1、 Solder paste stage
1.1 solder paste
Solder paste (or solder paste) is the key material for welding. As Wikipedia, it is a kind of material used in the manufacturing of printed circuit boards to connect surface mount components to pads on the board The paste is initially sticky. After heating, it is pasted on the components on the PCB board (strictly according to the temperature curve). It will melt and then solidify to form an electrical connection.
1.2 PCB template
The printed circuit board template is a steel plate (also known as steel mesh) with many holes. These holes are laser cut to allow the solder paste to flow.
1.3 solder paste printer
During the pasting process, we need to ensure that the template hole is aligned with the corresponding pad, and then brush the scream to ensure that all pads can be pasted with the scream. Of course, you can paste screams by hand, but that doesn’t apply to hundreds of boards, which is why you need a solder paste printer.
In the figure, we use an automatic solder paste printer to ensure that the solder paste is polished accurately in the correct position.
2、 Automatic component placement
After the solder on the printed circuit board screams, the next step is to place the electronic components on the bare board.
2.1 chip mounter
Chip mounter is the key equipment in PCB assembly and production. It determines the quality and speed of products, and also determines the maximum production capacity of products. Basically, it is used to “pick components and place them in the correct position”.
The engineer will first install all required components in the special position of the machine, such as resistors / Capacitors / integrated circuits / connectors, and then program the mounter and tell it “Hey, put this resistor / capacitor here”.
The key specification for a mounter is how accurate it can be. In the figure, we use sm481-pius, which can handle resistance / capacitor 0201 package and 0.3mm BGA IC.
2.2 vacuum compressor
The mounter works with a vacuum compressor, which pulls the rear of the part through a tiny nozzle, so you will usually see dozens of nozzles in the machine.
For PCBA prototype / small batch assembly, the process with the highest time cost is the installation / programming and debugging of components, which is why a project has a startup cost, while the unit welding cost is reduced from 10 to 50.
2.3 glue dispenser
Sometimes, there are components on both sides of the PCB. When one side is welded, the components on the other side may fall off, especially when the components are heavy or large. The glue dispenser applies glue to the PCB board to make the components sit firmly on the PCB board. This is very important for wave soldering. In wave soldering, the force of welding wave may shift large components, or for double-sided wave soldering or reflow soldering, to prevent components from falling off.
3、 Reflow soldering
3.1 reflow furnace
Typically, for surface mount components, reflow soldering is used. After the components are placed on the bare board, they are heated by this machine called “reflow furnace”.
Put the PCB board for placing components into the machine from one end. On the double track conveyor belt, the circuit board for newly placing components passes through the hot zone and cold zone of the reflow soldering furnace (most SMT plants have 10 zones) to accurately control the melting and cooling of solder to fill solder joints. The solder paste becomes hard after melting to form an electrical connection. When the PCBA board reaches the other end, the connection becomes firm.
The circuit board is very hot when coming out of the machine, so you need gloves to take it.
3.2 reflow temperature curve
The temperature curve related to reflow soldering is the most important parameter to ensure the correct connection of parts. The main temperature changes associated with the reflow soldering temperature curve can be divided into four stages / areas (shown below, and subsequent pictures).
Constant heating
High temperature welding
3.2.1 preheating
The preheating zone is used to volatilize the lower melting solvent in the solder paste. The preheating zone needs volatile solvent, but the temperature rise slope must be controlled.
3.2.2 constant heating
The setting of the constant temperature zone is mainly controlled within the parameters of the solder paste supplier and the thermal capacity of the PCB. At this stage, the printed circuit board reaches a uniform temperature, and the flux in the solder paste begins to react positively, thereby increasing the wettability.
3.2.3 high temperature welding
In this region, complete melting and wetting reactions occur. This part of the process also needs to be carefully controlled so that the slope of temperature rise and fall does not subject the component to thermal shock. The ideal temperature and time in this area are generally 220 ~ 260 ℃, 30s ~ 60s.
3.2.4 cooling
During the cooling process, the temperature drops and the solder squeal solidifies again. It plays a key role in the final result of the weld. Too slow cooling will raise the component, darken the solder joint or uneven the solder joint, while rapid cooling will cause thermal shock to the component.
The setting of temperature curve is the most important for experienced SMT engineers. There are many factors to consider, which depend on PCBA and components, including:
*The maximum temperature that the component can withstand. For example, if there is ws2812 on the board, makerfabs suggests that it should not exceed 220 degrees, just as we suggest: why ws2812 sk6812 SMT fails after welding
*Used solder paste. Different face cream may require different temperature curves.
*Maximum component size / maximum pin pins. For example, if they are a high-power sensor, the temperature needs to be higher to ensure that the solder paste is completely melted.
*PCB temperature specification
* ……