Laser reflow soldering

Freedman, G, Atmospheric Pressure Gaseous Flux-Assisted Laser Reflow Soldering, US. Patent No. 5,227,604, July 1993. 

Solder Sources and Defects Associated with Laser Reflow... 

Solder requirements are the same as for any other process. There are no ahoy composition requirements specific to laser soldering, as this soldering method is compatible with leaded or lead-free solder alloys. Even high-temperature alloys can be soldered by this technique. When single-point laser reflow is apphed, the board quality and integrity are not compromised if parameters are chosen and adequately controlled. 

In a similar process, known as polymer-film interconnect (PFI), an insulative thermoplastic film is laminated over the devices at the wafer stage, and vias are opened over the bonding pads using a laser. At that point, either the normal solder bumps can be formed or a silver-filled conductive adhesive can be stencil printed into the vias to form polymer bumps. After printing, the epoxy is B-staged and the flip-chip devices are diced. In assembly, the devices are heated to a temperature that completes the cure of the B-staged bumps and simultaneously reflows the thermoplastic underfill material. 

The presence of the hot liquid flux during soldering helps to promote thermal uniformity on the board.This is very important in mass reflow and may play an even more crucial role in some solder-in-place processes such as hot-bar laser. In some forms of laser soldering, the flow of hot flux may help to preheat leads and pads, readying them for solder joint formation. In the case of hot-bar soldering, a pool of liquid flux may help to even out thermal disparities over the length of the heated bar. 

This method requires a laser beam that is smaller in diameter than the length or width of the component lead or bonding pad. The beam is stepped to each lead-pad combination and delivered with sufficient energy to result in solder reflow. The beam can be CW, pulsed, or... 

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