LSR Overmolding on FPC Injection Molding Process: Technological Breakthroughs and Application Practices in Flexible Circuit Encapsulation

Liquid Silicone Rubber Overmolding on Flexible Printed Circuit (LSR Overmolding on FPC) is an advanced packaging technology that coats flexible circuit boards with liquid silicone rubber through precision injection molding. As the "neural center" of modern electronic devices, FPC offers advantages such as thinness, flexibility, and high-density wiring. However, its exposed copper traces and solder joints are highly susceptible to damage from moisture, dust, and mechanical shock.

Unlike traditional thermoset silicone compression molding, LSR Overmolding on FPC uses injection molding, offering significant advantages including high production efficiency, excellent dimensional accuracy, and high automation. It is particularly suitable for medical devices, consumer electronics, and automotive components requiring strict quality standards.

II. Core Process Principles and Technical Challenges

2.1 Interface Bonding Mechanism of LSR and FPC

FPC typically consists of three parts: polyimide (PI) substrate, copper foil conductive layer, and coverlay. The bonding between LSR and FPC faces unique challenges:

Chemical Inertness of PI Substrate: Polyimide has low surface energy (approximately 40-45 mN/m) and lacks active functional groups in its molecular structure, making it difficult to form chemical bonds with LSR. Surface treatment must be applied to introduce polar groups.

Oxidation Risk of Copper Foil: Exposed copper foil easily oxidizes to form Cu₂O/CuO under high temperature and humidity, leading to increased contact resistance or even open circuits. LSR coating must ensure complete sealing of copper foil areas.

Thermal Expansion Coefficient Mismatch: PI has a thermal expansion coefficient of approximately 20?10^-6/?C, copper foil 17?10^-6/?C, while LSR is as high as 250-300?10^-6/?C. During temperature cycling, interfacial shear stress may cause delamination.

2.2 Key Technical Challenges

Challenge 1: FPC Temperature Resistance vs. LSR Curing Temperature

Standard LSR curing temperatures are typically 150-180?C, while although FPC's PI substrate can briefly withstand 260?C (reflow temperature), long-term exposure above 150?C causes yellowing and embrittlement. The coverlay's acrylic adhesive also softens above 120?C.

Solutions:

• Select low-temperature fast-curing LSR formulas, such as Wacker ELASTOSIL? LR 3003/20, curing at 120-130?C with 60-90s cycles

• Use two-stage curing process: first stage 120?C?30s initial setting, second stage 100?C?60s full curing

• Design localized heating in the mold, heating only LSR runner areas while keeping FPC area ?100?C

Challenge 2: Uniformity Control of Ultra-Thin Coating Layers

Wearable devices require LSR coating thickness of 0.3-0.8mm, balancing protection performance while maintaining FPC bending radius (typically ?5mm).

Process Control Points:

• Mold cavity precision ?0.02mm, mirror polishing (Ra?0.1?m) to reduce flow resistance

• Injection speed 30-50mm/s to avoid FPC displacement from high-speed injection

• Hold pressure at 60-70% of injection pressure

• In-mold pressure sensors for real-time monitoring

Challenge 3: Precise FPC Positioning and Anti-Displacement Design

FPC thickness is typically only 0.1-0.3mm, making it prone to displacement, warping, or damage under injection pressure.

Engineering Solutions:

• Vacuum fixture for FPC fixing

• Alignment pins in mold matching FPC tooling holes

• CCD vision inspection before injection

• Optimized gate layout with multi-point balanced filling

Challenge 4: Electrical Performance Protection and Signal Integrity

Select low-dielectric-constant LSR (?r?2.8), reserve "window" openings in critical signal areas, equip injection machine with ionizing bars for static elimination.

III. Typical Applications and Technical Cases

3.1 Smart Wearable Devices

Smart wristband heart rate sensor FPC encapsulation achieves IP68 waterproofing through LSR coating, Shore A 20-30 skin contact hardness, and >50,000 bending cycles.

3.2 Medical Monitoring Equipment

Disposable ECG electrode FPC with medical-grade LSR coating achieves ISO 10993 biocompatibility certification and ?6N/mm peel strength.

3.3 Automotive Electronics

New energy vehicle battery temperature sensor FPC with temperature range -40?C to 125?C and UL94 V-0 flame retardant rating.

IV. Material Selection and Process Optimization

4.1 LSR Material Recommendations

Smart Wearable Window: Dow Corning SE 1700, Shore A 20-30, >92% light transmittance
Medical Skin Contact: Wacker LR 5040, Shore A 30-40, USP Class VI
Automotive High Temperature: Momentive 7-4050, Shore A 40-50, 200?C continuous use
High-Frequency Signal: Momentive 7-4030, low dielectric ?r?2.8
General Protection: Wacker LR 3003/20, Shore A 20-50 adjustable, low-temperature fast cure

4.2 Key Process Parameters

Mold Temperature: 120-130?C
Injection Pressure: 80-100MPa
Injection Speed: 30-50mm/s
Holding Time: 5-10s
Cooling Time: 15-25s

V. Quality Inspection and Reliability Verification

Online Inspection: AOI automatic optical inspection, X-Ray inspection, impedance testing
Offline Sampling: Bending test, peel strength, weathering test, salt spray test
Reliability Certification: ISO 10993 medical grade, AEC-Q200 automotive grade, IP68 consumer electronics

VI. Future Development Trends

Ultra-thin and miniaturization, intelligent process monitoring, environmental sustainability, multi-functional integrated packaging.

Keywords: LSR Overmolding on FPC

Long-tail Keywords: LSR overmolding on FPC process, FPC silicone encapsulation technology, flexible circuit LSR coating, wearable FPC protection, medical-grade FPC silicone encapsulation, automotive electronics FPC protection, FPC injection molding process