Selective soldering

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Selective soldering is the process of selectively soldering components to printed circuit boards, molded modules, etc. that could be damaged by the heat of a reflow oven in a traditional SMT assembly process. Consequently this process usually follows an SMT oven reflow process, but not always. Therefore parts to be selectively soldered are most often surrounded by parts that have been previously soldered in a surface mount reflow process, and the selective solder process must be sufficiently precise in order to avoid these components so as not to damage them.

[edit] Process

There is a number of assembly processes used to perform selective soldering; these include:

1. Selective aperture tooling over wave solder. These tools effectively mask off the areas previously soldered in the SMT reflow soldering process while exposing only those areas to be selectively soldered in the apertures of the tool. The tool/PCB assembly is then passed over traditional wave soldering equipment to complete the process. Each tool is specific to a PCB assembly.
2. Mass-Selective-Dip-Solder fountain, is the analog of selective aperture, whereby specialized tooling with apertures to allow solder to be pumped through it represent the areas to be soldered. The PCB is then presented over the selective solder fountain and all selective soldering of the printed circuit board are soldered simultaneously as the PCB is lowered into the solder fountain. Each tool is specific to a PCB assembly.
3. Miniature wave selective solder fountain(s) typically use a miniature pumped solder wave with a spherical shape similar to the end of a pencil or Crayola to sequentially solder the PCB. This process is somewhat slower than the two previous methods, but much more accurate. The PCB may be fixed and the wave/wave solder pot moved underneath the PCB In 3-axis, or alternately the PCB may be articulated over a fixed wave/solder bath to perform the selective solder process. This process is generally regarded to be tool-less unlike to the first two examples.
4. Laser Selective Soldering System are the newest systems and are able to import CAD- based board layouts and use that data to position a lasar to directly solder any point on the board. The major benefits are thermal stress is eliminated, non-contact soldering process, consistent high quality solder joints, and maximum flexibility. Solder time can typically average 1 sec per solder joint. It may even be possible to eliminate stencils and solder masks from the circuit board to reduce manufacturing costs.
5. Other less common selective solder processes include
   • hot-iron solder with wire-solder feed
   • induction solder with paste-solder, solder-laden pads, or pre-forms and, hot-gas including hydrogen with various methods of presenting the solder as above.

There are other selective solder applications that can be considered, some of which are non-electronic such as lead-frame attachment to ceramic substrates, coil-lead attachment, SMT attachment such as LED's to PCB's, fire sprinklers where the fuse is low temperature solder alloys and many more.