PCB Technical - PCB process COB and SMT process sequence relationship

PCB process COB and SMT process sequence relationship

Before performing the COB process, the SMT operation must be completed first. This is because SMT needs to use a steel plate (stencil) to print the solder paste, and the steel plate must be laid flat on an empty circuit board(bare PCB). It can be imagined as using a stencil to paint. But spray paint becomes paint. If there are already tall things on the wall to be painted, the template cannot be flat on the wall, and the protruding paint cannot be leveled; the steel plate is equivalent to the template, if the circuit board is above There are other parts that are higher than the surface, the steel plate cannot be flat on the circuit board, the printed solder paste thickness will be uneven, and the thickness of the solder paste will affect the subsequent parts to eat tin, too much tin Paste will cause a short circuit of parts (solder short), and too little solder paste will cause a solder skip (solder skip); in addition, a squeegee is required and pressure is applied when printing solder paste. If there are already parts on the circuit board, there is May be crushed.

If the COB 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 solder paste can be printed. The circuit board has to go through a high temperature reflow furnace of 240~250 degree Celsius. Generally, most of the epoxy of COB can not withstand such high temperature and become embrittlement, which finally causes instability in quality.

Therefore, the COB process is usually a process after SMT. In addition, COB sealing 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 the electrical characteristics of the board must be confirmed before performing COB. The manufacturing process.

In fact, from a purely COB perspective, 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 oven), and the high temperature of the reflow oven will also cause the board The phenomenon of bending and warping is not good for COB operations, but based on the current needs of the electronics industry, there are still some trade-offs.

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