Full analysis of installation methods for self-healing fuses
Self healing fuse (PPTC, polymer positive temperature coefficient thermistor) is an overcurrent protection component that can quickly heat up and enter a high resistance state when the current is overloaded, cutting off the circuit; After troubleshooting, it automatically returns to a low resistance state without the need for replacement. The installation method directly affects the protection performance and reliability, and needs to be selected based on the application scenario. The following system analysis will be conducted from three dimensions: installation form, process requirements, and application scenarios.
1、 Analysis of mainstream installation methods
Surface mount (SMD)
Technical features:
Suitable for automated SMT production lines, compatible with reflow soldering process.
There are various packaging forms (such as 1206, 1812, 2920), compact size, suitable for high-density PCBs.
Key points of the process:
The welding temperature should be controlled below 235 ℃ to avoid high temperature damage to PPTC material.
The pad design must meet the IPC-7351 standard to ensure welding reliability.
Typical applications: portable devices such as smartphones, tablets, TWS earphones, etc.
Plug in installation (Through Hole)
Technical features:
Using radial or axial pins for easy manual soldering or wave soldering.
High mechanical strength, suitable for vibration environments such as automotive electronics.
Key points of the process:
The cutting length of the pins should be kept at 2-3mm to avoid stress damage.
Welding time<3 seconds to prevent solder penetration and short circuit.
Typical applications: industrial controllers, power adapters, household appliances.
Direct welding of wires
Technical features:
Suitable for PCB free scenarios such as battery packs and wire harness protection.
The cross-sectional area of the wire needs to match the rated current of PPTC (such as 24AWG wire with 0.5A PPTC).
Key points of the process:
The welding points need to be insulated (such as heat shrink tubing) to prevent short circuits.
Wire bending radius>5 times the wire diameter to avoid mechanical fatigue.
Typical applications: power tools, drone batteries, LED light strips.
Installation of heat sink
Technical features:
For high-power PPTC (such as 60V/30A models), heat dissipation efficiency is improved through heat sinks.
The material of the heat sink should be aluminum or copper with a thermal conductivity greater than 200W/m · K.
Key points of the process:
Apply thermal conductive silicone grease on the contact surface, with a thermal resistance of less than 0.5 ℃/W.
The fixed torque is controlled at 0.6-0.8N · m to avoid pressure loss of PPTC.
Typical applications: electric vehicle charging stations, communication base station power supply.
Modular installation
Technical features:
Integrate PPTC into protection modules (such as USB Type-C modules) to simplify circuit design.
The module is equipped with multi-level protection for overvoltage, overcurrent, and overheating.
Key points of the process:
The module is connected to the PCB through spring pins, with a contact impedance of<10m Ω.
The module casing must comply with UL94 V-0 flame retardant rating.
Typical applications: laptops, power banks, smart speakers.
2、 Adaptive design for installation environment
High temperature scenario
Countermeasure:
Choose PPTC with a temperature resistance level>125 ℃ (such as TYCO's PolySwitch TR series).
Add heat dissipation holes or fins to reduce the impact of ambient temperature on PPTC.
Typical applications: automotive engine compartment, industrial frequency converter.
High humidity scene
Countermeasure:
The PPTC surface is coated with three proof paint, with a protection level of IP67.
The solder pad is designed in a "teardrop" shape to reduce the risk of water vapor infiltration.
Typical applications: outdoor LED display screens, agricultural IoT devices.
Vibration scene
Countermeasure:
The plug-in PPTC adopts a "U" - shaped bent foot design to enhance its anti vibration ability.
The spacing between wire fixing points should be less than 50mm to avoid resonance fracture.
Typical applications: construction machinery, aerospace equipment.
3、 Installation Failure Mode and Prevention
Failure mode
cause analysis
preventive measure
Poor welding
Pad oxidation, uncontrolled welding temperature/time
Gold plating treatment on solder pads, with welding temperature controlled at 235 ℃± 5 ℃ and time<3 seconds
Mechanical stress damage
Excessive bending of pins and cracking of the casing
Pin bending radius>2mm, shell material selected as LCP or PPS
Thermal aging failure
Long term high temperature leads to performance degradation of PPTC
Optimize heat dissipation design and choose low impedance models (such as R150<0.1 Ω)
Sulfide pollution
Corrosion electrode in sulfur-containing environment
Using gold-plated electrodes and packaging materials with added anti sulfurization agents
4、 Collaborative design case of selection and installation
Case 1: Portable Medical Devices
Requirement: Battery overcurrent protection, volume<5mm × 5mm, passed IEC60601 biocompatibility test.
Plan:
Select SMD PPTC packaged in 1206 (such as Littelfuse PTC1206 series).
PCB layout should be kept away from heating elements (such as DC-DC converters), and solder pads should be treated with solder mask and window opening.
The module is fully encapsulated with medical grade epoxy resin, with a protection level of IP68.
Case 2: BMS for New Energy Vehicles
Requirement: Battery pack overcurrent protection, capable of withstanding temperature surges from -40 ℃ to+125 ℃, certified by AEC-Q200.
Plan:
Choose plug-in PPTC (such as Bourns MF-NSML series) with rated current matching the maximum continuous current of the battery pack.
Installed in an independent heat dissipation channel, maintaining a distance of 10mm from the MOSFET.
Pin tin plating treatment, spray three proof paint after welding, salt spray test for 48 hours without corrosion. The installation method of self recovery fuses needs to comprehensively consider electrical performance, mechanical strength, and environmental adaptability. Through diversified installation forms such as SMD/plug-in/wire welding, combined with enhanced designs such as heat sinks and modularity, it can cover the full range of needs in consumer electronics, industrial control, automotive electronics, and other fields. In practical applications, it is necessary to strictly follow the welding process specifications and verify the robustness of the design through reliability tests (such as temperature shock, vibration, salt spray) to ensure the long-term stability of the circuit protection function.