How to Solder¶

There are mainly three types of soldering tasks you'll encounter: soldering SMT (Surface Mount) components, through-hole components, and joining two wires together. Each of these tasks requires a slightly different technique and approach, but the basic principles of soldering apply to all of them.

SMT Soldering¶

SMD (Surface Mount Device) components are soldered directly onto pads on the surface of a PCB.

Soldering SMD components can be more challenging than through-hole components due to their small size and lack of leads, but with the right tools and techniques, it can be done effectively.

There are a few ways to solder SMD components. We cover some of the processes here:

Single SMD component¶

This is the most basic method of soldering SMD components. You would use this method when you only have a few components to solder, and they are simple components, such as resistors, capacitors, or ICs whose legs are easily accessible.

Required:

• Soldering Iron

  Tip

  A fine tip such as a knife tip is recommended for SMD soldering. The sharp edge holds solder better and allows for more precise application of heat.

• Solder

  Tip

  Depending on how small your component or the pad is, you may want to use a thinner solder (like 0.3 mm) for better control and precision. Leaded solder can also be easier to work with for SMD soldering due to its lower melting point and better flow characteristics.

Optional:

• Flux
• Isopropyl Alcohol
• Tweezers

Steps:

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1. Place the PCB: Secure the PCB on a stable surface, such as a soldering mat or in a vise.
2. Prepare the PCB: Clean the PCB with isopropyl alcohol specially if there's dirt.
3. Apply flux: Using flux is highly recommended. The no-clean flux is preferred over the flux pen, but either will work. Apply it directly to the pads where the component will be soldered.
4. Apply solder: Touch the soldering iron tip to the pad and feed a small amount of solder onto the pad. The solder should melt and form a very thin layer on the pad.
5. Place the component: Use tweezers to position the SMD component accurately on the pads.
6. Solder the component: Heat the joint by gently touching the soldering iron tip to the pad and the component lead simultaneously. Since there is solder already on the pad, it should melt and form a good joint between the pad and the component lead.

Single SMD component with pads on the bottom¶

For components like QFNs or BGAs, the pads are on the bottom of the component, so you won't be able to heat the joint properly with a soldering iron. In this case, you can use solder paste and a hot air rework station to solder the component.

Required:

• Solder Paste or Solder
• Hot Air Rework Station
• Flux

Optional:

• Kapton Tape to protect nearby components from heat
• Tweezers

Steps:

Soldering a whole PCB at once¶

Soldering a whole PCB at once is typically done using a reflow oven, which allows for precise control of the temperature profile to ensure proper soldering of all components simultaneously.

You can reflow both sides of the PCB at once if both the sides have small enough components so that they would not fall off when the solder melts due to surface tension. Any larger or heavier components should be soldered separately after the reflow process.

Required:

• Solder Paste
• Reflow Oven

Steps:

1. Apply solder paste: Use a stencil to apply solder paste onto all the pads where components will be soldered. It's okay if there are some "bridges" of solder paste between the pads, because during reflow, the surface tension of the molten solder will help to pull the solder into the correct shape and position on the pads.
2. Place components: Use tweezers or a pick-and-place machine to position all the components accurately on the pads. The solder paste will help to hold the components in place.
3. Reflow the solder: Place the PCB in the reflow oven and run the appropriate temperature profile for the solder paste you're using. The oven will heat the PCB according to the profile.
4. Verify the joints: After the solder has cooled, inspect all the joints under a microscope to ensure that they are properly formed and there are no solder bridges or unflowed joints. If there are any issues, you can reheat the joint with a hot air rework station to fix them.

Through-Hole Components¶

Through-hole components have leads that pass through holes in the PCB and are soldered on the opposite side.

Required:

• Soldering Iron
• Solder

Optional:

• Flux

Steps:

1. Insert the component: Push the leads through the holes from the top side.
2. Secure: Bend the leads slightly on the bottom to hold the component in place.
3. Heat the joint: Touch the soldering iron tip to the lead and pad simultaneously.
4. Apply solder: Feed solder wire into the joint with your other hand until it flows and fills the hole.
5. Remove heat: Pull away the iron, then the solder wire.
6. Trim leads: Use cutters to remove excess lead length.

Two Wires Together¶

Joining two wires creates a secure electrical connection, often for repairs or extensions.

Required:

• Soldering Iron
• Solder
• Wire Strippers

Optional:

• Flux
• Heat-Shrink Tubing
• Helping hands or vise

Steps:

1. Strip insulation: Use wire strippers to expose about 1/2 inch of bare wire on each end.
2. Pass the wires through heat-shrink tubing: If you plan to use heat-shrink tubing for insulation, make sure to slide it onto one of the wires before twisting and soldering.
3. Clamp: Use helping hands or a vise to hold the wires in place while soldering.

4. Twist or place on top each other: Either way is fine, but make sure the wires are in good contact with each other to ensure a strong solder joint.

   

5. Apply flux: Dab flux on the twisted wires to improve flow.

6. Apply solder: Use the soldering iron and solder on the joint until it flows into the twists.