As one of the most common surface coating forms for circuit board surface treatment, tin spray (SMOBC & HAL) is widely used in circuit production. The quality of tin spray directly affects the quality of soldering during subsequent customer production and solderability. Therefore, the quality of spray tin has become a focus of quality control for PCB circuit board manufacturers.
There are currently two types of tin spraying: vertical tin spraying and horizontal tin spraying.
The main function of spray tin:
① Prevent oxidation of bare copper surface
② maintain solderability
Other surface treatment methods include hot melt, organic protective film OSP, chemical tin, chemical silver, chemical nickel gold, electroplated nickel gold, etc, but the cost-performance ratio of spray tin plate is the best.
Vertical tin spraying mainly has the following disadvantages:
① The board is heated unevenly up and down, last in first out, and the defects of board bending and warping are prone to occur.
② The thickness of tin on the pad is uneven. Due to the blowing force and gravity of the hot air, the lower edge of the pad produces solder sag, which makes the soldering of the SMT surface mount parts difficult to stick firmly, and it is easy to cause the soldering of the parts after welding and tomb stoning.
③ The contact time between the pad on the bare copper on the board and the hole wall and the solder is long, generally more than 6 seconds. The copper melt increases rapidly in the soldering furnace. The increase in copper content will directly affect the solderability of the pad, because the resulting thickness of the IMC alloy layer is too thick, which greatly shortens the shelf life of the board.
Compared with vertical tin spraying, the horizontal spray tin greatly overcomes the above defects, and has the following advantages:
① The contact time between molten tin and bare copper is short, about 2 seconds, the thickness of IMC is thin, and the shelf life is long.
② Wetting time is short, about 1 second.
③ The board is heated evenly, the mechanical properties are kept well, and the board warps is small.
The process flow of horizontal spray tin:
Pre-cleaning treatment—preheating—flux coating—horizontal tin spraying—hot air knife scraping tin—cooling—post-cleaning treatment
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