lifting the SMD components. Potting is performed with a dispense needle of 1.2 mm internal diameter at 0.3 MPa pressure; the part is then rested for 5 min at atmospheric pressure before final cure at 80°C for 20 min. Compliance is demonstrated against UL 8750, IEC 61347-2-13, and UL 94 V-0 at 3.0 mm thickness; surge-protected drivers are also tested for tracking resistance according to IEC 60112 at 600 V CTI. The terminal product is an assembled LED driver module sealed against moisture ingress at IP66 with thermal conductivity in the filled state of 0.3 W/m·K as measured by ASTM D5470. A process boundary arises where the clearance is below 1.0 mm; below this gap, flow-induced shear can orient filler platelets and reduce local thermal conductivity, so cavity design rules specify a minimum 1.5 mm gap over high-heat components.

Unlike epoxy encapsulants, the silicone remains sufficiently low in modulus to accommodate differences in coefficient of thermal expansion between the aluminium housing and glass-fibre PCB across the -40°C to 85°C outdoor range of IEC 60068-2-14. The main failure mode on production lines is incomplete coverage beneath the toroidal inductor, which is mitigated by tilting the housing 5° from horizontal during dispense and by allowing the fluid to settle for 60 s before gelation begins. Use of amine-bearing conformal coatings beneath the potting is not permitted because residual amine groups retard the platinum cure and create a tacky interfacial layer.

The same 1:1 addition-cure silicone is applied to a transmission-mounted speed sensor where the PBT housing must survive continuous oil immersion at 125°C and short excursions to 150°C. The potting compound is introduced into an annular cavity of 0.8 mm width after the sensor chip and lead frame are welded, using a low-pressure jet valve at 0.15 MPa and a dispense shot of 1.2 mL. Chemical resistance is screened by immersion in Dexron VI ATF and polyalphaolefin gear oil for 1,000 h at 125°C according to ISO 16750-3, with hardness change limited to ±5 Shore A points. The sensor is subjected to thermal shock from -40°C to 150°C per ISO 16750-4 and vibration at 20 g random profile per IEC 60068-2-64. Because the cavity is narrow, the material is degassed to 2.0 kPa before the shot; otherwise microvoids appear at the interface with the lead frame after the first thermal shock. The terminal part is a potted speed sensor ready for transmission bell-housing installation.

What Limits Pot Life When an Addition-Cure Silicone Contacts ENIG-Coated Copper Traces?

For a 1200 V IGBT gate-driver board potted into a polycarbonate shell, the primary production risk is not thermal failure but cure inhibition at the interface between the silicone and the printed-circuit-board metallization. Addition-cure silicones rely on a platinum coordination complex and hydride-functional crosslinkers; if the substrate carries organic solderability preservative films, rosin residues, or sulfur-bearing solder masks, the platinum catalyst is partially deactivated and the interfacial layer remains liquid after the bulk material has reached full hardness. This failure mode is observed as a sticky residue around the ENIG edge connectors after a nominal cure of 80°C for 20 min. The corrective action is to eliminate the residue source: bare copper is re-passivated with ENIG, solder flux is cleaned with an isopropanol-based saponifier, and the board is baked at 85°C for 4 h before potting. The pot life at 25°C for the mixed compound is typically 45–60 min; if the material is held in a static mixer at 35°C, the usable dispense window drops to 20–25 min. The dispenser is therefore configured with a chilled static mixer jacket at 20°C to maintain a stable viscosity of 1.8 Pa·s and to prevent gel formation in the mixer.

The gate-driver assembly is potted with the 1:1 by weight mix ratio specified for RH-AS1100 and cured in a staged profile of 40°C for 30 min, 60°C for 20 min, and 80°C for 30 min to reduce stress on the gate resistors. After cure, volume resistivity is checked at 500 V DC according to ASTM D257, with acceptance at 1×10^14 Ω·cm or higher; dielectric strength is measured according to ASTM D149 at 18 kV/mm on 2 mm specimens. The terminal product is a potted 1200 V gate-driver unit for compressor drives, with a flame classification of UL 94 V-0 at 3.0 mm.

A 300 W roof-mounted microinverter contains a copper-wound MPPT inductor and aluminium electrolytic capacitors that are potted together in a powder-coated aluminium shell. Because the module is exposed to condensation and rapid thermal swings on the roof, the potting compound must provide a continuous moisture barrier without imposing mechanical stress on the capacitor pressure-relief vents. The 1:1 by weight mixture is degassed at 1.5 kPa and dispensed into the shell at 0.4 MPa using a needle valve; the fill is carried out in two stages with a 30 s dwell between shots to allow air to escape from the inductor core. The cure profile is 60°C for 30 min followed by 7 days ambient post-cure for full adhesion to the powder-coated aluminium. Electrical safety is evaluated under IEC 62109-1 and UL 1741, with dielectric withstand tested at 3.0 kV AC for 60 s after 84 h at 85°C and 85% relative humidity. Thermal cycling follows IEC 60068-2-14 from -40°C to 85°C for 1,000 cycles, and the acceptance criterion is zero visual cracking and no loss of adhesion on the capacitor side walls. The resulting terminal product is a sealed microinverter in which the silicone provides both dielectric isolation and vibration damping for the MPPT stage.

When a BMS Cell-Monitoring Unit Requires Thermal Stability at 105°C Continuous Service

In a high-voltage battery management system, the cell-monitoring unit is potted after electrical test to protect the cell-voltage sense lines and chip resistors from corrosion and mechanical fretting. The continuous service temperature at the board surface is 105°C, which demands that the cured silicone retain its dielectric properties and softness after long-term thermal aging. The 1:1 material is dispensed at 0.25 MPa through a 0.8 mm needle into a polycarbonate cover, then cured at 80°C for 30 min; the relatively low dispense pressure prevents jetting damage on the 0603 resistor terminations. Thermal aging is conducted at 125°C for 1,000 h according to UL 746B, with a retained Shore A hardness within 5 points of the initial value. Volume resistivity after aging is measured at 500 V DC by ASTM D257 and must remain above 1×10^13 Ω·cm to avoid leakage currents across adjacent cell-voltage channels. The terminal product is a potted CMU with creepage and clearance design verified under IEC 60664-1 for pollution degree 2 and installed inside the battery enclosure.

The compliance matrix for this application is given below.