Optocouplers (optocouplers) play a crucial role in circuit design as a key electro-optical signal isolation device, used to achieve signal transmission while blocking electrical connections (such as high and low voltage isolation, eliminating ground loop interference, protecting sensitive circuits, etc.). Its performance not only depends on the internal photoelectric conversion efficiency, but its external packaging form also deeply affects the physical size, installation method, heat dissipation capacity, isolation and voltage resistance level, and application scenarios of the device. This article will systematically introduce the common types and characteristics of optocoupler packaging on the market.
1、 The core function of optocoupler packaging
Physical support and protection: Provide a sturdy casing for internal LEDs (light emitting diodes) and photosensitive devices (such as phototransistors, photodiodes, photosensitive thyristors, etc.) to resist environmental influences such as mechanical stress, dust, and moisture.
Electrical isolation: The packaging structure (especially the plastic encapsulation material and internal structure design) is a key barrier to achieve high isolation voltage (such as 2500Vrms, 3750Vrms, 5000Vrms or even higher) between the input side (LED) and the output side (photosensitive device).
Heat dissipation channel: Partial packaging (such as packaging with heat sinks or larger volumes) helps dissipate the heat generated by LED operation, ensuring device performance and lifespan.
Pin definition and connection: Provide standardized pin layout for easy soldering or installation on PCB (printed circuit board).
Optical channel: Ensure that the light emitted by the LED is efficiently and reliably transmitted to the photosensitive device.
2、 Detailed explanation of common optocoupler packaging types
According to the pin arrangement, external dimensions, and installation method, it can be mainly divided into the following categories:
Dual In Line Package (DIP)
Description: This is the most classic and oldest form of optocoupler packaging. The pins are led out parallel downwards from both sides of the package.
Common models:
DIP-4: Standard 4-pin optocoupler package (2 inputs+2 outputs). The most common, relatively large in size.
DIP-6: 6-pin package, usually used for optocouplers that require more functions (such as phototransistor optocouplers with base lead out pins, convenient for external gain or speed control) or special types (such as optical relays, thyristor drives).
DIP-8: 8-pin package, commonly found in more complex optocouplers (such as high-speed logic gate optocouplers, optocouplers with Schmitt triggers) or optocoupler arrays that integrate multiple channels (such as dual channel and four channel optocouplers).
Characteristics:
Advantages: The structure is sturdy, the pin spacing (usually 2.54mm/0.1 inch) is suitable for through-hole soldering (THT), manual soldering and maintenance are convenient, and the heat dissipation performance is relatively good (with a large volume).
Disadvantages: Large size, occupying a lot of PCB area, not suitable for high-density installation. Automated mounting efficiency is not as good as surface mount devices.
Typical applications: Industrial control equipment, power supplies (primary/secondary feedback isolation), instrumentation, traditional household appliances, and other applications that do not require strict volume requirements.
Small Outline Package (SOP)
Description: This is the mainstream optocoupler packaging form of surface mount technology. The pins are horizontally bent outward in a Gull Wing or J-Lead shape from both sides of the package.
Common models:
SOP-4: Standard 4-pin surface mount optocoupler package.
SOP-5: Less common, used for specific functions.
SOP-6: Used for output terminals with base pins or optocouplers with specific functions.
SOP-8: Used for multi-channel optocouplers (such as dual channels) or optocouplers with more complex functions.
Characteristics:
Advantages: The volume is significantly smaller than DIP packaging (usually in the width range of 3.9mm-7.5mm), the weight is light, PCB area is saved, suitable for high-density SMT mounting, and the automation production efficiency is high.
Disadvantages: The pin spacing is small (such as 1.27mm/0.05 inches), making manual soldering and maintenance slightly difficult. The heat dissipation capability is usually weaker than DIP packages with the same function (which can be optimized through PCB heat dissipation design).
Typical applications: consumer electronics products (air conditioners, televisions, chargers), communication equipment, computers and peripherals, modern industrial control boards, and other space limited situations.
Thin Small Outline Package (TSOP)
Description: It is a thinner version of SOP encapsulation. Mainly to further reduce the height (thickness) of the packaging.
Characteristics:
Advantages: While maintaining SOP width and pin spacing, the thickness is significantly reduced (usually around 1mm), meeting the needs of ultra-thin devices.
Disadvantage: The ultra-thin design may pose certain challenges to heat dissipation.
Typical applications: portable electronic devices with strict thickness restrictions, ultra-thin display driver boards, etc.
Shrink Small Outline Package (SSOP)
Description: A more compact version of SOP encapsulation. By reducing the pin spacing (such as 0.65mm/0.0256 inches) and overall size, a smaller footprint can be achieved.
Common models: SSOP-4, SSOP-5, SSOP-6.
Characteristics:
Advantages: Smaller packaging size than standard SOP, further saving space.
Disadvantages: Smaller pin spacing, higher requirements for PCB manufacturing (line width/line spacing) and soldering processes (SMT accuracy, rework).
Typical applications: Miniature electronic products, wearable devices, high-density modules, etc. that require extremely strict space requirements.
Very Thin Fine Pitch Ball Grid Array (VFBGA)
Description: An advanced, high-density packaging form. The chip is placed on the substrate, with a regular array of solder balls (Bumps) arranged at the bottom as pins.
Characteristics:
Advantages: Extremely small size, ultra-thin thickness (can be less than 1mm), excellent electrical performance (small parasitic inductance and capacitance, conducive to high-speed transmission), good thermal performance (can conduct heat to PCB through bottom solder balls).
Disadvantages: Relatively high cost, extremely high requirements for PCB design (blind buried holes, HDI), manufacturing process, and inspection (X-Ray, AOI), and extremely difficult repair.
Typical applications: High end smartphones, tablets, ultra-thin laptops, high-speed data communication interfaces (such as USB isolation, Ethernet PHY isolation), and other applications that require extremely high volume, speed, and performance. At present, it is mainly used for high-speed optocouplers (such as 10Mbps, 50Mbps or even higher).
Other special packaging
Surface Mount Device (SMD): This is a broad term that generally refers to all surface mount forms of packaging such as SOP, TSOP, SSOP, VFBGA, etc. Sometimes it also refers to non-standard SMT optocouplers with customized external dimensions.
Optical relay packaging: Some optical relays (a type of solid-state relay that uses optocouplers as inputs to control MOSFET outputs) use special packaging similar to traditional electromechanical relays (such as DIP type with heat sinks or SOP type with different pin functions) to facilitate replacement or meet specific installation requirements. The output pin spacing may be larger to carry higher currents.
Direct insertion power type package: For optocouplers that require driving high current loads (such as certain optical relays and thyristor driven optocouplers), direct insertion packages with metal heat sinks (Tab) (such as variants similar to TO-220) are used to enhance heat dissipation capabilities.
3、 Key packaging parameters and selection considerations
When choosing optocoupler packaging, the following factors should be considered comprehensively:
Installation method:
Through hole insertion: DIP packaging is preferred.
Surface mount: Preferred packaging options include SOP/TSOP/SSOP/VFBGA. Consider the capability of SMT equipment on the production line.
Space limitations:
PCB area: SOP/SOP/VFBGA is superior to DIP.
Height limitation: TSOP/VFBGA is the preferred choice for ultra-thin applications.
Isolation voltage requirements:
The maximum isolation voltage (such as 3750Vrms) and creepage distance/electrical clearance that can be achieved by different packages are different. DIP packaging usually has a slight advantage in high isolation voltage specifications (due to its larger physical size). High specification SOP/VFBGA can also meet the vast majority of industrial needs.
Heat dissipation requirements:
For applications with high driving current and high power consumption (such as LED side current or output side driving power devices), DIP or special packages with heat sinks provide better heat dissipation. SMD packaging relies on PCB heat dissipation design (copper plating, heat dissipation vias).
Number of channels:
Single channel: DIP-4, SOP-4, SSOP-4.
Equipped with base control: DIP-6, SOP-6.
Dual/quad channel: DIP-8, SOP-8, VFBGA.
Speed requirement:
High speed optocouplers (>1Mbps) are increasingly being packaged in SOP, SSOP, and even VFBGA due to their optimized internal structure and smaller parasitic parameters.
Cost and manufacturability:
DIP costs are usually lower, while SMT packaging (SOP/SOP) has a cost advantage in mass production. VFBGA has the highest cost. PCB manufacturing costs need to be considered (such as HDI boards used for VFBGA).
Environmental factors: For high temperature, high humidity, and high vibration environments, the structural strength and sealing of the packaging are also important.
4、 Packaging development trend
Continuous miniaturization: The proportion of smaller packages such as SSOP, ultra small size SOP, VFBGA, etc. continues to rise.
High integration: Multi channel optocouplers and optocouplers that integrate more functions (such as Schmitt triggers and gate drivers) require SOP or VFBGA packaging with more pins.
High speed packaging: To meet the isolation requirements of high-speed data transmission (such as USB4, PCIe isolation), low parasitic parameter packaging such as VFBGA has become the preferred choice for high-speed optocouplers.
Enhanced heat dissipation design: As power density increases, SMD packaging addresses heat dissipation challenges by improving internal structure and optimizing PCB heat dissipation design.
Standardization and compatibility: Mainstream packaging (such as SOP-4, SOP-6) tends to standardize size and pin definitions, making it easier to design, replace, and procure.
Optocoupler packaging is an important bridge connecting the core photoelectric conversion function inside the device with external application circuits. From classic DIP to mainstream SOP and its variants (TSOP, SSOP), to advanced VFBGA, each package serves specific design requirements, space constraints, and performance goals. Engineers must consider the packaging form as one of the key factors when choosing optocouplers, balancing space, cost, isolation, heat dissipation, installation methods, and performance requirements in order to select the most suitable "optoelectronic bridge" for circuit design. With the continuous development of electronic devices towards miniaturization and high performance, surface mount, especially smaller and higher density packaging forms, will continue to lead the trend of optocoupler packaging technology.