Brompton frames are made with a mixture of fillet brazed joints and brazed lugs. In terms of strength, fillet brazing and TIG both produce a joint that is stronger than the parent tubes, when done correctly. There are arguments made that brazing uses less heat and thus causes less damage to the tubes. TIG uses more heat, but that heat is concentrated in a smaller area and generally results in a smaller heat affected zone. I'm not a metallurgist, so I don't know exactly what happens to the metal. I have built frames using both methods and none have failed yet, even the early ones where I know some of the joints were overheated.(those all belong to me)