A soldering iron is just a heat source. Any iron that produces enough heat to meet the thermal requirements of the components plus solder will work perfectly well. Just be sure it is electrically grounded to protect against ESD. Spending more than around $100 for a soldering iron is a waste of money. And, yes, I know the advertising claims that the extra money buys better performance, but it’s wrong. As I demonstrate in Science of Soldering, the temperature of the iron is less important in heat control than how the iron and solder are used together.
I began my career developing wave soldering processes. It was a simpler time, before bottom side surface mount components up. Solder mask covers exposed metal which you don’t want to solder. By reducing the metal area, the tendency of bridging is reduced. Selective soldering originally referred to the use of pallets to cover the surface mount components during wave soldering but more often today refers to robotic assists that apply solder to just a small area of the PCB assembly. The pallet approach changes the fluid dynamics of the wave solder flow producing conditions that range from sdr skips (shadowing by the pallet) to bridges. Some selective soldering machines do a nice job soldering rows on I.C. leads, for example. Success always depends on understanding fluid dynamics and the role of flux in preventing bridging.
Once upon a time, most of the parts we “soldered” were tin-plated. And life was easy. The tin melts close to the melting temperature of solder, so the melted plating and liquid solder simply flow together. It’s important to realize that this is not truly soldering (application of solder to metal surfaces that don’t melt). Because tin can sprout hairs (“tin whiskers”) capable of causing shorts, tin plating has largely disappeared on smaller parts, especially multi-leaded surface mount packages.
The obvious difference between surface mount and through-hole parts is the lack of anything to hold the surface mount parts in place. The same wetting forces apply to all parts but surface mount parts are easier in the sense that there is no plated hole to fill. High efficiency placement equipment, solder paste and advanced stencil technology (screen printing) or jet printing have made high volume surface mount soldering very easy.
Interesting question that actually has three parts. As far as the solder alloy is concerned, flux is irrelevant; any flux that can deoxidize the part will meet the needs of any alloy. (Different alloys have different wetting properties but they are independent of the flux.) A properly manufactured ENIG plated board is easy to solder with very mild flux. (RMA is the old ratings system, ROL0 in the newer J-STD-004 system). The gold dissolves into the solder and the solder bond is made with the underlying nickel. If the gold covers oxidized or contaminated nickel, however, the solder will not wet (often called “black pad”) and there is no cure. With respect to the THT, the tin plating will melt and expose the underlying metal. If the tin plating was applied over clean, oxide-free nickel, wetting will occur. If the nickel is oxidized or contaminated, dewetting forces will take over and there will be defects. The defects are a parts issue, not a reflection on the flux. Again, very mild fluxes will work nicely with a properly plated tinned part but no flux, regardless of strength, will result in proper wetting if the underlying metal is oxidized or contaminated.
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I left out the most important feature of tin plated parts. The plating must be applid to oxide-free, clean base metal. If the base metal is oxidized or contaminated, the melted tin surface will expose the unsolderable underlying metal and cause dewetting (solder being pushed away from the surface). Very important. This was the primary soldering issue in the pre-RoHS days.
Indium is an interesting metal. It has a low melting temperature (roughly 350°F), so is an attractive solder material for very heat-sensitive components. On the other hand, it lacks tensile and ductile strength, so the resulting joints can be rather fragile. Other metals are generally mixed with indium to produce a more robust joint. One aspect of your question, however, appears to be poor wetting (flow over component surfaces). That’s due to the attractive force between indium and other metals; it has weak interatomic attraction, so doesn’t wet as well as more active solder metals. That also contributes to your hand soldering into PTHs. SAC305 does have stronger wetting forces because tin is a very active metal. You will never get the same wetting with indium as with SAC305… Your question is a bit puzzling, however, because you seem to be conflating hand soldering and wave soldering.
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That’s helpful, thank you Jim!
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It was fun. Thank you.
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Thanks, Jim!
Hi, Steve.
Thanks for the support today. I am unhappy with my response to your question but suspect it was designed to elicit what I did write. Everyone thinks soldering is low tech and simple. In many respects, it was. But it’s not anymore.
If you ever need real answers to specific soldering questions, you know where to find me. I would be happy to help. Sierra Circuits is part of my family.
Best wishes,
Jim
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Thanks Jim.
Thank you for your response, Jim 
Thanks a lot, Jim
Thank You, Jim!
Thank you, Jim!
Honest is good. Thank you Jim.
Thank you Jim
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