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 process of printed circuit board, which is used to connect surface mount components to the pad on the board ' The paste is initially sticky. After heating, it will be pasted on the components on the PCB board (in strict accordance with the temperature curve). It will melt and then solidify to form electrical connection.

1.2 PCB template

The printed circuit board template is a steel plate (also called steel mesh) with many holes. These holes are cut by laser to make the solder paste flow.

1.3 Solder paste printing machine

During the pasting process, we need to ensure that the template hole is aligned with the corresponding pad, and then brush the squeal to ensure that all pads can be pasted with squeal. Of course, you can paste the scream manually, but that doesn't apply to hundreds of boards, which is why you need a solder paste printer.

We use automatic solder paste printing machine to ensure that the solder paste is polished accurately in the correct position.

2、 Automatic component placement

After the solder squeal on the printed circuit board, the next step is to put the electronic components on the bare board.

2.1 Chip mounter

Chip mounter is the key equipment in the production of circuit board assembly. 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 the required components in the special position of the machine, such as resistors/capacitors/integrated circuits/connectors, and then program the placement machine and tell it 'hey, put this resistor/capacitor here'.

The key specification of a mounting machine is how accurate it can be. In the figure, the SM481-PIUS chip mounter we use can handle 0201 package of resistor/capacitor and 0.3mm BGA IC.

2.2 Vacuum compressor

The mounter works together with the vacuum compressor. It pulls the rear of the part through a tiny nozzle. Therefore, you can usually see dozens of nozzles in the machine.

For PCBA prototype/small batch assembly, the most time-consuming process is the installation/programming and debugging of components, which is why a project has a start-up cost, while the unit welding cost is reduced from 10 to 50.

2.3 Gluing machine

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 gluing machine applies glue to the PCB board to make the components firmly sit on the PCB board. This is very important for wave soldering. In wave soldering, the force of the welding wave may cause large components to shift, or for double-sided wave soldering or reflow soldering, to prevent components from falling off.

3、 Reflow soldering

3.1 Reflow soldering furnace

Generally, for surface mount components, reflow soldering is used. After placing the element on the bare plate, heat it with this machine called 'reflow furnace'.

Put the PCB board where the components are placed into the machine from one end. On the double-track conveyor belt, the circuit board where the components are newly placed accurately controls the melting and cooling of the solder through the hot and cold zones of the reflow soldering furnace (most SMT factories are 10 zones) to fill the solder joint. The solder paste melts and hardens to form an electrical connection. 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 it needs gloves to take it.

3.2 Reflow soldering 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 related to the reflow temperature curve can be divided into four stages/regions (as shown below, and the following pictures).

preheat

Constant heating

High temperature welding

cooling

3.2.1 Preheating

The preheating zone is used to volatilize the lower melting solvent in the solder paste. Volatile solvent is required in the preheating zone, but the heating 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, 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 will not make the parts subject to thermal shock. The ideal temperature and time in this area are generally 220~260 ℃ and 30s~60s.

3.2.4 Cooling

During the cooling process, the temperature drops and the solder squeals solidify again. It plays a key role in the final result of the weld. Too slow cooling will make the parts rise, solder joints darken or solder joints uneven, while rapid cooling will cause thermal shock to the 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 module can withstand. For example, if there is ws2812 on the board, Makerfabs recommends not to exceed 220 degrees. As we suggest, why does the WS2812 SK6812 SMT fail after welding

*Solder paste used. Different face cream may require different temperature profiles.

*Maximum component size/maximum pin number. For example, if they are a high-power sensor, the temperature needs to be higher to ensure that the solder paste completely melts.

*PCB temperature specification

4、 Inspection and test

4.1 AOI (Automatic Optical Inspection)

There, you can't guarantee that all components are welded, for many reasons, such as component loss/lifting/cold connection... AOI is used to detect these common problems.

The 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. Then compare the captured image with the image of the correct reference plate to identify various defects, including incorrect components, missing components, cold connections, etc.

4.2 Programming tools

For functional testing, especially the controller board, some programming tools or burning/writing tools, such as Jlink/offline downloader, are required.

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.

*PIC chip

*ATMEL series

*ARM series

*Arduino bootstrap and sketch

* FPGA

*MicroPython and Circuit Python are related

*STM series

*ESP/ESP32 series

4.3 Test fixture

Function test is a key step to ensure that the final PCBA board works as expected. Although physical defects such as solder bridge or tombstone make PCBA 'should work', functional test makes circuit board 'ready'.

As a PCB assembly factory, it is difficult or even impossible to know how the customer's circuit board should work. It is usually necessary for the PCBA designer 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 packaging. After completing all machine steps, the most basic/simplest visual inspection is required to check whether there are any scars/special needs of customers, and any other problems that the machine cannot detect.

What are the processing steps of the circuit board? The introduction of the four major steps of circuit board processing is expected to help you. At the same time, if you want to know more about circuit board processing information, you can pay attention to the update of Linzhuo proofing.