An optocoupler, as shown in Figure 1, is a device that transmits electrical signals using optical signals as the transmission medium. Typically, the input side (LED) and the output side (phototransistor) are packaged together via specific packaging technologies. When a voltage is applied to the infrared input side, the LED chip emits light, and the photosensitive side receives the signal to generate a photocurrent, realizing the "electricity–light–electricity" conversion. Owing to its excellent properties such as fast switching speed and high insulation voltage resistance, optocouplers have been widely used in new energy vehicles and other fields in recent years. The reliability and stability of optocouplers directly affect the overall reliability of end products like new energy vehicles, which are closely related to packaging processes.

Analysis of Delamination Locations and Causes in Optocouplers

For optocouplers, delamination occurs at different locations due to distinct delamination mechanisms. Studies have identified the following vulnerable delamination sites:

(1) Delamination between the chip and the internal white epoxy resin

(2) Delamination between the lead frame and the epoxy molding compound

(3) Delamination between the chip and the die attach pad

(4) Delamination between the internal white epoxy resin and the external black epoxy resin

(5) Delamination between the silicone gel and the internal white epoxy resin

The packaging process of encapsulated devices largely determines product quality. Every step from dicing to final electroplating and trim forming impacts packaging performance. For instance, the dispensing volume affects the internal bonding strength; excessive dispensing may cause device rupture due to moisture absorption and expansion. Oxidation and corrosion of the chip and lead frame during wire bonding also degrade product quality. Peak temperature control in reflow soldering defines the overall solderability; improper temperature settings lead to electrical parameter drift caused by delamination. Lastly, plasma cleaning is required prior to the injection molding process, and its parameter settings influence the bonding strength between black and white epoxy molding compounds. All these processes demonstrate that process parameters are critical to packaging quality.

Plasma Cleaning Process

Plasma, known as the fourth state of matter, is an ionized gas composed of charged particles (positive ions and electrons) and neutral particles (neutral atoms and molecules). It contains highly active species, including free radicals, excited molecules or atoms, positive ions, and electrons. Physical and chemical modifications of material surfaces occur through interactions between solid surfaces and these high-energy active species in plasma. Stable plasma can be generated by sustaining a steady discharge in a gas under controlled conditions and applied for surface modification of materials.

Plasma cleaning removes molecular-level contaminants from material surfaces via physical bombardment or chemical reactions with active plasma. When applied to optocoupler packaging, it effectively eliminates organic residues and microparticle contaminants on epoxy molding materials. Failure to remove these contaminants results in air bubbles inside the material during epoxy molding, which form voids as temperature changes and eventually cause delamination. For double-molded optocouplers, the two molding steps are usually performed consecutively. However, release agents and other solvents are sprayed to facilitate demolding of the first-molded device. Practical production shows that direct second molding without solvent treatment significantly increases delamination risks. Plasma cleaning effectively improves the bonding strength between black and white epoxy resins.