Machines are naturally used for manufacturing circuit boards, but accurately placing a component smaller than the lead of a mechanical pencil on a circuit board in a specific position is difficult. Joining such components on the board is even more difficult. In many cases, solder is printed on the board so that it simultaneously works as an adhesive and an electrical connection between the board and the component. The component is placed on the printed solder, and the entire board is heated to melt the solder to connect the component and the board.
Solder Paste Printing Machine
In this case, the solder is made to resemble a cream-like paste by combining several types of metal alloys prepared in small particles of several microns to several tens of microns mixed with organic material. The width of the solder paste pattern required for printing on the circuit board should be as small as possible, as long as the solder provides electrical conductivity and mechanical strength. A large width pattern tends to spread the solder when heated so that it touches the adjacent circuit and fails to form a high-density circuit. In this case, skillfully using the solder paste or controlling the solder behavior is essential.
To print solder paste on a circuit board, we use a screen mask with many small holes that reflect the design. By supplying solder paste on this screen mask and pressing with a squeezer, it is pushed out of the small holes and placed on the circuit board in the designed pattern. Solder paste resembles toothpaste with much higher viscosity. Skillfully using this highly viscous solder paste is the key to produce high-density circuit boards.
Panasonic further plans to develop technology for producing a circuit pattern only with solder. In this case, we require more accurate and uniform printing of solder paste than the level needed for device connections. However, screen masks are generally made from a thin metal plate, and the shape of the printed solder tends to become non-uniform between the center and edge parts of the board. Variation in the width and height of the solder paste can create defects in the circuit boards.
Generating Non-uniform Shape of Solder Paste
Panasonic invented a method of printing a variety of solder patterns on circuit boards by pushing solder paste out from a cartridge like macaroni pasta from a machine nozzle. This method reduces the risk of exposing solder paste to the atmosphere compared with the previous method, and high quality solder paste can be maintained for many hours. This method also simplifies temperature control, and consistent viscosity can produce more uniform printing.
The cartridge and the Out View of the Printer Head
This method, however, is different from the previous printing principle and presents a new problem of creating "threads" of paste when the cartridge is lifted after printing paste on the circuit. On the other hand, solder paste has a unique characteristic that reduces its viscosity when shearing force is applied. Panasonic focused on this characteristic and solved the above problem by applying a special design at the cartridge's tip and by lifting the cartridge from the circuit board in a special motion developed by Panasonic.
This development enables a 20-µm wide circuit pattern to be printed with 20-µm spaces. This method can also print uniform shaped patterns relatively easily even when the board has uneven flatness.
The Cross Sectional View of Printed Solder Pattern by a Conventional Method
The View by Panasonic's Solder Paste Cartridge
In a smartphone, many varieties of small components are laid out on a complex electronic circuit. Many solder paste printing machines made by Panasonic are used in the world to connect components on complex high-density circuit boards. Panasonic's technology will accelerate the achievement of higher-level electronic products using solder for producing electronic circuits and accurately controlling the height and the width of circuit patterns.