By: Jerry Lui (Manufacturing Engineer)
The reason for this post is to create a guide on how to solder SMD components onto your fabricated board.
By the last quarter of the semester, custom PCB’s should have been fabricated and all components on hand. Our PCB is shown above with a total dimension of 1’’x1.425’’. The majority of the components are SMD with the exception of the 2 through hole dielectric capacitors, piezo speaker, and phoenix pins.
First the PCB should be cleaned thoroughly with at least 70% rubbing alcohol (preferably 90%+) by soaking either Kim-wipes or cotton swabs.
Next flux should be applied to the contacts of the board. Generally use a no-clean water based flux for ease of cleaning but a typical rosin based flux will suffice. These come in small containers to syringes to pen applicators.
Now that the flux has been applied each contact pad has to have solder paste or normal solder.
A stencil can be used but it needs to be fabricated so the next step will be hand application
DO NOT put too much solder paste. There only needs to be enough to thinly cover the pads. For the multi-pin packages like opamps, a thin line of solder paste can be applied perpendicularly to the pads instead applying it to each pad individually. Once all the pads have been covered in solder paste it’s time to add the components.
Here I use tweezers to place the SMD components onto the prepared board. To help with placing the SMD properly, grip the components from the top or use a angled tweezer. The components don’t need to be pressed all the way down, give them a light press and the capillary effect of the solder will automatically align them.
To have a proper solder joint the board needs to be warmed up properly. An increase of will be fine. If a reflow oven isn’t being used a hot air station is adequate as well.
Set the temperature to around 310C (depends on the flux paste used) with the air setting to 1~3 and hold the hot air reflow gun 3~5 inches over the board while rotating the gun for a few seconds. The point of this is to heat up the board (especially the ground plane) and to allow the components/leads to come up to the proper temperature. Once the board is warmed up (the paste will also start to become more fluid) hold the reflow gun 1~2 inches over the board while wiggling the gun. The solder will start to melt and align the components to the pads.
Once the board is completely soldered, clean it again with rubbing alcohol and kim wipes or cotton swabs.
Soldering surface mount components is very straightforward and isn’t as hard as many people believe it to be, especially with a hot air gun or reflow oven. As long as you have a steady hand (for hand soldering) and patience (hot air gun) you’ll be able to solder SMD components with ease.
EEvlog has great tutorials on how to re-flow: