High power SMD (surface mounted device) LEDs are normally soldered to a printed circuit board that has a metal core to help conduct the heat generated away. SMD devices are usually small, sometimes very small. Soldering them with a soldering iron is to say the least challenging as unlike components installed with pins going through holes in the board they just sit on its surface.

The use of electronic devices in “trough-hole” packages is decreasing rapidly, so availability is limited. I recently bought a mini-hot plate and the photographs below shows how it works. Alternative approaches are re-flow ovens and hot air guns. The hot-plate I show here (MHP30 from Miniware, Guangzhou City, China) is specially handy when working with small individual boards and not too many of them. The size of the ceramic plate is 30 mm \(\times\) 30 mm, and can reach 300 C.

Mini hot plate, circuit board with an SMD LED soldered onto it.

When using a soldering iron one adds the solder using a lead/tin “wire” that has a core of flux, when using a hot plate for re-flow soldering one uses a paste with the flux as matrix carrying small beads of lead/tin or other alloy. As temperature increases gradually the flux melts first, cleaning the metal surface and acting as a wetting agent for the metal that melts at a higher temperature. The solder paste can be applied with a stencil, but in this example with a “large” LED \(4 \times 4\) mm device I applied it directly with a syringe and a very small tip. I could have used slightly less solder paste, but not much less as the metal pads on the board are larger. Through this series of images the temperature of the board raises from ambient to nearly 250 C.

The LED sitting on the solder paste (cold)

The solder paste spreads as the flux becomes more fluid but the metal beads are still solid.

The first solder beads in the paste start to melt while the flux remains semi-liquid.

The metal melts and surface tension changes

Surface tension pulls the molten metal to the contact pads

Surface tension pulls the LED into place, the surface has a mask that “rejects” the molten metal as it separates from the flux. It only remains for the board to cool down.