Sauter GmbH, Technology Center for mold- and toolmaking, from Swabia (Germany), recently delivered the 250th hot bar. The family-owned company is one of the leading manufacturers of thermodes worldwide due to the capability of forming very difficult to machine materials into soldering brackets with any desired geometry. Initially, the blanks are roughly formed into the desired shape by milling at low feed rates and then precisely shaped by spark erosion. Normally tolerances are below 15 µm.
Pulsed-heated hot-bar reflow soldering is a quite robust process, requires little space, is energy efficient and has a long service life. It is therefore particularly suited for parts that are manufactured industrially in large quantities. However, the quality of the soldered joints depends directly on the precise geometric shape of the thermode. Standard accuracy requirements in production are 0.5 ± 0.01 for the lateral legs, surface Ra 0.2, in order to adjust the immersion depth with micrometer accuracy during the process. Otherwise, cable insulations, for example, can burn, semiconductor dies can be mechanically damaged, or the solder joint can fail prematurely.
Thermodes generally consist of low-carbon alloys which do not deform at usual soldering temperatures of around 300°C and can therefore – in addition to the heat – also transfer the necessary pressing forces (e.g. 60 N on only a few mm²). The material is highly resistant to corrosion and wear; however, the latter is a decisive disadvantage for mechanical processing: it is tough and at the same time quite abrasive. The chip does not absorb the heat generated during milling. This leads to friction problems on the tool blades. Even at very low feed rates it is hardly possible to give the workpiece a highly precise, repeatable shape.
Therefore, Sauter produces only the blanks by CNC machining. To reach the high precision required for thermode soldering, the blank is then spark eroded. The material properties result in comparatively short erosion times, which require highly focused work to ensure that the tight tolerances, often below 15 µm, are met.
Thermode soldering, or pulsed-heated hot-bar reflow soldering, is a reflow processes in which a solder paste is applied first. Later in the process, solder is melted to create a permanent electrical connection. A thermode is a stamp or clip adapted to the shape of the components to be soldered (hence the term clip soldering). This is heated and then pressed onto the soldering point until the solder melts. The heating is now switched off; the stamp remains on the soldering point until the solder has solidified. Only then is the stamp lifted off.
Springy parts can also be reliably soldered by thermodes. The process is therefore well suited for soft-soldering of series-production parts that do not involve a PCB, for example when attaching cables to battery clips.