- Surface Mount Technology SMT (Surface Mount Technology) is a new generation of electronic mounting technology. It compresses traditional electronic components into devices with only a few tenths of the volume, thereby realizing high-density, high-density electronic product mounting. High reliability, miniaturization, low cost, and automation of production.
- Surface-mounted components are mainly divided into two categories: chip passive components and active components. Their main features are: miniaturized, leadless (flat or short leads, suitable for surface assembly on PCBs.
- The packaging form of surface-mounted components directly affects the efficiency of assembly production, and must be optimized in combination with the type and number of feeders of the placement machine. There are four main packaging forms of surface mount components, namely tape, tube, tray and bulk.
- Solder is a fusible metal, which can form an alloy on the surface of the base metal and connect with the base metal as a whole, not only for mechanical connection, but also for electrical connection. In soldering, it is customary to call soldering with a soldering temperature lower than 450 °C as soldering, and the solder used is also called soft solder. The soldering temperature of electronic circuits is usually between 180 ° C and 300 ° C. The main components of the solder used are tin and lead, so it is also called tin-lead solder.
- Solder paste is a mixture of solder powder and paste flux with fluxing function. Usually, the alloy solder powder accounts for about 85% to 90% of the total weight and about 50% of the volume.
- SMD glue is also called adhesive. In the hybrid assembly, the surface-mounted components are temporarily fixed on the pad pattern of the PCB, so that the subsequent process operations such as wave soldering can be carried out smoothly; Surface mount components can drop due to vibrations during and process operations. Therefore, before mounting the surface mount components, it is necessary to apply the patch glue on the PCB to set the pad position.
- There are two most common types of manual welding: contact welding and heated gas welding.
- Heating control is a key factor in the desoldering process, the solder must be completely melted to avoid damage to the pads when the components are removed.
- The contents of the desoldering design include: according to the type, size and packaging material of the dismantled device, use the standard database parameters provided by the rework device to basically determine the temperature range and air volume of the top and bottom heating, and select the appropriate hot air nozzle and vacuum suction. Remove the peripheral components that affect the operation or apply necessary heat resistance means according to the scope of the desoldering operation, and determine the heating time, etc.
- After the ball grid array package is removed, it is necessary to carry out solder ball reforming, which is usually called ball mounting. When a BGA package type device is removed from the PCB, there will always be some solder balls remaining on the device and others on the pads. The solder balls remaining on the pads are usually like solder icicles. If the device is still required to be remounted on the PCB, it is required to carry out all solder ball reforming and cleaning preparation of the PCB pads.
- The repair process of BGA packaged devices can be divided into four steps: ① It is to clean the pads on the BGA and the residual solder balls or solder on the surface of the PCB pads, and organize the original solder ball pads to keep them flat. When handling, try to use the same chemical composition of flux and solder tape as in the original assembly process, which will reduce the possibility of depositing residues on the ball grid array surface and the underlying village of small packaged devices such as CSP or flip-chip;
② is to evenly apply the prepared flux to the pad;
③ is to manually transplant the prepared solder ball particles corresponding to the diameter of the original device solder ball to the corresponding pad;
④ According to the temperature requirements of the solder balls and flux, the BGA that has been implanted is placed in a suitable temperature atmosphere to "solidify" so that the solder balls and the pads are tightly and reliably connected.