Here are a few papers on aluminum soldering, written by our aluminum soldering expert:
Direct Aluminum Solder Paste (IMAP Thermal Management Workshop – Palo Alto, CA, 2011)
Soldering Aluminum (SME International Brazing & Soldering Conference – Orlando, FL, 2009)

Superior AL26-33-75 direct aluminum solder paste combines aluminum flux and tin-silver solder powder to create an all-in-one aluminum soldering solution, ideal for applications where precision dispensing is required This water-soluble aluminum solder paste was designed for aluminum to copper soldering. The aluminum solder paste creates a true inter-metallic bond between the solder and aluminum substrate.  Superior AL26-33-75 solder paste residues are water soluble and must be washed off after soldering.

Superior AL26-193-75 is blended with a proprietary low-melting solder alloy that is specifically formulated for the direct soldering of heat sinks. It solders at temperatures under 200°C, thus protecting copper piping built into the heat sink. Most importantly, Superior AL26-193-75 allows for direct Aluminum-to-Aluminum or Aluminum-to-Copper soldering in heat sink fabrication, thus negating the need for plating in heat sink applications. Eliminating plating not only translates into greatly reduced costs, but eliminates the associated hazardous chemicals from the work environment.  Superior AL26-193-75 post-solder flux residues are water-soluble and should be cleaned post soldering.

Superior AL28-33-75 is our strongest Aluminum Solder Paste.  This solder paste is effective in soldering 1000-series and 3000-series Aluminum, but can also solder 6000-series Aluminum when soldering the Aluminum to alloys such as steel, stainless steel, copper, and brass.  To activate the flux in this solder paste, it is important to bring the temperature of the surface of the Aluminum up to at least 300°C.  Superior AL28-33-75 solder paste residues are water soluble and must be washed off after soldering.

Superior AL26-33-25 tinning paste uses a combination of aluminum soldering flux, binder and a Tin-Silver solder to tackle applications using harder to solder aluminum alloys. This direct aluminum tinning paste eliminates the need for costly plating techniques. Superior AL26-33-25 can also be used when soldering thick aluminum surfaces to smaller parts. After it is applied, the larger part is rapidly heated, and the surface cleaned, the smaller or more delicate part can be easily soldered on.  Superior AL26-33-25 tinning paste residues are water soluble and must be washed off after use.