Smt Reflow Soldering
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]:
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 [1]:
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 [1]:
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).
[1]
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,
1. Solder Alloy
Considerations on selecting solder alloy should be based on the following criteria [1]:
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 [1]:
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 [1]:
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).
[1]
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,
References: 1. Harper, Charles, A. Electronics Packaging and Interconnection Handbook 2. Indium Corporation 3. Wikipedia 4. Directive on the restriction of the use of certain hazardous substances (RoHS) in electrical and electronic equipment 2002/95/EC 5. Siewert, Liu, Database for Solder Properties with Emphasis on New Lead-free Solders. 6. SJ/T 11186-2009 General specification for solder paste, China Electronic Industry Standards