How to: Reflow Surface Mount (SMD) Soldering Tutorial

As devices get smaller and smaller, surface mount devices (SMD) are becoming more popular.  As an alternative to traditional, larger through hole components, SMD components save space on PCBs.  One glance at the tiny size of the components can be intimidating at first but the reflow soldering method makes it simple.  Reflow soldering is not only quick but also inexpensive, especially for batch jobs.

New to soldering? No problem, check out basic soldering tutorial and surface mount soldering guides.

Reflow Soldering Materials:

• An old toaster oven
• Putty knife
• Rubbing alcohol
• Tweezers
• Solder paste
• Kapton Film stencil to apply solder paste over PCB pads.

How to Solder Surface Mount Components:

1. The project starts with a Kapton stencil, a PCB holder and the PCB to be soldered. The PCB is placed on the holder; the stencil goes on top, fixed with scotch tape. The holder is really useful as it keeps the PCB in place for the stencil to do its job.

   

2. Solder paste is applied on the stencil with a putty knife. Using the putty knife, excess solder pastes is scraped off from the stencil template.  The larger components like 1206 resistor/capacitors may require thicker solder paste while a thinner layer is good for the smaller parts. Excess paste, if stored properly in an airtight container can be reused again.

   

3. Mounting the PCB with the components is straightforward with a pair of tweezers and a steady hand.
4. A very important component called the temperature marker is a useful addition on the board for the reflow process. The marker is a crayon which melts at the precise temperature of reflow, thus making it a visual indicator to stop the reflow heating.
   
5. The board is now ready for the toast. As the author shows, place boards on a shelf so that temperature marker is visible through the oven door, slide the shelf in and set toaster to ‘Bake’ at a point less than 400F for 5 minutes. Gradually move up to 500F and watch the temperature marker for the first signs of melt down.
   
6. The reflow process stops at the exact point the temperature marker liquefies and becomes transparent. Allow the board to cool, take it out and remove the remains of the marker with alcohol.