Issues with Solder Alloys, Solder Pastes and Lead-free Solders
1. Solder Alloy
Considerations on selecting solder alloy should be based on the following criteria : 1) The melting range of solder alloy, considering application temperature; 2) The mechanic properties of solder alloy, considering to application conditions; 3) Metallurgic compatibility, considering leaching phenomenon and possible intermetallic compound; 4) Compatibility with application environment;
5) Wetting performance on specific substrate;
6) Eutectic or non-eutectic.
Melting points of common solder metal elements :
Eutectic Alloy is a mixture of different metal ingredients with a specific compound ratio, so that the alloy has a single chemical composition that solidifies / liquefies at a single temperature, which is lower that the solidifying / liquefying temperature of any other kinds of composition made up with the same ingredients.
Melting range of common solder alloy :
For example, 63Sn/37Pb solder alloy is a Eutectic composition with a single melting temperature of 183℃. There are also other Sn/Pb solder with different mixture ratio, while they are not eutectic. Non-eutectic mixtures will display solidification of one component of the mixture before the other. 60Sn/40Pb solder alloy has a melting range of 183s-188l (s: completely solidus/freeze temperature; l: completely liquidus /melt temperature. In this temperature range, the metal particles in the alloy display to be partly solidus and partly liquidus). 
2. SMT Solder Pastes
To reach best printing quality, we need not only the right solder paste characteristics (viscosity, metals combination ratio, powder size, flux activity), but also the right equipments and tools (printer, stencil, squeegee), as well as the right control process (reflow profile, locating, cleaning).
2.1. Components of Solder Pastes
Solder paste is a sticky mixture of Solder Powders and Flux. In SMT reflow process, solder paste is used to temporarily attach surface mount electronic components to their pads on the PWB. When the reflow process begins, according to specified Reflow temperature profile, the solder inside of the paste is melt and so that the components are soldered on the PWB.
The alloy material should be carefully selected according to the materials of PWB substrate and SMT components.
There are several different types of solder paste: from type 1 to type 6, categorized by the size of powder particle. Type 1 has largest particle size, while type 6 has the smallest, corresponding to the application for different components pitches. One important requirement on solder powders is that the size of powder particles in the solder paste must be well-distributed. For instance, type 3 solder paste (45-25mm) is normally required that at least 60% powder particles should around 35mm . Additionally, the shape of the powder is required to be spherical or subsphaeroidal.
Three kinds of flux are Rosin Based , Water Soluble, and No-clean.
Rosin based pastes are made of rosin, a natural extract from pine trees. These fluxes need to be cleaned after the soldering process using a solvent. Water soluble fluxes are made up of organic materials and glycol bases. There is a wide variety of cleaning agents for these fluxes. A no-clean flux is made with resins and various levels of solid residues. No-clean pastes save not only cleaning costs, but also capital expenditures and floor space. The backward is no-clean pastes require a very clean assembly environment and may need an inert re-flow environment.
Generally speaking, there are four main elements in a flux: Activators, Thixotropic, Solvents, and Resins.
In the reflow section, the Activator in the flux removes oxidation on pads of PWB and leads of electronic components, and at the same time reduces surface tension at the juncture of the metals to accomplish metallurgical bonding, allowing the...
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