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Soldering to aluminum is the opposite of soldering to gold; both difficult and simple to solder to do. Whereas soldering to gold can use almost any flux and almost any solder (depending on the thickness of the gold), soldering to aluminum has a much narrower selection for both the flux and the solder types. I don’t know if this makes it harder, but having a limited selection sure sounds easier in my book.
There is a very specific type of flux that is needed when soldering to aluminum. Said flux must contain Zinc Chloride, which is the specific activator that effectively etches the aluminum and allows the formation of the intermetallics needed for a strong solder joint.
Also the opposite to gold soldering, aluminum soldering works best with a high Tin content alloy, such as Indalloy 121 (Ind121; 96.5Sn 3.5Ag, 221°C eutectic) or any of the “SAC alloys” (Tin-Silver-Copper or SnAgCu) like Ind256 (96.5Sn 3.0Ag 0.5Cu, 220°C liquidus).
Just as soldering to nickel, soldering to aluminum with a high Tin content alloy takes time to form a good Tin-Aluminum intermetallic. The longer the Time Above Liquidus (TAL) and higher peak temperature, the stronger the Tin-Aluminum intermetallic should be, hence the stronger the solder joint. A quick cool down also aids the solder joint strength by having a tighter grain structure in the final solidified joint.
Of course, aluminum soldering is not limited to electronics, and the procedure for soldering to aluminum may be used for other application as well. In fact, we normally do not recommend the use of Zinc-Chloride containing fluxes in electronics soldering, because they are very corrosive the residues are difficult to control on the final assembly.
More information may be found at our Online Help: Indium Knowledge Base or Soldering to Aluminum Application Note.