1. Surface mount technology SMT (Surface Mount Technology) is a new generation of electronic mount technology, which compresses the traditional electronic components into only a few tenths of the volume of the device, thus realizing the high density, high reliability, miniaturization, low cost of electronic product mounting, and the automation of production.
2. Surface assembly components are mainly divided into two categories of chip passive components and active devices. Their main characteristics are: miniaturization, no lead (flat or short lead, suitable for surface assembly on PCB.
3. Surface assembly components packaging form directly affects the efficiency of assembly production, must be combined with the type and number of feeders to optimize the design of the mounting machine. Surface assembly components packaging forms are mainly four kinds, namely, braided tape, tube, tray and bulk.
4. Solder is a fusible metal, it can form an alloy on the surface of the base material, and connected to the base material as one, not only to achieve mechanical even, but also for electrical connection. Welding science, the customary welding temperature below 450 ℃ welding called soft brazing, with the solder is also known as soft solder. Electronic circuit welding temperature is usually between 180 ℃ ~ 300 ℃, the composition of the solder used is tin and lead, so also known as tin-lead solder.
5. The solder paste is the solder powder and flux with flux paste mixed, usually alloy solder powder proportion of the total weight of about 85% to 90%, accounting for about 50% of the volume
6. SMD adhesive is also called adhesive. In the mixed assembly in the surface assembly components temporarily fixed in the PCB pad graphics, so that the subsequent wave soldering and other process operations can be carried out smoothly; in the case of double-sided surface assembly, auxiliary fixed surface assembly components, to prevent the flip board and process operations when the vibration leads to surface assembly components fall. Therefore, in the mount surface assembly components before, should be set on the PCB pad position coated with SMD adhesive.
7. The most common manual welding has two kinds: contact welding and heating gas welding.
8. Heating control is a key factor in the desoldering process, the solder must be completely melted, so as not to damage the pad when taking away the components.
9. Desoldering design are: according to the type of device to be removed, size and package material, using the standard database parameters provided by the rework device to basically determine the temperature range and air volume of the top and bottom heating, select the appropriate hot air nozzle and vacuum nozzle; according to the scope of desoldering operations to remove the surrounding components that affect the operation or impose the necessary thermal resistance means to determine the application of heat time, etc.
10. The ball grid array package after taking down the need for tin ball reforming, the process is often called ball implantation. bga package class devices from the pcb when removed there will always be some tin balls remain on the device, and some remain on the pad. Residual tin balls on the pads is usually like solder icicles, if the requirements of the device will still be reinstalled on the PCB, the requirement for all the tin ball restructuring and PCB pad cleaning preparation.
11. BGA class package device rework process can be divided into four steps.
① is to clean up the BGA on the pad and PCB pad surface residual solder ball or solder and other substances, finishing the original solder ball solder plate to maintain flat. Processing to try to use the same chemical composition as the original assembly process flux and absorbing tin tape, which will reduce the possibility of depositing residues on the surface of the ball grid array with the following village of CSP or flip chip and other small package devices.
② It is the uniform application of the prepared flux to the pads.
③ is to hand transplant the prepared solder ball particles corresponding to the diameter of the solder ball of the original device onto the corresponding pads.
④ is according to the solder ball, flux temperature requirements of the completed implantation of the BGA in the appropriate temperature atmosphere "curing" to make the solder ball and pad close and reliable connection.
