The sequence of COB bonding processing and SMT processing in PCBA processing
In general PCBA processing, before COB processing, SMT patch processing must be completed. This is because SMT processing needs to use steel plate (stencil) to print solder paste, and the steel plate must be laid flat on the empty circuit board (bare PCB). ) Above, it can be imagined to use a template to spray paint, but the spray paint becomes paint. If there are already tall things on the wall to be painted, the template will not be flat on the wall, and the protruding paint will not be leveled; The steel plate is equivalent to the template. If there are other parts above the surface on the circuit board, the steel plate cannot be flatly attached to the circuit board, and the printed solder paste thickness will be uneven, and the thickness of the solder paste will affect Subsequent parts eat tin, too much solder paste will cause a short circuit (solder short), too little solder paste will cause solder skip; in addition, a squeegee is needed and pressure is applied when printing solder paste. If there are parts on the circuit board, they may be crushed.
If the COB bonding is completed first, a small circular hill will be formed on the circuit board, so that the steel plate cannot be used to print solder paste, and other electronic parts cannot be soldered to the circuit board, and the printed tin The paste circuit board has to go through a high temperature reflow furnace at 240~250℃. Generally, the epoxy can not withstand such a high temperature during COB bonding, which will cause embrittlement, resulting in unstable quality.
Therefore, COB bonding processing is usually a process after SMT chip processing. In addition, the COB bonding sealant is generally an irreversible process, that is, it cannot be repaired (repair), so it is usually placed in the last step of all circuit board assembly, and it is necessary to make sure that the electrical characteristics of the board are not problematic. Carry out the COB bonding process.
In fact, if purely from the perspective of COB bonding, the COB process should be completed as soon as possible, because the gold layer (Au) on the circuit board will be slightly oxidized after the SMT reflow (reflow furnace), and the high temperature of the reflow furnace will also The phenomenon of board bending and warping is not good for COB bonding. However, based on the current needs of the electronics industry, there are still some trade-offs.
