PCB Technical - About the three main processes of SMT placement

The technical staff of the solution company often communicate with the SMT patch production factory, but people who have not actually been in the factory do not necessarily understand the basic production process and important processes, so here is the basic process and important of the SMT patch factory Make some introduction to the process to facilitate the relevant people to understand.

First of all, SMT (abbreviation of Surface Mounted Technology) in Chinese means "Surface Mounted Technology", which is currently the most popular technology and process in the electronics assembly industry. The process flow of SMT patch is very complicated, and different product processes are different. The basic process is as follows: incoming inspection-burning-printing-inspection-placement-pre-furnace inspection-reflow soldering- AOI inspection-repair-test-assembly.

In the process of SMT patch processing, no matter how many processes or processes there are, the three main processes of SMT are indispensable: printing-patch-reflow (soldering). This is the most traditional process and also the most basic process. In the past ten years, no matter how it has evolved, these three processes cannot be separated. Of course, all three processes are now completed by equipment automation.

SMT patch printing solder paste printing

Printing is the same as the principle of printing ink on paper through typesetting in a printing factory, except that we are talking about printing solder paste on PCB substrates. The equipment and tools used in printing are:

Printing machine: Fully automatic printing machine and semi-automatic printing machine, for example, the automatic printing machine is used for papaya patch proofing;

Solder paste: solder paste is a kind of material, a special material used to fix the material and the PCB board;

Stencil: Simply put, it is a mold that hollows out the positions of the pads on the PCB, so that the solder paste can penetrate into the pads from these hollowed out positions. It is a very thin steel sheet, which is fixed by a screen frame. It is very flat. The most commonly used thickness is 0.10mm. Different steel sheet thicknesses and manufacturing processes are selected according to the different components on different products. This is made according to a paste mask file in the Gerber file provided by R&D or the customer. This work needs to be completed before production, because the steel mesh production has another production process and production process, and the quality of the steel mesh determines the paste. The product quality of the film is therefore crucial. The more precise the components, the more important the stencil is, and different printing machines have slightly different requirements for the opening of the stencil. According to personal experience, if there are similar precision components such as 0.4pitchBGA on the product, it is recommended not to The customer-supplied steel mesh can be made by professionals in the production plant, because there is no strict standard for the process, and the process engineer of the plant is the most clear and understanding of the specific details.

After understanding the tools needed for printing, I believe everyone has some understanding of the basic operations. The simple operation is to install the stencil in the printing machine, add solder paste on the stencil, the PCB board enters the track of the printing machine, scan the mark points on the PCB board and the stencil through the camera of the printing machine, and after the alignment is completed, The printing machine platform rises and the stencil is attached. The scraper on the printing machine is inclined at 45° to scrape the solder paste from the stencil, and the solder paste is scraped onto the pads on the PCB through the hollow position of the stencil. This is a complete The printing process. If there is no defect, it is perfect. If there is any defect, it needs to be fine-tuned by the field equipment engineer. According to years of on-site process analysis, the printing process is the most important of the three SMT processes, because 70% of SMT process defects are related to this process.

SMT patch display

Patch

Mounting, also known as SMT, is to mount components on a printed PCB board with a mounter equipment. The reason why the word "sticking" is used in the process of patching is that the solder paste contains a flux composition, which has a certain degree of viscosity, and can stick to the components when it is not melted. SMT is also called patch, which means to paste materials on the circuit board.

The principle of SMT is simple and complicated at the same time. It is simple because it is evolved from the original manual soldering, that is, the components are placed on the circuit board with tweezers, and the placement machine uses the placement head to pass through the vacuum. The suction components are attached to the PCB board; the complexity is because the actual patching situation is very complicated, and the equipment is also very sophisticated. Through the improvement of technology, all the traditional hand plug-ins are changed into patch parts, which greatly improves the production efficiency., The supply chain of the entire industry has changed accordingly, and has undergone earth-shaking changes.

So what is the working principle of the placement machine?

Product placement program: Any type of placement machine will need to use the Gerber, coordinate file, BOM, and location map provided by the customer to make the placement program in advance. Then through the placement head (suction nozzle), feeder and track of the placement machine to complete the entire placement process.

Suction nozzle: There are 12 suction nozzles above the patch head, the center of the suction nozzle is empty, and the material is sucked by vacuum.

Feeder: It is the feeder. According to the placement program made by the placement machine programmer, it is printed as a station table. The operator installs the materials on the feeder according to the order of the station table, and rows of feeders are installed on the placement table. On the machine, plug in, the gear drives the material tape, and the material tape advances the program instruction to specify the suction nozzle to suck the material at the specified position, and then paste it to the position specified by the coordinates.

Backflow

After the two processes of printing solder paste and patching, it is reflow soldering. After all the components are pasted, the PCB board will be sent to the docking station by the placement machine for manual visual inspection or by the AOI machine in front of the furnace to check if there is any defective placement of the components. If there is no problem, You can go through the furnace.

When it comes to the name of reflow soldering, many people may not know what "reflow" is. It does not mean that solder paste flows from here to there. Reflow soldering comes from "Reflow Soldering". The solder paste becomes a flowing liquid and then solidifies into an alloy state.” The reflow oven is an "oven" with a bicycle chain, but it is a rectangular oven that transports the PCB board through the chain, heats and melts the solder paste, and solidifies the components on the PCB board pads. There is a hot air device in the reflow oven, which is divided into multiple temperature zones and gradually heated. A curve is used to describe it and it is generally divided into four key zones.

Preheating zone: Preheating the PCB and components, referring to the heating effect of the first one to three heating zones for the reflow oven. Higher preheating makes the material to be soldered reach thermal equilibrium, the solder paste starts to move, and the flux and other components begin to volatilize due to the rise in temperature, mainly to pave the way for good soldering in the future.

Constant temperature zone: the surface oxide is removed, and the solder paste starts to become active. At this time, the solder paste is in an undissolved state, and it is heated for the 563 heating zone of the reflow oven.

Reflow area: It is also the soldering area. It is the area with the highest temperature in the entire reflow furnace, so that it reaches the melting point of the solder paste. The commonly used lead-free solder paste with a melting point generally starts to melt at 220°C and takes about 40 seconds.

Cooling zone: From the melting point to about 50 degrees slowly, the formation process of alloy solder joints.

In this way, even if the entire reflow process is completed, this process usually takes about 6 minutes.

The above explanation and description of the three main processes of printing, patching, and reflow in the SMT patch processing process, I believe that relevant people will have a deeper understanding of the three main processes of SMT patching.