Durable Soldering Joints in Sheet Metal Joseph D. Rogers, P.E., Keith B. Nelson and David S. Slick, P.E., CFM
In the February 2021 issue of Florida Roofing, we discussed material selection and quality control measures to improve the quality of soldered architectural sheet metal joints and minimize vulnerability to failure due to thermal movement and water infiltration. In this issue, we will discuss the procedure and quality control measures for joint preparation and soldering techniques that must be implemented to avoid soldered architectural sheet metal joints that are vulnerable to failure. Construction of durable and watertight soldered sheet metal joints is difficult to achieve in a field environment where dust, dirt, debris and other contaminants can limit the effectiveness and quality of the soldered joint. Although proper material and equipment selection is crucial in preparation for soldering architectural sheet metal, detailed forming, cleaning and soldering techniques are also necessary to ensure durable and watertight soldered joints.
Base Metal Forming
Proper forming techniques begin with cutting and folding of the base metals. If the sheet metal components do not have straight and crisp cuts and folds, the joints between the sections of metal cannot be “dressed” tight to one another, which may inhibit flow of the solder into the joint. Where feasible, perform the majority of any sheet metal cutting and forming operation on a shear and bending brake, respectively. Limit hand shaping, trimming and seaming in the field as much as practicable and ensure that any field work is performed with proper sheet metal hand tools. Form flat lock seams with 1/2 to 3/4-inch legs on the seams. Initially, leave bends in base material under-bent (partially open) to permit proper cleaning and pretinning of joint interior surfaces. Pre-drill any joints to be riveted prior to cleaning the sheet metal to avoid metal shavings remaining within the joint that could inhibit uniform solder flow.
Cleaning
Clean the sheet metal base material immediately prior to pre-tinning and joining sections. Wipe all surfaces to receive solder with a clean, dry rag and use a wire brush, wire wool or emery cloth to mechanically remove embedded contaminants and oxidation from the base material. Apply flux to all desired surfaces of the joint prior to pre-tinning, applying flux only where solder will be placed. Properly cleaned and fluxed metal surfaces will be bright and shiny in comparison to the surrounding surfaces that have not been cleaned (photo 1). 32
FLORIDA ROOFING | March 2021
Photo 1 – properly prepared flat-seam joint in soldering prequalification testing sample of zinc-tin coated copper.
The soldering iron must also be cleaned immediately prior to commencement of soldering. Once the soldering iron(s) are heated, use a wire brush and file to remove oxidation and previous solder remnants from the tip (bit). Rub all surfaces of the soldering iron tip on a block of Sal Ammoniac (ammonium chloride) and dip the tip of the iron in a container with a water and flux mixture (3:1 ratio) to complete the soldering iron cleaning procedure. Repeat this procedure as required during the soldering process to maintain a clean soldering iron tip.
Pre-Tinning and Heat Control
The soldering iron and base sheet metal should be pre-tinned prior to soldering to facilitate solder flow and promote bond of the solder to the desired surfaces. To pre-tin the soldering iron, melt a small amount of solder on the block of ammonium chloride and rub the soldering iron tip surfaces over the solder so that a thin layer of solder remains on all parts of the tip to be used. Avoid pre-tinning the downward facing horizontal surfaces of an iron when used to solder vertical joints in place, to prevent solder from flowing readily down and off of the bottom of the iron. Once the iron is pre-tinned, heat the sheet metal individual sections at the respective common joint surfaces, applying the same type of solder that was used to pre-tin the iron near the soldering iron tip, forming a small puddle of liquid solder on the sheet metal. Drag the tip of the iron in one direction along the metal, adding solder as necessary, so that a thin layer of solder remains on the desired surfaces of the sheet metal sections that will form the joint. Apply only as much heat to the base metal as necessary to melt the solder. If too much heat is applied to the base metal, the metal may warp or oxidize, which will prevent bonding of solder to the base metal and will require additional mechanical cleaning and fluxing to remove the oxidation prior to re-soldering.