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The industry-standard flip-chip Mini COB production line consists of five major stages with 13 core processes, covering the full workflow of substrate pretreatment, chip mounting & soldering, circuit assembly & initial testing, coating & post-testing, and finished module assembly.

As the first process of the entire production line, dehumidification lays the foundation for stable soldering quality. PCB substrates such as FR-4 are hygroscopic. If residual moisture remains inside the boards, rapid vaporization under high reflow temperature will trigger batch defects including PCB delamination, surface blistering, solder voids and solder balls.

This process removes adsorbed moisture inside and on the surface of boards through constant-temperature baking to control board moisture content, preventing substrate and solder joint damage caused by high-temperature soldering from the source.

High-precision stencils and automatic printing machines are adopted to evenly and accurately coat solder paste onto PCB pads corresponding to chip positions. Solder paste is a mixture of tin alloy powder and flux, serving as the key medium for reliable bonding between chips and substrates. The thickness, uniformity and alignment accuracy of printed solder paste directly determine subsequent soldering yield, making this one of the most precision-critical processes in flip-chip technology.

3D optical SPI equipment is deployed immediately after solder paste printing as an upstream quality control procedure. Based on laser triangulation measurement, the system automatically detects the thickness, volume, coverage area, offset and forming state of solder paste on each pad, pre-screening printing defects such as missing printing, excessive solder, alignment offset and pad bridging. Industry data shows that 70% of soldering defects stem from abnormal solder paste printing. SPI inspection significantly reduces downstream rework costs and serves as the first critical barrier to stabilize production yield.

High-precision die bonders equipped with visual positioning systems accurately align metal bumps on flip-chip LED chips with solder paste on PCB pads and place chips steadily. Mini COB requires die attach alignment accuracy within ±10μm to ensure all chip bumps fully contact solder paste, laying a foundation for subsequent fusion soldering.

Reflow soldering is the core process to form reliable mechanical and electrical connections between chips and substrates. PCBs with mounted chips are sent into a reflow oven, passing through four temperature zones sequentially: preheating, soaking, reflow and cooling.

• Preheating zone: Slow temperature rise to evaporate flux solvent;
• Soaking zone: Remove oxidation layers on pads and chip surfaces;
• Reflow zone: Temperature rises above the melting point of solder paste (approx. 217℃ for SAC305 solder paste). Molten solder paste wraps chip bumps and PCB pads under surface tension;
• Cooling zone: Rapid cooling to solidify solder paste and form stable solder joints.

Precise control of temperature curve parameters is vital. Excessively high temperature will burn chips, while insufficient temperature leads to cold solder joints and poor bonding.

After LED chip soldering, standard SMT processes are applied to mount peripheral electronic components including driver ICs, resistors and capacitors to build a complete driving circuit for display modules. Consistent with conventional circuit board SMT processing, mounters place components accurately, followed by secondary reflow soldering to realize full circuit conduction of modules.

Full-circuit soldering is followed by the first power-on test. Inspectors comprehensively measure electrical parameters (voltage, current), lighting performance (dead lamps, dim lamps, color cast), and screen for open/short circuit defects to eliminate defective semi-finished products in advance.

Semi-finished products marked NG (Not Good) in initial testing enter the rework station. Specialized rework equipment repairs failed solder joints or replaces faulty LED chips and electronic components. Repaired products are retested to minimize board scrap loss.

Modules passing initial testing undergo long-time power-on accelerated aging before coating. Under fixed temperature, humidity and standard driving current, continuous lighting accelerates exposure of early latent failures of chips and and solder joints. Defects detected after coating will incur drastically higher rework difficulty and material loss; therefore, aging screening is a core procedure to guarantee long-term stable operation of finished products.

Also known as potting, it is the signature process of COB packaging. Optical epoxy resin or polymer encapsulant is evenly coated across the entire lamp surface to fully wrap all LED chips and solder joints. The three major functions of coating are as follows:

1. Physical protection: Isolate moisture and dust, improve anti-collision and anti-static performance of modules;
2. Optical optimization: Adjust light emission angle, enhance light mixing uniformity and unify black color performance;
3. Structural reinforcement: Boost overall mechanical strength of modules and reduce the risk of chip detachment.

Glossy or matte coating surfaces can be customized according to application demands to fit various display scenarios.

After full curing of encapsulant, the second round of comprehensive testing is carried out. Inspectors focus on checking whether the coating process damages chips and circuits, and simultaneously test optical indicators including brightness, emission wavelength and color temperature to ensure unified compliance of optical parameters across all modules.