The manufacturing process of optocouplers is a precise process that integrates semiconductor technology, microelectronic packaging, and materials science. Its core lies in achieving the conversion of input electrical signals to optical signals and output electrical signals, while ensuring strict electrical isolation between input and output terminals. The following is a typical manufacturing process flow, and the logic is clearly explained as follows:
1、 Core chip preparation stage
Manufacturing of light-emitting device chips (usually infrared LEDs):
Material selection: Commonly used III-V semiconductor materials, such as gallium arsenide and aluminum gallium arsenide, have efficient luminescent properties in the near-infrared band (typical wavelengths of 850nm or 940nm).
Epitaxial growth: On a single crystal substrate, multiple layers of epitaxial structures (N-type layer, active layer, P-type layer) forming a PN junction are precisely grown using processes such as MOCVD or MBE.
Lithography and etching: The specific shape of the LED device (usually circular or square) is defined on the wafer surface through photolithography technology, and then dry or wet etching is performed to form a tabletop structure.
Electrode preparation:
N-type electrode: A fully contacting metal electrode (such as AuGeNi/Au) is usually made on the back of the substrate.
P-type electrode: A ring-shaped or finger shaped electrode (such as Ti/Pt/Au) is made at the top of the table to allow effective emission of light from the central area.
Passivation and Protection: Deposition of a passivation layer (such as SiO ₂ or SiNx) on the surface of the chip to protect the sidewalls of the table from environmental influences and reduce leakage.
Thinning and cutting: Thinning the backside of the wafer to a suitable thickness (conducive to heat dissipation and light transmission), and then cutting it into individual LED chips using a cutting machine.
Manufacturing of photosensitive device chips (common types: photosensitive transistor, photosensitive IC, photosensitive Darlington, photosensitive thyristor):
For example, a phototransistor:
Materials and structures: typically based on silicon materials. Epitaxial growth of N-epitaxial layer on N-type substrate, forming P-type base region and N+emitter region through ion implantation or diffusion, forming NPN structure. The collector electrode is usually composed of a substrate and a bottom electrode.
Light sensitive area: A large area of light window is required at the top (usually without metal cover or using transparent electrodes such as ITO) to ensure that light can effectively shine on the base area.
Passivation and anti reflection film: Deposition of high-quality passivation layer (SiNx) and anti reflection coating (such as SiO ₂/SiNx stack) in the light window area to maximize light entry and photoelectric conversion efficiency.
Electrode preparation: Prepare metal electrodes (usually Al or Al alloy) for the emitter (E) and base (B). The collector electrode (C) is located on the back of the chip.
Lithography and etching: Similar to LED technology, defining device regions and electrodes.
Photosensitive IC: The process is more complex, integrating photodiodes (usually PIN structures) and signal processing circuits (such as amplifiers, Schmitt triggers, logic gates, etc.) on silicon wafers, requiring standard CMOS or BiCMOS process lines to complete.
Cutting: The wafer is ultimately cut into individual light receiving device chips.
2、 Packaging and isolation structure construction stage (core process)
Lead frame preparation:
Using stamping or etching processes to produce specific lead frames, typically consisting of two or more pairs of separated pins (input and output sides), the frame design needs to ensure sufficient creepage distance and electrical clearance between the internal conductive parts.
Electroplating (such as silver plating, tin plating) may be performed to improve welding performance and conductivity.
Chip mounting:
Accurately die bond the prepared LED chips and light receiving device chips onto the designated and isolated pads on the lead frame.
Use conductive adhesive (under the light receiving device chip, which needs to be grounded or connected to the collector) or insulating adhesive (under the LED chip) for bonding. Applications with high thermal conductivity requirements will use solder (such as AuSn eutectic solder).
Wire bonding:
Establish electrical connections between the chip electrodes (P/N terminals of LEDs, E/B/C terminals of light receiving devices, or IC pads) and the corresponding pins of the lead frame using fine gold or copper wire bonding.
This process requires high precision to avoid short circuits or excessive length of the gold wire affecting performance.
Transparent insulation medium filling and forming (isolation layer formation - the most critical step):
Purpose: To fill a layer of high transmittance and high insulation strength material between the LED chip and the light receiving device chip to achieve effective transmission of optical signals and electrical isolation between input/output terminals.
Materials: commonly used silicon gel, transparent epoxy resin or polyimide (PI). requirement:
Extremely low light absorption rate (especially at the wavelength of LED emission).
Extremely high volume resistivity and dielectric strength (usually>20kV/mm).
Good thermal stability, mechanical stability, and long-term reliability.
Compatible with materials such as chips, frames, and gold wires.
Process:
Create temporary enclosures around the bonded semi-finished products or use precision dispensing equipment.
Accurately inject/dot apply liquid transparent insulating material into the gap between the LED and the light receiving device chip, completely covering the chip and bonding wires (but usually not covering the top emitting/light receiving surface of the chip).
Strictly control the filling amount and avoid the generation of bubbles.
Cure at a specific temperature to change the material from liquid to stable solid/gelled state, forming optical transmission channels and high-voltage isolation barriers. The thickness (usually between 0.2mm and 1mm) and uniformity of this medium directly affect the isolation withstand voltage (such as AC 3750Vrms, AC 5000Vrms) and current transfer ratio.
External packaging molding:
Place the components that have been filled with transparent media into the mold.
Use opaque black epoxy resin (or other molding materials) for transfer molding.
The plastic enclosure completely wraps around the internal structure (transparent medium, chip, bonding wire, partial lead frame), providing mechanical protection, environmental protection (moisture-proof, dust-proof), enhanced insulation (increasing external creepage distance), and preventing external stray light interference.
Curing after encapsulation.
Post curing and separation:
Post curing of the plastic enclosure to eliminate stress and ensure stable material properties.
Cut off the connecting ribs (Dejunk) on the lead frame and separate multiple connected device units into individual optocoupler devices (Singations).
3、 Testing and screening phase
Electrical performance testing:
Input side test: LED forward voltage, reverse breakdown voltage, leakage current.
Output side testing: Dark current, photocurrent/saturation voltage drop of photosensitive devices (transistor), output logic level (optocoupler IC).
Key parameter testing: current transfer ratio, isolation resistance, and capacitance between input and output.
Time parameter testing: response time (rise/fall time), transmission delay.
High voltage isolation test:
Apply a high voltage (such as AC 3750Vrms or DC 5000V, lasting for 1 second) between the input pin group and the output pin group, and strictly test the insulation medium's voltage resistance and whether there is leakage exceeding the standard. This is a key test to ensure safe isolation.
Aging screening and final testing:
Possible high-temperature aging (Burn in) to screen for early failure products.
Conduct a final comprehensive electrical performance retest and visual inspection.
Laser marking, marking model, batch number and other information.
Summarize key points
Dual chip structure: The core consists of independent light-emitting chips (LEDs) and light receiving chips (photosensitive devices).
Precision optical alignment: During manufacturing, it is necessary to ensure good spatial alignment between the LED emitting surface and the photosensitive area of the light receiving device to optimize the light coupling efficiency (CTR).
Transparent insulating dielectric layer: This is the core for achieving electrical isolation and optical transmission. The material selection, filling process, thickness control, and curing quality directly determine the isolation withstand voltage, long-term reliability, and CTR stability of the device.
Double insulation: The internal transparent dielectric layer provides functional insulation, while the external black plastic enclosure provides basic/reinforced insulation and increases creepage distance.
High reliability requirements: The process requires strict control of cleanliness, material purity, and process parameters (temperature, time, pressure) to ensure that the device can operate stably in harsh environments for a long time
The entire manufacturing process embodies how to cleverly utilize light as a medium to achieve safe, reliable, and high-speed electrical isolation and signal transmission between strong and weak electricity in a small space through precise material engineering and manufacturing technology.