Unsourced material may be challenged and removed. A majority of the defects in circuit-board assembly tyco connectors catalog pdf caused due to issues in the solder-paste printing process or due to defects in the solder paste. There are many different types of defects possible, i.
Insufficient amounts of paste result in incomplete circuits. BGA sphere and reflowed paste deposit. An electronics manufacturer needs experience with the printing process, specifically the paste characteristics, to avoid costly re-work on the assemblies. The paste’s physical characteristics, like viscosity and flux levels, need to be monitored periodically by performing in-house tests. SPI systems measure the volume of the solder pads before the components are applied and the solder melted. SPI systems can reduce the incidence of solder-related defects to statistically insignificant amounts.
Solder paste viewed under a microscope. Flux is added to act as a temporary adhesive, holding the components until the soldering process melts the solder and makes a stronger physical connection. The paste is a gray, putty-like material. The composition of the solder paste varies, depending upon its intended use. Depending upon the formulation of the paste, it may be very important to stir the paste before it is used, to ensure that the viscosity is appropriate for proper application. The size and shape of the metal particles in the solder paste determines how well the paste will “print”. Irregular particle sizes are not used, as they tend to clog the stencil, causing printing defects.
To produce a quality solder joint, it’s very important for the spheres of metal to be very regular in size and have a low level of oxidation. Solder pastes are classified based on the particle size by IPC standard J-STD 005. Loctite descriptions are given for comparison. There is a wide variety of cleaning agents for these fluxes.
No-clean pastes save not only cleaning costs, but also capital expenditures and floor space. However, these pastes need a very clean assembly environment and may need an inert reflow environment. The degree to which the material resists the tendency to flow. In this case, varying viscosities of solder paste are desired at different levels of shearing force. When the stress on the paste is removed, it regains it shape, preventing it from flowing on the circuit board.
The characteristic of a material’s tendency to spread after application. Theoretically, the paste’s sidewalls are perfectly straight after the paste is deposited on the circuit board, and it will remain like that until the part placement. If the paste has a high slump value, it might deviate from the expected behavior, as now the paste’s sidewalls are not perfectly straight. A paste’s slump should be minimized, as slump creates the risk of forming solder bridges between two adjacent lands, creating a short circuit. The amount of time solder paste can stay on a stencil without affecting its printing properties. The paste manufacturer provides this value.
A reasonably rapid cool-down period is required after this time. For a good soldered joint, the proper amount of solder paste must be used. Too much paste may result in a short-circuit, too little may result in poor electrical connection or physical strength. This is due to the presence of flux and other non-metallic agents in the paste, and the lower density of the metal particles when suspended in the paste as compared to the final, solid alloy. This will be less so with lead-free solders. It should be warmed to room temperature for use. Recently, new solder pastes have been introduced that remain stable at 26.