1. Bare board loading

The first step of circuit board assembly is, of course, to load the bare board onto the SMT pipeline. At present, the most common technology is to stack and arrange the empty plates neatly, place them on the material rack, and then send them to the conveyor belt of SMT production line from the top plate by the mechanism device, just like the paper transportation of the printer. However, this operation sometimes causes the problem of surface scratch on some plates in the process of pushing, so sometimes empty plates will be placed in the magazine, In this way, there will be no scratch problem during the stacking of the machine, but there is an additional action to put it into the distribution rack.

2. Solder paste printing

The first operation of printed circuit board (PCB) entering the SMT production line is "solder paste". In fact, it is a little similar to girls smearing facial mask on their faces. In this step, the solder paste will be printed on the pads / pads of PCB parts to be welded through the steel plate. The position and volume of the solder paste will affect the subsequent welding quality. These solder pastes will melt when the subsequent SMT process flows through the high-temperature area of the "reflow oven", and the electronic parts will be welded on the circuit board during the re solidification process.

3. Solder paste inspector (SPI)

The quality of solder paste printing will directly affect the quality of subsequent parts welding. Therefore, most leading EMS factories will first set up an additional optical instrument after solder paste printing to check the quality of solder paste printing, which is called "solder paste inspector (SPI)", or remove excess solder paste by repair.

4. Chip placement, pick and place speed machine

The electronic parts on the circuit board are generally divided into active components (IC parts) and passive components (inductors, capacitors, resistors and other parts), and this kind of SMD passive components (such as small resistors, capacitors, inductors) is also known as "small chip", which is usually small in size, and generally only two endpoints need to be welded, so the relative position accuracy requirements are relatively low when placing this kind of small parts on the circuit board, Therefore, a fast / high-speed chip placement machine was designed. This kind of placement machine usually has several suction nozzles, and the speed is very fast. It is as fast as a rotary machine gun, and it can hit several parts in a second.

5. Pick and place general machine

This kind of universal printer and mounter is generally called "slow machine". It can be applied to almost all SMD parts, but because its demand is not speed, but accuracy, the slow machine is generally used to hit some electronic parts with large volume, heavy or multi foot position, and then adjust the position and angle of the parts before placing, so the overall speed is relatively slow.

6. Hand place component or visual inspection or AOI in front of furnace

When all parts are printed on the circuit board, before entering the high-temperature reflow furnace, a check point is usually set to pick out the shortcomings of the offset or falling of the printed parts... Because after the high-temperature furnace, if there are welding problems again, the iron must be used, which will affect the quality of the product and also have additional costs; It will also manually swing parts here.

7. Reflow

The purpose of reflow is to melt the solder paste and form a non-metallic compound (IMC) between the part foot and the circuit board, that is, to weld the electronic parts on the circuit board. The temperature profile often affects the welding quality of the whole circuit board. According to the characteristics of solder, general reflow furnaces will set pre heat, soak, reflow Cool area to achieve the best soldering effect.

8. Optical inspection (AOI, auto optical inspection) option

Although "after furnace AOI" has almost become the standard configuration of today's SMT, it is not necessarily that every SMT production line will be equipped with "optical inspection machine (AOI)". One of the purposes of setting "after furnace AOI" is to replace it with AOI because some circuit boards with too high density cannot effectively carry out subsequent short circuit circuit test (ICT). However, because AOI is optical interpretation, it has its own blind spot, For example, the soldering tin under the parts cannot be checked, and there will be shadow effect near the high parts, which cannot be effectively checked. At present, AOI can only check whether there are tombstones, side standing, missing parts, displacement, polarity direction, tin bridge, empty welding, etc. for the visible parts, but it cannot judge the quality of parts such as false welding, BGA welding, resistance value, capacitance value, inductance value, etc, Some AOI can't even check the side solder of QFN or castellated terminals.

9. Unloading

When the board is assembled, it will be returned to the magazine. These shelves have been designed to allow the SMT machine to automatically take and place the board without affecting its quality, but during operation, we still have to pay attention to the upper and lower spacing of different boards to avoid the problem of electronic parts hitting.

10. Visual inspection of finished products

Whether there is an AOI station or not, the general SMT line will still set up a visual inspection area for PCBA assembly boards, and the confidence is still insufficient! The purpose is to check whether there is any defect after the assembly of the circuit board. If there is an AOI station, the number of visual inspectors can be reduced, because it is still necessary to check some places that cannot be read by AOI, or recheck the defects that AOI decides to reject. At present, AOI still has a certain misjudgment rate.

11. Touch up or wave welded parts

If some parts cannot be pasted and punched with SMT, it is necessary to touch up hand welded parts or use "wave soldering" or "selective welding" to weld traditional through-hole parts. This step is usually placed after the finished product inspection of SMT, in order to distinguish whether the defect comes from SMT or the process after SMT.