Thermal Conductivity in Silicone Molds: Heat Sink Potting and Encapsulation

Thermally conductive silicone is filled silicone. Unfilled platinum RTV and unfilled LSR are electrical insulators and poor heat spreaders. To dump heat into a heat sink you add ceramics, usually alumina or hexagonal boron nitride (BN). Graphite can move heat and can ruin dielectric behavior.

The mold and the bond line decide whether filler does any work. A void is an insulator. A thick brick of “thermal” paste is often a worse cooler than a thin path onto nested metal. Do not invent a watt number or a filler percent. This is heat-sink potting: platinum RTV for pours, LSR for overmold. Tin-cure is the wrong default around copper.

Heat follows a path, not a filler nickname

Ceramic particles raise bulk conductivity by building a particle network through the gum. That network still has polymer at every contact. Unfilled silicone stays in the thermal-insulator class. Filled pastes are better, not aluminum. Metal heat sinks, copper pours, exposed pads, and thickness still dominate. Halving a bond line often does more than chasing the next filler nickname.

Design the path on purpose:

• A defined thin layer of filled silicone between a hot package and a heat sink or chassis
• A nest in the potting mold (or in the housing) that locates the sink so the rubber thickness is a dimension, not a puddle
• Keep-out where metal must seat metal-to-metal
• Soft unfilled gel elsewhere if solder joints cannot live in a stiff, high-filler brick

A uniform 8 mm jacket of filled rubber around a PCB is a mechanical clamp and a mediocre cooler. The mold can do the useful work by holding the sink and by forcing fill from one side so the last air leaves at a vent, not under the die.

Name a test method if you put a conductivity or impedance target on the RFQ. ASTM D5470-style thermal impedance work is common for gap pads and greases (specimen thickness and pressure matter). A plaque number from a pressed disk is not the impedance of a potted cavity with voids. Do not copy a competitor’s marketing watt class onto a pourable RTV.

Alumina versus boron nitride, conceptually

Alumina is the workhorse thermal ceramic in silicone. It is hard, relatively cheap, electrically insulating, and available as irregular grind or as more spherical grades that pack and flow better. It raises viscosity, raises density, cuts elongation, and abrades tools. High alumina loadings are pastes. They settle in a cup. They sand LSR screws and aluminum tryout cavities. Color is white to off-white unless tinted.

Hexagonal boron nitride is the premium platelet. It can deliver more thermal performance per a given process window in many silicone thermal compounds, especially when platelets align. Alignment is a feature and a trap: in-plane conductivity can look better than through-thickness conductivity if flow orients flakes parallel to the heat-sink face (the wrong way for a through-bond-line job). BN is electrically insulating, often kinder on dielectric constant than some dirty alumina packages, and more expensive. It can feel lubricating. It still raises viscosity and still settles if the paste is thin.

Other ceramics (zinc oxide, aluminum nitride, silicon carbide) show up in specialist compounds. Aluminum nitride cares about hydrolysis. SiC can be abrasive and is not automatically dielectric-safe. Do not dump shop ceramics into platinum RTV.

Carbon/graphite thermal silicones can move heat and can percolate electrically. If the potted module has creepage and clearance, carbon is the wrong family unless the drawing wants a conductive path. See the conductive-filler article. Do not assume “thermally conductive” means “still an insulator.”

Filler loading is the compounder’s recipe. More ceramic generally means higher conductivity, higher viscosity, worse elongation, more abrasion, more settling. There is no honest universal percent. Ask for the thermal target and a viscosity or rheology the process can fill.

Bond line, voids, and heat-sink nests

Potting failures that look like “the compound is not thermal enough” are often geometry:

• Air under the package. The mold or the housing must push paste from one side and vent the last corner. Vacuum potting (chamber or vacuum-assist) is a process choice. A pretty cavity will not degas a blind pocket under a QFN.
• Sink not located. If the heat sink can float, the bond line becomes a random thick gap. Pins, steps, or a nest in the tool hold the metal. The silicone thickness over the die is then a drawing dimension.
• Wicking into connectors. Low-viscosity unfilled RTV runs into a pin field. Filled thermal paste may still wet a header. Dam, gasket, or thixotrope.
• Strain. High-filler pastes are stiff. Thermal cycle against a large PCB can crack solder. Sometimes the right stack is a thin thermal path plus a soft gel, not one hardness everywhere.

Alumina and BN pastes trap air in the particle bed. Mix, wet, then vacuum. A dry powder dump into platinum RTV is a foam generator under vacuum. Pressure pots compress remaining bubbles; they do not create a thermal path through a void that still exists as a low-density region.

Viscosity, fill, and which silicone

Pourable “thermal RTV” is already a paste at useful ceramic loadings. If it pours like water, it is probably not carrying much ceramic, or it will settle before gel. Thixotropic thermal RTVs stay on a vertical sink and may not wet under a low-standoff package without vacuum assist or a designed flow leader.

LSR overmold of a thermal grade needs an injectable thermal LSR, larger gates, more pressure, and steel that can live with alumina. Weld lines in a ceramic-filled grade can be polymer-rich (a thermal bottleneck) as well as weak. Do not quote a cup paste through a micro-gate.

Inhibition: platinum against sulfurated gaskets, some fluxes, amines, and some tapes. A steel nest sprayed with a sulfur-bearing release is a new failure mode. The board prep is part of the encapsulant spec.

Dielectric still sits next to the heat path

Alumina- and BN-filled silicones are chosen because they remain electrical insulators while moving some heat. That is why they belong on a live module. Confirm dielectric strength or volume resistivity on the filled grade, same cure, named method. A thin flash of filled paste into a creepage slot still shortens distance. Carbon-filled “thermal” paste in that slot is worse.

UL 94, if the housing file needs a specimen class, is a bar at a thickness, not a potting certificate. ISO 10993 applies only if the device story requires it; a heat-sink brick is usually an industrial electrical part.

RFQ language that a compounder can use

“Process: platinum RTV potting or thermal LSR overmold [pick one]. Filler family: alumina and/or boron nitride as in the named grade; no carbon unless dielectric is waived. Thermal: impedance or conductivity by [named method], specimen thickness/pressure stated, and a defined bond line in the tool. Dielectric: [method] on the same grade. Heat-sink nest and keep-out on the drawing. Vacuum or fill path described. Mechanicals on the filled paste. Ban shop percents and copied watt classes.”

If the supplier offers “thermally conductive electronic silicone” with no method and no dielectric line, you have a color, not a compound.

FAQ

Should I specify alumina or boron nitride on the PO?

Specify the thermal job and the dielectric job, then let the compounder pick. Alumina is the common, abrasive, insulating workhorse. BN is a platelet that can perform well and can orient the wrong way for a through-thickness bond line. Do not dump either powder into a 1:1 platinum kit as a shop percent.

Why is a thick pour of thermal silicone still hot?

Heat cares about thickness and voids. A tall brick of filled rubber is a long thermal path. A void under the die is an insulator. Locate the heat sink, control the bond line, vent the last corner, and vacuum if the pocket is blind. Chasing a marketing watt class will not fix a floating sink.

Can graphite thermal silicone be used next to live copper?

Only if you have accepted electrical conductivity or a percolation risk. Graphite and some carbons can move heat and can ruin insulation. Alumina and BN families are the usual insulating thermal path. Demand resistivity or dielectric strength on the filled grade.

Will alumina-filled LSR run in a printed prototype tool?

Poorly, and not for long. Alumina is hard. Thermal LSR belongs in steel with larger gates. A pourable RTV paste in the housing, or a machined aluminum tryout with abrasion accepted, is the prototype path. Micro-gates and resin tools are the wrong cavity.

How do I keep BN or alumina from settling in a potting cup?

Use the thixotropic grade the compounder sells for that filler, mix until wet, vacuum, and pour before the paste stratifies. A water-thin RTV will drop ceramic to the floor of the cup. Density or a cut-and-look coupon from top and bottom of a dummy shot is the check, not a stir-time story.