The SMT circuit board installation scheme adopts the SMT installation method and process completely different from the installation method and process of through-hole plug-in components. At present, in the products applying SMT chip technology, some of them are circuits that all use SMT components, but the so-called "mixed assembly process" can also be seen, that is, on the same printed circuit board, there are existing plug-in circuits. There are traditional THT components installed, as well as surface-mounted SMT components. In this way, there are many types of circuit mounting structures.

1. Three SMT installation structures and assembly welding process flow

⑴ The first assembly structure: All surface-mounted printed circuit boards have no through-hole plug-in components, and various SMD and SMC are mounted on one or both sides of the circuit board.

⑵ The second assembly structure: double-sided mixed installation on the A side of the printed circuit board (also called "component side"), with both through-hole plug-in components and various SMT components; On the B side of the board (also called the "welding side"), only small SMD transistors and SMC components are assembled.

⑶ The third assembly structure: the two sides are respectively installed on the A side of the printed circuit board and only the through-hole plug-in components are installed, while the small SMT components are mounted on the B side of the printed circuit board. It can be considered that the first assembly structure can fully reflect the technical advantages of SMT, and this printed circuit board will eventually be the cheapest and smallest in size. However, many experts still believe that the latter two mixed-assembly printed circuit boards also have good prospects, because they not only take advantage of the advantages of SMT chip processing, but also can solve the problem that some components cannot be surface-mounted so far. . From the perspective of the printed circuit board assembly and welding process, the third assembly structure is not much different from the traditional through-hole insertion method except for the use of patch glue to paste the SMT components on the printed circuit board. In particular, the wave soldering equipment that is now more popular can be used for soldering, and the process technology is relatively mature; and the first two assembly structures generally require the patch processing plant to add reflow soldering equipment.

Two, smt printed circuit board wave soldering process flow

Under the above-mentioned third SMT assembly structure, the printed circuit board adopts the process flow of wave soldering.

⑴ Making the adhesive screen According to the position of the SMT components on the printed circuit board, make the screen for the missing adhesive. ⑵ Screen missed printing adhesive The adhesive screen is covered on the printed circuit board, and the adhesive is missed. It is necessary to accurately ensure that the adhesive is printed in the center of the component, especially to prevent the adhesive from contaminating the pad of the component. If you use a dispenser or manually dispense the adhesive, the first two procedures must be changed accordingly.

⑶ Mounting SMT components Mount the SMT components on the printed circuit board so that their electrodes are accurately positioned on their respective pads.

⑷ To cure the adhesive, use heating or ultraviolet radiation to dry and cure the adhesive, so that the SMT components are firmly fixed on the printed circuit board.

⑸ Insert THT components Turn the printed circuit board 180°, and insert traditional THT lead components on the other side.

⑹ The welding process of wave soldering is the same as that of ordinary printed circuit boards, and the wave soldering equipment is used for soldering. During the soldering process of printed circuit boards, SMT components are immersed in molten tin. It can be seen that SMT components should have good heat resistance. If dual-wave soldering equipment is used, the soldering quality will be much better.

⑺ Printed circuit board (cleaning) test to clean the soldered printed circuit board to remove the residual flux residue (now no-clean flux is generally used, unless it is a special product, it is generally not necessary to clean). The SMT processing plant finally conducts circuit inspection and testing.

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