How many times have you stopped and looked at a PCB (Printed Circuit Board)? Most are covered in many small components, and this makes it quite intimidating to work with them – especially given the fact that there are so many different kinds, with each offering distinct functions. However, if you are working on a custom project you may need to try your hand at surface mount soldering, but before we get into the juicy details of how to take advantage of surface mount soldering we first need to understand what surface mount soldering is.
With so many terms being thrown around here is a breakdown of terms used in this post:
SMD – Surface Mount Device
THD – Through-Hole Device
PCB – Printed Circuit Board
Surface Mount Devices differ from Through-Hole Devices in a few key areas. Surface mount devices are often noticeably smaller than through-hole, are mounted onto the surface of the PCB, and allow for components to be mounted to both sides of boards. This reduces the need to drill holes through PCBs, and of course, means that they can be made smaller, denser, and with a much more efficient form factor too. This doesn’t mean that Through-Hole Devices are redundant though, and you will often find a combination of SMD and THD on a PCB. So now that we have a better understanding of surface mount devices, it’s time to get started with the actual surface mount soldering.
Surface mount soldering is the process of soldering an SMD onto a PCB through a variety of methods. The soldering method can vary with the user and with different types of SMD with some methods being more common than others. In this blog, we will be looking at the well-known drag and dab methods used by electronic technicians.
Recommended tools and equipment:
– Digital or analog soldering iron (50W or higher recommended)
– Old/Practice PCB
– Variety of SMD components
– 0.46MM Solder (Rosin Core)
– Desoldering braid/wick
– Flux w/ applicator
– Isopropyl alcohol (90% or above is recommended)
– Magnifier or microscope
– PCB stand/clamp/vice
Important note: It is important to remember that the soldering iron will get hot so please don’t touch the tip when heated. Remember to keep the soldering tip clean with a wet sponge or solder cleaning wire for best result. Due to the heat of the soldering iron it is recommended that a soldering mat is used to help prevent damage to work surfaces, fire and possibly injury. Practice makes perfect so we strongly recommend you use old or practice PCB and old SMD so that you do not waste money or kill components that still deserve a good lifecycle.
- Prepare the soldering iron and PCB:
Heat the soldering iron to approximately 330oC. If you have a temperature you are comfortable with you can also try that. Remember to not set the temperature too high as this might damage the SMD, the PCB or the soldering iron. Attach the PCB to the stand or clamp and take this opportunity to clean it with the isopropyl alcohol. Let the PCB stand for a bit to ensure it is dry.
- Apply flux onto the pins on the PCB:
Using the flux applicator apply flux to the PCB pins in the area you intend to mount the SMD. In this step, it’s better to put a bit too much rather than too little since flux is inexpensive compared to some components, and your main goal is really just to ensure that your solder joints are clean and operating like they should. Flux can greatly improve your soldering as it reduces oxidation, improves the electrical contact between pins, and also helps ensure even solder flow. Remember that some flux may leave a residue though, but that can be cleaned off with isopropyl alcohol when finished (Step 7).
- Tack down one corner:
To avoid having the SMD move while trying to solder we recommend you tack a corner pin to the PCB. To do this you first need to apply some solder to the chosen solder pad on the PCB. Using your tweezers hold the SMD in position and place the soldering iron tip on the tack (solder you applied to the solder pad). Once the SMD is in place with the pin in the solder, remove the soldering iron, let the solder joint cool and harden, and then repeat this process for the other corners if necessary. Note: When working with IC (Integrated Circuits) components, such as an ATMEGA MCU chip, it is important to ensure you have the correct orientation between IC and PCB.
- Apply some solder to the soldering tip:
Now that the SMD is secured to the PCB it is time to solder the rest of the pins using the drag or dab methods. For these methods we recommend trying the following, beveled, hoof, or chisel tip for best results although other tips may work just as well. Once you have your chosen soldering tip you need to add some solder to it, but be careful not to put excessive amounts though, since the components you’re working with typically have very tiny pins. As you practice you will learn the ideal amount of solder although it is easier to add extra compared to removing the excess with solder braid.
- Drag method (Beveled or hoof tip):
For the drag method take the soldering iron and place it on a corner pin of the SMD, and then move the soldering iron along the pins in a single smooth movement and at a consistent rate. It’s important to avoid moving too fast or too slow and do not put too much pressure on the pins while moving as this may cause damage. Try to avoid multiple movements as this can cause bridging and messy soldering, which of course can leads to shorts between the pins or problems relating to bad solder joints later.
Dab method (Chisel tip):
For dab method take the soldering iron and place the tip against the pins of the SMD, then move the tip away from the SMD in a smooth movement without applying too much pressure, try not to pull away too far otherwise you might get solder on other solder pads or components. This method is slower but can allow for more accurate soldering and neater end results.
Note: These methods are very similar and some electronic technicians may not differentiate them, and may instead refer to both methods simply as drag soldering. We have just made sure to split it into 2 distinct methods so that the information and instructions are easier to understand for makers who are newer to surface mount soldering.
- Check the pins:
Using the magnifier or microscope check the pins for any bridges in the solder and any other errors that may have occurred. If an issue is found, its time to bring out the desoldering wick. Place the desoldering wick against the problematic section and press the soldering iron tip onto the wick, the solder should be absorbed directly by the wick. If the problem is small you can also try just reheating the joint using the soldering iron tip, in which case the solder may flow back onto the pads or pins with minimal extra effort.
- Clean up:
When you have finished soldering the surface mounts it time to clean the PCB of any flux residue. Apply some isopropyl alcohol to an anti-static cloth and clean up any flux residue you find. You can also use an electronic component anti-static brush along with the alcohol to clean the PCB.
Note: Not all SMD can be soldered with drag or dab method. These components need to be soldered with more traditional methods so we recommend using the method you are comfortable with.
We know that surface mount soldering can be a daunting idea for new makers. The idea of working with such tiny components can often put off makers of all ages. We know that there are many methods for surface mount soldering not mentioned above. However, for makers looking to try their hand at surface mount soldering we hope this post has helped. We are always eager to learn and would love to hear what soldering methods you use. If you found this post helpful please consider sharing it on social media to spread the knowledge. If you want to keep up to date with the DIYElectronics brand check out our social media, Facebook, Instagram, Twitter, and website.
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