Firstly, SMT (Surface Mounted Technology) is the abbreviation for “Surface Mounted Technology” in Chinese. It is currently the most popular technology and process in the electronics assembly industry. The process flow of SMT patching is highly intricate and varies for different products. The basic process is as follows: incoming inspection, burning, printing, inspection, placement, pre-furnace inspection, reflow soldering, AOI inspection, repair, test, and assembly.
Regardless of the number or complexity of processes involved in SMT patch processing, the three essential processes of printing, patching, and reflow soldering are indispensable. These processes have remained unaltered over the past decade, despite any evolutionary changes. Moreover, all three processes are now fully automated.
SMT patch printing and solder paste printing involve the same principle as printing ink on paper through typesetting in a printing factory. However, in this case, solder paste is printed on PCB substrates. The equipment and tools used for printing include:
**Printing Machine:** There are fully automatic and semi-automatic printing machines. For instance, an automatic printing machine is often used for papaya patch proofing.
**Solder Paste:** Solder paste is a specialized material used to secure components to the PCB board.
**Stencil:** Essentially, a stencil is a mold that outlines the positions of the pads on the PCB, allowing solder paste to flow into these positions. It is a very thin steel sheet, fixed by a screen frame, and is extremely flat. The most commonly used thickness is 0.10mm, though different thicknesses and manufacturing processes are selected based on the components used in various products. Stencils are made according to a paste mask file from the Gerber file provided by R&D or the customer. This preparation must be completed before production since the stencil production involves its own set of processes, and the quality of the stencil directly impacts the quality of the printed paste. The precision of the stencil is crucial, especially for components like 0.4-pitch BGA. It is often recommended to have such stencils made by professionals in the production plant rather than using customer-supplied ones, due to the lack of strict standards and the plant’s expertise in handling specific details.
**Printing Process:** Understanding the necessary tools for printing gives insight into the basic operations. The process involves installing the stencil in the printing machine, applying solder paste onto the stencil, then feeding the PCB board through the machine. The machine’s camera scans the alignment marks on the PCB and stencil. Once aligned, the machine platform rises, and the stencil is pressed against the PCB. A scraper on the machine, angled at 45°, removes the solder paste from the stencil, depositing it onto the PCB pads through the stencil’s openings. This completes the printing process. If defects occur, adjustments are needed by the field equipment engineer. Based on years of experience, the printing process is the most critical of the three SMT processes, as 70% of SMT process defects are related to this stage.
**SMT Placement Process:** Mounting, or SMT (Surface Mount Technology), involves placing components onto a PCB using a mounter. The term “patch” refers to the fact that solder paste, which contains flux with a degree of viscosity, holds the components in place before melting. SMT is an advanced version of manual soldering, where components were placed with tweezers, while the placement machine uses a vacuum to pick and place components onto the PCB. The complexity arises from the sophisticated equipment and processes involved. Advances in technology have replaced manual placement with automated systems, significantly enhancing production efficiency and transforming the industry supply chain.
**Placement Machine Operation:** To operate a placement machine, the Gerber files, coordinate files, BOM, and location maps provided by the customer are used to create the placement program. The machine then uses its placement head (suction nozzle), feeder, and track to complete the placement.
**Suction Nozzle:** The placement head features 12 suction nozzles, which use vacuum to pick up components.
**Feeder:** The feeder system, based on the placement program, is set up with materials according to a station table. The operator installs the materials on the feeders, which are then arranged on the placement machine. The gear drives the material tape, advancing it as per the program instructions, allowing the suction nozzle to pick and place components at specified coordinates.
**Reflow Soldering:** After printing solder paste and placing components, the next step is reflow soldering. The PCB, with components placed, undergoes manual inspection or AOI (Automated Optical Inspection) before entering the reflow oven. The term “reflow” refers to the process where solder paste melts and then solidifies to form solder joints. The reflow oven is a rectangular chamber that heats the PCB using a chain conveyor. It contains a hot air system divided into multiple temperature zones, typically described by a curve and divided into four key zones:
– **Preheating Zone:** Heats the PCB and components, preparing them for soldering by achieving thermal equilibrium and initiating the movement of solder paste.
– **Constant Temperature Zone:** Removes surface oxides and activates the solder paste, which remains undissolved but begins to become active.
– **Reflow Area:** The area with the highest temperature, where solder paste reaches its melting point. Lead-free solder paste typically starts to melt at around 220°C, taking about 40 seconds.
– **Cooling Zone:** Gradually cools the PCB from the melting point to about 50°C, solidifying the solder joints.
The entire reflow process usually takes about 6 minutes. This description of the three main SMT processes—printing, placing, and reflow soldering—should provide a comprehensive understanding of SMT patch processing.
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