Brief Introduction about solder Paste in PCB Assembly
Solder paste is typically used in a stencil-printing process by a solder paste printer, in which paste is deposited over a stainless steel or polyester mask to create the desired pattern on a printed circuit board. The paste may be dispensed pneumatically, by pin transfer (where a grid of pins is dipped in solder paste and then applied to the board), or by jet printing (where the paste is ejected onto the pads through nozzles, like an inkjet printer).
As well as forming the solder joint itself, the paste carrier/flux must have sufficient tackiness to hold the components while the assembly passes through the various manufacturing processes, perhaps moved around the factory.
Printing is followed by a complete reflow soldering process.
The paste manufacturer will suggest a suitable reflow temperature profile to suit their individual paste. The main requirement is a gentle rise in temperature to prevent explosive expansion (which can cause “solder balling”), yet activate the flux. Thereafter, the solder melts. The time in this area is known as Time Above Liquidus. 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. Although solder paste typically contains around 90% metal in solids by weight, the volume of the soldered joint is only about half that of the solder paste applied. 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.
As with all fluxes used in electronics, residues left behind may be harmful to the circuit, and standards (e.g., J-std, JIS, IPC) exist to measure the safety of the residues left behind.
In most countries, “no-clean” solder pastes are the most common; in the United States, water-soluble pastes (which have compulsory cleaning requirements) are common.