Solder paste printing process BASIC:
Basic Operation of dispenser V-0.
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Solder paste is typically used in a screen-printing process, 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 sprayed on 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 pre-heating and reflow (melting).
The paste manufacturer will suggest a suitable reflow temperature profile to suit their individual paste; however, one can expend too much energy on this. The main requirement is a gentle rise in temperature to prevent explosive expansion (“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.
A good tin/lead solder joint will be shiny and relatively concave. This will be less so with lead-free solders.
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.
Solder paste should be stored in an airtight container at low, but above freezing, temperatures. It should be warmed to room temperature for use. Exposure of the solder particles, in their raw powder form, to air causes them to oxidize, so exposure should be minimized.