Flex PCB Silicone Encapsulation: RTV vs LSR Heat

A flexible printed circuit in a silicone boot is not a gasket with a surprise inside. It is an electronic assembly you are about to clamp and surround with a thermoset.

Insert-molding flex PCBs (polyimide most often, sometimes PET that cannot take LIM heat) into LSR is how some wearables, medical cables, and sensor skins get a sealed shell — on a parts cell, with cavity heat around 160–200 °C. On this site the analog is room-temperature platinum encapsulation in a fixture or RTV mold: mix A/B, vacuum degas, pour or inject by hand around a nested flex, cure without a hot tool. Name the process. If the electronics cannot take LIM heat, stop talking injection.

The rubber is the easy part. The stack (copper, adhesive, coverlay, solder, components, stiffener) has to survive clamp, flow, and then years of flex at the exit. If you ignore the stack, the defect is an open trace the first continuity test missed because the crack opened after cool-down.

Heat is the first gate

LSR cavities typically sit around 160–200 °C. Cycle time is seconds to a couple of minutes, plus whatever post-cure you specified. Polyimide flex can live there. Many PET flexes, low-temp adhesives, and electrolytic capacitors cannot.

Walk the BOM before the box or the tool:

• Flex dielectric and adhesive temperature rating versus *process* temperature, not versus a 125 °C app note.
• Solder alloy and whether joints will reflow or go brittle in a LIM cycle.
• Component packages: tall parts the rubber will shear, MEMS that hate pressure, LEDs that need a clear window of optical-grade silicone.
• Coatings (parylene, silicone conformal) already on the board. Some inhibit platinum systems. Test the actual coating against the actual grade.
• Stiffeners and FR-4 tails. They change how the insert sits and how heat soaks.

If the electronics cannot take LIM heat, pour platinum RTV at room temperature in a fixture, or use a two-step: low-stress room-temp gel around the chips, then a structural jacket or a later LSR overmold on a designed dam. Do not “turn the LSR tool down to 120 °C” as a fantasy unless the grade and the cycle still cure the interface you need.

ISO 10993 on the *encapsulation grade* does not qualify the solder or the flux. Medical devices need the whole stack in the file.

Mechanical: the board is a core that can move

LSR viscosity is low, but the flow still drags. RTV pours drag less violently and still fold an un-nested flex. A circuit that is not located will skim copper over a shut-off, or leave a dry pocket on the back of a chip (air, then a bubble, then a leak path).

DFM that belongs in CAD:

• Nest and pins that locate holes in the flex without punching traces. Oversized slots if the polyimide will see a hot LIM tool.
• Shut-offs on designed lands, not on copper. LSR flashes through about 0.005 mm. RTV will still flash a gap at a poorly seated fixture. A land that bites a trace is an open. A land that leaves a gap is a leak.
• Strain relief where the flex exits the rubber. A sharp rubber edge on polyimide is a fatigue razor. A molded radius, a secondary clamp, or a longer bonded length beats a pretty flush exit.
• Component keep-outs under shut-offs, gates, and pour paths. Do not gate or pour onto a QFN.
• Windows for optical or connector faces: the fixture must seal around them without crushing gold fingers.

Vacuum helps air behind chips and under flex. It also wants a seal. Hold the insert, then pull. If vacuum can lift a loosely nested circuit, it will.

Gates belong on rubber that can take a vestige, not on a chip-scale package. On LSR, a valve-gated cold-runner is the usual choice so you are not dragging a cured sprue across a populated flex. On RTV, a pour gate at a designed well, vacuum degas of the mix, and a slow fill from the low point.

HCR transfer onto electronics is uncommon and harsher. Skip it unless you have a reason.

Electrical and materials after the shot — or the pour

Platinum RTV and platinum LSR are insulators. Filled grades (carbon, metal) can be not. If the product needs ESD or a grounded shield, that is a second material or a designed conductor, not a hope that “silicone is rubber.” Clear optical silicone over an LED is a grade pick (yellowing, refractive reality) and a vent pick (bubbles on the die are the defect).

Ionic residues (flux) plus moisture plus a poorly bonded interface are a corrosion cell the encapsulation was supposed to prevent. Cleanliness of the insert is a process step.

Continuity and hi-pot after molding, after thermal cycle, and after flex at the exit: first article is not one meter probe on a still-warm part. Copper and polyimide shrink differently from silicone; cracks show up later. RTV shrink is small (platinum often about 0.1–0.3%); LIM shrink is a different band. Do not copy an LSR residual-stress story onto a room-temp pour without testing the exit.

FDA 21 CFR 177.2600 is food-contact rubber language. It is not an electronics standard. IP ratings are tested on the assembly.

RUUIPON Mold Structure Decision: Flat back? → open-back fixture. Deep undercuts the skin must peel? If no → two-part / plug. Large wet pour over a populated flex? → mother mold / rigid nest so the board cannot flutter.

Common mix-up: “insert mold” when you needed a pour

A lot of RFQs send a flex STEP to an LSR shop and a pour shop with the same title. In the shop that usually means one bidder quotes 180 °C steel and the PET dies. Name LSR insert molding or room-temperature platinum encapsulation.

What to ask when you're buying

• Max temperature the assembly can take — cavity heat or room-temp pour?
• Gerber/keep-outs: where rubber may and may not go?
• Inhibition risk: coatings, tapes, legends, tin-cure residue on the bench?
• Strain-relief intent at the exit?
• Continuity after cool-down, thermal cycle, and bend — not only on a warm first article?

Worked example

You're encapsulating a 0.12 mm polyimide flex, 18 × 8 mm, two 0402 parts, PET is not in the stack, 50 prototypes. Pour platinum RTV in a two-part nested fixture, vacuum the mix, hold the flex on pins, strain-relief radius at the exit. Skip LIM. If the annual later is 40,000 and the BOM is polyimide-rated, that is a parts-OEM insert-mold quote with a cold-runner — qualify heat on the real BOM first.

We would not recommend putting PET flex in an LSR tool

We would not recommend gating onto a QFN, vacuum-lifting an unclamped circuit, or raising temperature to “force” a platinum cure against a conformal coat that inhibits. We would not treat a warm continuity pass as the ship test.

Typical shop values

Reference values only. Confirm material-specific limits against the supplier’s technical data and agree project acceptance criteria before production.

| Item | Typical shop value | | --- | --- | | RTV mix | Platinum 1:1 or 10:1; vacuum degas until the foam collapses | | LSR cavity (contrast) | About 160–200 °C; cold-runner feed | | Platinum RTV shrink | About 0.1–0.3% | | LSR shrink (contrast) | About 2–3% | | LSR leak band | About 0.005 mm — do not shut off on copper | | ISO 3302-1 | M2 common on the rubber boot |

Copy this into a flex-in-silicone RFQ

• Process: LSR insert molding or room-temperature platinum encapsulation
• Flex stack-up, max temperature, Gerber/keep-outs, connector faces
• Strain-relief intent; inhibition risks (coatings, tapes, legends)
• Continuity / hi-pot after equilibrium, thermal cycle, and bend at the exit

Send the flex stack, keep-out drawing, and the temperature the BOM can take. We'll quote a nested pour fixture, or tell you this is a hot-cavity insert-mold job. Custom RTV molds.

Related: 2-shot PC/ABS, threaded inserts, electronic encapsulation molds.

FAQ

Can a PET flex circuit go into an LSR tool? Usually no. LSR cavity heat is in the 160–200 °C class, and polyimide is the common flex dielectric that can sit there for a cycle. PET and many adhesives cannot. If the circuit is PET, pour room-temperature platinum silicone in a fixture, or change the flex.

Will vacuum in the tool or box damage components? Vacuum itself is rarely the killer; the nest and the flow are. Vacuum will lift an unclamped flex and leave voids behind chips if the insert can flutter. Hold the board, pull vacuum, then inject or pour. Fragile MEMS still need a pressure and heat review.

Why did continuity pass after molding and fail after a week? Thermal mismatch and residual stress. Copper, polyimide, solder, and silicone shrink on different clocks. A micro-crack at a pad or at the rubber exit can open after cool-down or after the first flex. Test after equilibrium, thermal cycle, and bend at the strain relief.

Can platinum silicone cure against a conformal-coated board? Some coatings inhibit platinum systems (the same family of problems as sulfur, tin RTV, and certain adhesives). Test the actual coating with the actual grade. If it inhibits, change the coating, mask, or chemistry. Do not raise temperature to “force” a poisoned interface.

Is insert-molded silicone waterproofing the same as an IP rating? No. IP tests the finished assembly: exits, connectors, and any flash path. Good encapsulation helps, but a flash line to a via, a cut trace, or a dry pocket under a chip is still a leak. Specify the IP test on the product, and design shut-offs so rubber cannot leave a capillary to the copper.