Solder paste is a substance made of metal alloy and flux. The alloy part is a combination of tin and lead. However, scientific studies prove that excessive exposures to lead and products with lead components can cause harmful effects to humans and other living things. Due to the studies shown, legislative movements across the world urge the manufacturing sector of solder paste and other solder products to develop metal alloys that are free of lead traces. Nowadays, solder pastes are a combination of tin and silver, copper, bismuth or zinc.
Flux medium is also critical in manufacturing and maintaining a quality solder paste. It is composed of rosin, activator, thickening agent, and solvent. Each composition is essential in making the solder paste work according to its function. Flux medium adds color, determines shelf life, establishes viscosity, and activates the electrical and chemical reliability of the solder paste.
Effect of Changes in Solder Paste Composition
With the changes of metal alloy composition in solder pastes, PCB assembly houses have to alter the reflow or soldering processes. Most metal alloys that are lead free have higher melting point. Higher melting point exposes electrical components to longer heating stress during the soldering process. In conjunction with the changes in metal alloy composition, the manufacturing sector for electronics has to strengthen the insulation packaging of the parts to withstand increased heating point.
Aside from higher melting point, metal alloys with silver components have higher cost of manufacturing. As of date, tin and copper offers the least increase in manufacturing cost of solder paste alloys and has lower melting point compared to other lead-free alloy combinations.
Uses of Solder Paste
Solder paste is used during the soldering of the components in to the board. As a conveyor belt transports the printed circuit board to the reflow oven, solder paste acts as temporary adhesive for the electrical components attached. After soldering, it helps established electrical connections between the components and the copper traces of the board.
Solder Paste in PCB Assembly
It is the “squeezing” of solder paste into the board. It is also referred to as stencil printing, wherein a stencil or a template is attached to the machine. With the use of squeegee and pressure inside the machine, the solder paste is squeezed through the stencil. During this process, placing the right amount of solder paste is important. Too much paste covers up the electrical component. Too little may not established electrical connections within the board.
After paste printing, the printed circuit board is transferred to the reflow oven. The reflow oven heats the board until the paste softens, enough to bond the electronic components onto the board.
Solder Paste Proper Handling
Solder paste is important in PCB assembly. Thus, the assembly team must maintain quality to ensure print ability, solder ability, and work ability of the solder paste. The substance is best kept in airtight containers at low temperature but not below freezing point. Once opened, application of the solder paste is immediate to maximize functionality. The longer the paste is exposed the lower is the rate of excellent performance due to oxidation. Aside from this, solder paste has self-life and should be used on or before expiration date.
Whether the PCB assembly is for prototypes or mass production, solder paste is very useful. With care and proper handling, its quality and intended purpose can be maximized. In addition, the right amount of the substance should be applied during the paste printing. Other factors that affect solder paste quality performance are manufacturing quality of the paste, equipment functionality, and stenciling process. With all aspects functioning well, the overall outcome is a quality printed circuit board.