While most SMT soldering in industry is done using solder paste stencils,
should the need arise there are ways in which these parts can be soldered with a soldering iron. Knowing how to solder SMT’s by hand is a valuable skill if your project requires it or in the case that you need to rework or repair a machine.
This guide aims to teach soldering techniques for flat chips, QFP's, and SOIC's
Now, before we get started, here’s what you will need for this guide:
- Soldering Iron with chisel tip
- Wire Solder
- Solder Flux
- Isopropyl Alcohol and tissues for cleaning
- Acid Brush
- Printed Circuit Board (PCB)
- Solder Wick
- SMT components
If you're looking for a kit to help you with learning some of these concepts, consider looking into the "How to Solder Training kit" at http://www.soldertools.net/learning-how-to-solder... It provides hands-on soldering experience with a wide variety of SMT and through-hole parts.
The first type of SMT we’re going to talk about are 5-sided rectangular components. As the name suggests, these are typically flat, rectangular chips with metallization covering 5 sides of their two opposite ends. Soldering these SMT’s requires a steady hand and for you to take care with the amount of heat applied.
Step 1: Clean the PCB
Clean the lands for soldering using isopropyl alcohol and then wipe them dry to ensure that the PCB is clean. This will help make sure a good solder joint is formed.
Tin the tip of the soldering iron by melting a small bead of solder onto the tip and wiping it off.
Step 2: Prepare the Lands
Reflow a small amount of solder onto the tip of your soldering iron. Use this to tack a small amount f solder onto one of the pads of the PCB. This will be the first side of the components that you solder down.
Set the part down so that both leads are making contact with the pads. Make sure the part is properly aligned, because after you solder down one side the part it cannot be moved.
Step 3: Solder Down One Side
Apply liquid flux appropriate for the type of alloy (typically lead or lead-free) to the side of the part that has the tacked down solder.
Using the soldering iron, apply heat to reflow the solder that’s already on the pad. Make sure to hold down the part using tweezers to prevent it from flipping up as the solder cools and contracts. The tendency of SMT’s to do this is called “tombstoning.”
Step 4: Soldering the Opposite Side
On the other side, apply liquid flux, then place your cored solder wire at the junction where the SMT meets the pad. Use the soldering iron to reflow the solder wire onto the pad while holding down the part with tweezers until you can see good wetting to the part. This will not take much solder to accomplish.
Step 5: Clean and Inspect
Clean and inspect the final product for any errors. You should observe a shiny, concave fillet that has good wetting to the lead. Bear in mind that lead-free solders will be more dull in appearance than tin-lead solders.
A complete how to in video format can be found here.
Step 6: Soldering QFP's and SOIC's
The other type of SMT you are likely to come across are leaded components such as SOIC's and QFP's. Rather than have flat connections, these parts have many small leads. Because of this, they are more difficult to solder and are easy to damage. To avoid bent or broken leads, handle these parts gently.
Clean the lands and pads with the alcohol method before beginning to solder, as before.
Step 7: Preparing to Solder
Now, tin the lands to prepare them for soldering. There are two ways that you can approach this task. You can tack solder to every lead one by one, which is an easier task, but can become tedious. Alternatively, you can use a method called “drag soldering.” Drag soldering is the faster of the two options, but is more difficult.
To drag solder, apply a small bead of solder to the tip of your soldering iron, then drag it back and forth across the lands to spread it out over every pad. The difficulty is in being able to apply the correct amount of solder and in moving slow enough to get good coverage and to not pull off pads, but fast enough to avoid burning the pads or creating solder bridges. As a general rule, it is easier to put another small amount of solder on the iron and run over the pads again quickly than it is to wick off solder if you put on too much.
Step 8: Tack Down the Component
After the pads have been tinned, place the part, making sure you have the correct polarity as indicated by the notch on the chip. Failing to line up the part with the proper polarity could damage it.
Once you’re satisfied that the part is in the correct place, tack down two diagonally opposite leads to hold down the part. Lay the tip of the soldering iron on one of the leads and apply a little solder at the junction where the lead meets the lands to tack it down.
Step 9: Solder the Part Down
Drag solder or solder the pins one-by-one, depending on which method you chose earlier. Start with a side that doesn’t have a tacked lead to make sure the chip stays in place.
Step 10: Clean and Inspect
Clean and inspect the component as before. Make sure the solder joints exhibit a smooth, shiny surface and concave whetting at the heel of the leads.