This thread has rambled, yet raised many valid points as well as a few yet-unanswered questions. It also made me realize that even though soldering is like second nature to me, I hadn’t done anything to advance my knowledge of the science of the art since…..let’s just say it was SNLs first season and leave it at that….So, I spent some time sifting the bits, snagging enough objective data for a refresher for myself, as well a decent foundation for a beginner. The following material is copied text compiled from several non-retail, industry sources, so there’s no smoke or mirrors. I focused only on data applicable to electronics, so we’re not talking oven elements or 7KV lines (no one mentioned underground transmission lines yet) Oh yeah, the answer to one of the original questions is in there too.:
"Soldering filler materials are available in many different alloys for differing applications. Traditionally, the eutectic alloy of 63% tin and 37% lead (or 60/40, which is almost identical in performance to the eutectic) has been the alloy of choice for most copper-joining applications. A eutectic formulation has several advantages for soldering; chief among these is the coincidence of the liquidus and solidus temperatures, i.e. the absence of a plastic phase. This allows for quicker wetting out as the solder heats up, and quicker setup as the solder cools. A non-eutectic formulation must remain still as the temperature drops through the liquidus and solidus temperatures. Any differential movement during the plastic phase may result in cracks, giving an unreliable joint. Additionally, a eutectic formulation has the lowest possible melting point, which minimizes stress on components during the soldering process.For environmental reasons, 'no-lead' solders are becoming more widely used. Unfortunately most 'no-lead' solders are not eutectic formulations, making it more difficult to create reliable joints with them.
Though the base material is not melted in a soldering process, some of the base material's atoms do dissolve into the liquid solder. This dissolution process enhances the soldered joint's mechanical and electrical characteristics. Lead and tin form a simple eutectic system with a small solid solubility of lead in tin and a rather larger solubility of tin in lead. In consequence, a solder surface may display reactions of either metal with some tendency, not always quite sharp, for the properties of the metal present in the higher proportion to predominate. Both metals are attacked by acids and alkalis but the presence of lead, which forms many more insoluble compounds than does tin, creates further possibilities for the formation of protective layers in near-neutral aqueous media.
The Tin/Lead Process
The tin/lead process is what is currently being done for the majority of electronic assembly in the US and throughout North America and Europe. What happens when lead-free components are put in place in assembly of this current technology?
Fortunately for assemblers the news here is good. Studies indicate that there are no reliability or processing issues when lead-free plating and finishes are used on components. In fact, lead-free components finishes have been used in electronic assembly for many years. The addition of extra tin does not raise the reflow temperature of the alloy. While more tin may dilute the amount of tin/lead intermetallics present, the tin/lead intermetallics do not go away and so the reflow temperature of the resulting alloy does not change.
The Lead-free Process
When assemblers move to lead-free connection materials the same question arises concerning using components with tin-lead finishes. The news here isn’t so good. The addition of small amounts of lead to lead-free connections can have a dramatic affect on the alloy integrity. When lead is added to a lead-free system the lead reacts with the tin forming the same low temperature intermetallics that are present in tin/lead systems. Even small amounts of tin/lead intermetallics will lower the reflow temperature of the alloy. The solder alloy temperature is usually described in terms of liquidus and solidus. Liquidus is the temperature at which the alloy changes from a plastic state to a liquid. Solidus is the temperature at which the material changes from a plastic state to a solid. The addition of lead does not affect the liquidus but it does change the solidus of the resulting alloy. An example of how much the reflow temperature can be affected by the addition of as little as 1% lead is shown below:
1% lead will drop the solidus by 40-50°C. SnCu0.7; 227°C to 183°C
SnAg3.0Cu0.5; 220°C to 179°C
SnAg3.5; 221°C to 179°C
SnBi57; 138°C to 96°C
When mixing tin/lead and lead-free technologies it is important to note which technology you are trying to achieve. For a tin/lead technology adding lead-free components will not change the reflow temperature. For lead-free technology adding leaded components will lower the reflow temperature.
TIN
Tin (Sn) is a member of Group IV of the Periodic Table, along with carbon, silicon, germanium and lead. As a metal, the most important properties of tin are its low melting point, its non-toxicity, its resistance to corrosion, its' attractive appearance and the ability to readily form alloys with most metals to create useful materials.
Property
Atomic mass 118.69
Atomic number 50
Melting point 232°C
Boiling point 2625°C
Density 7.28g/cm3
Electrical resistivity at 20°C 12.6 μΩ cm
Young’s Modulus at 20°C 49.9GPa
SILVER
Silver is a very ductile and malleable (slightly harder than gold) univalent coinage metal with a brilliant white metallic luster that can take a high degree of polish. It has the highest electrical conductivity of all metals, even higher than copper, but its greater cost and tarnishability has prevented it from being widely used in place of copper for electrical purposes.
Phase solid
Physical properties
Density (near r.t.) 10.49 g·cm−3
Liquid density at m.p. 9.320 g·cm−3
Melting point 1234.93 K (961.78 °C, 1763.2 °F)
Boiling point 2435 K (2162 °C, 3924 °F)
Heat of fusion 11.28 kJ·mol−1 Heat of vaporization 258 kJ·mol−1
Heat capacity (25 °C) 25.350 J·mol−1·K−1
Applications
Q; Sn60 vs. Sn63; When is the use of one of these two alloys more appropriate than the other?
The Sn60Pb40 has a plastic range and puts down a slightly thicker coating of solder. Sn60 is often preferred for lead tinning and other solder coating applications. Sn63Pb37 is eutectic and as such has no plastic range. Generally it flows better than the Sn60 and is the preferred alloy for wave soldering and surface mount applications.
Q We are soldering to gold and we heard that we need a silver solder. Is this true?
This is a common misconception. You need a small amount of silver in your solder only if you are soldering to silver or silver plated components/leads. The small percent of silver in the solder prevents the silver on the leads from migrating into the solder resulting in a weak or brittle solder connection. The two most common situations are silver plating on component leads and silver palladium substrates. In both these cases the Sn62 alloy should be used. The high melting point of silver should also be considered when soldering temperature-sensitive electronic components.
When soldering to a gold plated surface the thickness of the gold is important. If the gold is thicker than 40-50 micro-inches, the solder most likely may not dissolve all the gold and bond to it. The solder will be dull looking and, if the gold content in the solder exceeds about 5%, the solder joint will be brittle. If the gold is thin, less than 20 micro-inches, it easily dissolves into the solder, making the solder joint look grainy. If the metal that was under the gold is not oxidized, the gold-contaminated solder will bond to it. However, as gold plates usually in a columnar structure, the gold should be at least 10 micro-inches thick to protect the base metal (in most cases nickel) from oxidation.
Q; What Does No Clean flux Mean?
Many fluxes fall into this category because the flux residues are not harmful to assemblies. It does not mean there will be no residues. All fluxes leave residues (the solids are the active portion of the flux that does all the work). Some flux residues are conductive or corrosive and must be removed. Other fluxes like the no-clean fluxes leave residues that do not need to be removed."
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Originally Posted by Thom
