Solder paste is commonly used to create electrical connections and mechanical bonds between printed circuit board pads and surface mount devices, such as resistors and capacitors. It is made up of powdered solder in a flux paste. The flux serves several purposes. It removes oxidation from metallic surfaces, it protects the components to be soldered, as well as the metallic surface on the board. Flux will also act as a temporary low-tac adhesive to hold electronic surface mount devices in place until the board and its’ components are heated, which melts the powdered solder and creates the connection and bonds the devices to the printed circuit board pads. This heating process is referred to as reflow.
Each surface mount device has its’ own footprint and the solder connections between the board and the devices are critical. Many variables affect the performance of the solder paste, which is why it is important to have a proven process. The variables include the paste quality, the percentage of solder powder in the flux, the flux viscosity, the application of the solder paste, commonly referred to as stencil printing, and the reflow process. The process and procedure, or a lack there of, will determine the level of your product success since the majority of printed circuit board failures can be attributed to solder paste process issues.
When engineers design a printed circuit board, they determine the required surface mount devices and how they will connect and interact based on the desired function of the board. A circuit diagram, typically referred to as a schematic (see figure 1) is digitally laid out, representing the connectivity relationship between each device. This schematic will then be used to design a digital layout of the printed circuit board, referred to as a Gerber file (see figure 2). The size of the printed circuit board is most often determined or limited by the available space in the finished product, and the size of the surface mount devices will be determined by the available space on the printed circuit board. This is why the right solder paste procedure is critical. As electronic devices get smaller, they require designs that take full advantage of the space available. Smaller boards typically require smaller surface mount devices, that have smaller footprints. So, the board layout will have smaller pads for mounting. As spatial tolerances shrink, precision placement of surface mount devices becomes more critical. Today’s high-speed pick and place equipment can accurately place devices, but if the soldering process is not carefully and accurately executed, you run the risk of diminishing any advantages your pick and place equipment offer. A solder paste that is too thick, or that is applied too light or too heavy on the pads may prevent surface mount devices from making a solid connection. Too much solder paste may also result in bridge shorts, where devices make unwanted connections with other devices in close proximity. If it is too light you may have inferior connections that may not fail right away, but the life of the product can be dramatically shortened because of a weakened connection. So, it is important to use a manufacturer that understands the relationship between surface mount devices, pad size, solder paste application, and the required heat to properly reflow the solder paste, making a solid connection. A high-quality solder paste, properly applied will assure proper board function and increase the longevity of the board and its’ product.