In the manufacturing process of electronic circuits, “surface mounting” is an important process for attaching electronic components to printed circuit boards. In this article, we are going to read how the surface mount manufacturing process works.
What is surface mounting?
Surface mounting is a typical method of attaching components to a printed circuit board. In the past, electronic components were connected by extending legs called leads and inserting them into printed circuit boards. However, there was a problem that it was not possible to reduce the size of the components and the board because holes had to be made in the board.
A method developed to solve this problem is to solder the parts directly to the copper pads on the surface of the pcb board without attaching leads to the parts. It came to be called “Surface Mount” because components can be mounted on the surface of the board and soldered without drilling holes in the board.
Today, the equipment for automating surface mounting has been put in place, enabling ultra-high-speed and high-precision mounting, so surface mounting is used in most electronic circuits.
Flow of Surface Mounting
Now, let’s explain step by step how surface mounting is done.
Prepare a printed circuit board
First, prepare a printed circuit board for mounting the parts. A printed circuit board is a component that has copper foil patterns wired on the surface and inside of the board in order to connect the components as designed.
Printed circuit boards have patterns called “lands” for soldering so that components can be correctly mounted on the board. The land shape is technically devised to improve solder quality and yield when mounting components.
Substrate cleaning
After preparing the printed circuit board, cleaning is performed to remove dust and dirt from the circuit board before starting to mount the components. A compact cleaning device is attached to the component mounting device, and it is common to blow away the dust with the wind.
Although it is a comprehensive work, there is a large difference in the defect rate, so it is important to devise a process that prevents dust from adhering.
Cream solders application
After cleaning, start the mounting process. The first step is to apply solder to attach the electronic components to the board. For surface mounting, “cream solder” is used, which is a paste that maintains cream-like softness even at room temperature.
If the cream solder is applied as it is, the solder will also adhere to unnecessary places, so a jig called “metal mask” made based on the CAD data is used so that the solder sticks only to the lands.
It is the most difficult process to maintain the quality of cream solder because the application conditions change depending on the external environment such as temperature and humidity. The metal mask is also full of know-how to accurately apply cream solder while avoiding short circuits between terminals, solder cracks, voids, and solder balls.
Component Mounting on Printed Circuit Board
After applying cream solder, electronic components are placed on the printed circuit board. It is common to use a machine called a “chip mounter” that automatically places electronic components in place.
The electronic components to be mounted are purchased on reels and set in a machine called a “feeder” to supply the chip mounter. After that, if you create a dedicated program for each type of board, the chip mounter will automatically recognize the parts and mount them in the specified location.
High-speed, high-precision component mounting is achieved by using the image recognition technology of the camera to determine the mounting position of the component and by picking up the small component with the nozzle head and steadily transporting it.
By the way, the chip mounter automatically performs mounting, so mass production is easy, but if there are many parts, the setup becomes complicated. When making a small number of printed circuit boards, such as for prototyping, or when using loose parts for electronic components, chip mounters cannot handle them, so it is necessary to place the mounted components by hand.
Reflow Soldering Melting Temperature
After mounting the components, put the printed circuit board in a reflow oven to apply heat and melt the solder. The melting temperature of solder is 183°C for solder, and 217°C for lead-free solder.
However, some electronic components will break if left in a high-temperature environment where solder melts, so it is essential to control the temperature so that the high temperature remains below a certain period of time. In general, preheating to about 150°C and then heating to 250°C within about 30 seconds minimizes heat damage.
In addition, in most cases, the temperature rise rate and heating time are strictly specified so that the solder melts correctly and cracks and voids do not occur.
By the way, the name “reflow furnace” comes from the fact that it is paired with a “flow furnace” that solders by directly flowing molten solder.
Process after Surface Mounting
The above is the surface mounting process, but the process of “lead component mounting” and “appearance/conductivity inspection” is included before the printed circuit board is completed.
Lead Component Mounting
Some parts, such as coils and electrolytic capacitors, are large in size and are only manufactured as leaded parts. Since lead parts cannot be soldered by surface mounting, they are mounted in the process after surface mounting.
First, insert the lead component into the board and cut off the excess lead length. This process is often done manually. After inserting into the board, solder is applied in a flow furnace.
The flow furnace is a method of soldering by directly contacting the back surface of the substrate with molten solder. After applying flux, the board is heated with a heater, and the melted solder is poured around the lead parts for mounting.
Automatic Inspection of Printed Circuit Board with AI Technology
After all the parts are mounted, the board is inspected to see if it works properly. In the post-mounting inspection, it is common to perform a “visual inspection” to check if there are any defects in the mounting of the parts and the shape of the solder, and a “continuity inspection” to check if the current flows normally in the circuit.
Automatic inspection using image recognition is widely used for visual inspection. However, since there is a limit to the accuracy of recognition, human visual inspection is often performed at the same time. Recently, efforts are being made to improve the quality of automatic inspections using AI learning functions, so full automation is expected in the future.
Continuity testing is a test that applies electrodes to the test pads on the board and checks the capacitance and resistance values of each pattern to ensure correct continuity. Since there are many inspection items, during mass production, we design according to the mounting board and use an automatic inspection machine that automatically conducts continuity inspection.
Summary
In this article, we have explained in detail about the surface mounting process of electronic circuits. Surface mounting is a mounting method in which components without leads are placed on a printed circuit board and fixed with solder.
Since surface mounting is currently the preferred method for most board manufacturing, equipment, and technology for surface mounting are well established. However, know-how to improve the quality and yield of soldering is packed here and there, so be careful when changing the conditions.
