1、 Solder paste stage
1.1 solder paste
Solder paste (or solder paste) is the key material for welding. As Wikipedia, it is a material used in printed circuit board manufacturing to connect surface mount components to pads on the board ” The paste is initially viscous. 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), which has many holes. These holes are laser cut to allow the solder paste to flow.
1.3 solder paste printer
During the bonding 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 a full-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.
Chip mounter is the key equipment in circuit board assembly and production. It not only determines the quality and speed of products, but also determines the maximum production capacity of products. Basically, it is used to “pick components and place them in the right place”.
The engineer will first install all the required components in a 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. It pulls the rear of the part through a tiny nozzle, so you 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 start-up fee, while the unit welding fee 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 adding machine 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, so as 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”.
After the solder is placed on the cold transfer zone of the machine, most of the solder is placed on the cold transfer zone of the PCB and the solder return zone of the new PCB, so as to accurately place the solder on the cold transfer zone of the machine. When the PCBA board reaches the other end, the connection becomes firm.
The circuit board is very hot when it comes out of the machine, so you need gloves to get 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 temperature curve can be divided into four stages / regions (shown below and subsequent pictures).
High temperature welding
The preheating zone is to volatilize the lower melting solvent in the solder paste. Volatile solvent is required in the preheating area, but the heating slope must be controlled.
3.2.2 constant heating
The setting of constant temperature zone is mainly controlled within the parameters of solder paste supplier and the heat capacity of PCB. At this stage, the printed circuit board reaches a uniform temperature, and the flux in the solder paste begins to react positively, thus 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 degrees and 30s ~ 60s.
During the cooling process, the temperature drops and the solder screams solidify again. It plays a key role in the final result of the weld. Too slow cooling will raise parts, darken solder joints or uneven solder joints, while rapid cooling will cause thermal shock to parts.
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 recommends that it should not exceed 220 degrees, as we suggest: why ws2812 sk6812 SMT fails after welding
*Solder paste used. Different facial creams may need 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 melts completely.
*PCB temperature specification
The very rough temperature is:
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4、 Inspection and testing
4.1 AOI (automatic optical inspection)
There, you can’t guarantee that all components are welded because of many reasons, such as component loss / lifting / cold connection… AOI is used to detect these common problems.
AOI machine uses optical methods to detect defects. It uses a high-definition camera to capture the surface of the circuit board and establish its image for analysis. The captured image is then compared with the image of the correct reference board to identify various defects, including incorrect components, missing components, cold connections, etc.
4.2 programming tools
For the function test, especially the controller board, some programming tools or burning / writing tools are needed, such as JLINK / offline downloader and so on.
As an important part of production, it is necessary to master the programming experience of the following chips and skillfully use programming tools and related software.
*Arduino bootstrap and sketches
*Micropthon is related to circuit python
*ESP / esp32 series
4.3 test fixture
Functional testing is a key step to ensure that the final PCBA board works as expected. Although physical defects such as solder bridges or tombstones make PCBA “should work”, functional testing makes the circuit board “ready”.
As a PCB assembly factory, it is difficult or even impossible to know how the customer’s circuit board should work. PCBA designers usually need to provide detailed test steps and how to judge whether the circuit board works normally.
For users, factory testing of test boards is usually very helpful.
A test fixture is composed of many pins to contact the pads on the PCBA, so the tester can program and test the pads if they work as expected. Of course, make the PCBA board work before the firmware / software is loaded.
4.4 visual inspection
Yes, this is the last step before packing. After completing all machine steps, the most basic / simplest visual inspection is required to check for any scars / special needs of the customer and any other problems that the machine cannot detect.
The above is the processing steps of circuit board. What are the steps? I hope the introduction of the four steps of circuit board processing can help you. At the same time, if you want to know more information and knowledge of circuit board processing, you can pay attention to the update of lingzhuo proofing.
Xinfeng Mingxin Electronic Technology Co., Ltd