How to Solder Wire to PCB

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Soldering wires to a printed circuit board (PCB) is a common task in electronics assembly and repair. With the right techniques and a little practice, anyone can learn to solder wires to a PCB quickly and reliably.

In this guide, we will cover everything you need to know to solder wires to a PCB successfully, including:

  • Safety precautions
  • Tools and supplies needed
  • Prepping wires and PCB
  • Tinning wires and pads
  • Soldering techniques
  • Inspecting solder joints
  • Troubleshooting tips

Follow these steps and you will be soldering like a pro in no time. Let’s get started!

Safety Precautions

Whenever soldering, it’s important to work safely. Follow these basic safety guidelines:

  • Work in a well-ventilated area to avoid breathing solder fumes.
  • Wear safety glasses to protect your eyes from debris.
  • Secure wires and PCB so they don’t move during soldering.
  • Allow soldering iron and PCB to cool before handling to avoid burns.
  • Keep a fire extinguisher and first aid kit nearby.
  • Wash hands after handling solder, which contains lead.

Tools and Supplies

You’ll need a few basic tools and supplies:

Soldering Iron

  • A 15-40W electric soldering iron with a fine conical tip works best. The tip should come to a sharp point for precision soldering.

Solder

  • Lead solder with a rosin core is commonly used for electronics. A 0.031″ diameter works for most PCB wiring.

Soldering Stand and Sponge

  • Essential for keeping the soldering iron safely in place while hot and cleaning the tip.

Wire Strippers and Cutters

  • For prepping and trimming wires to length. Automatic strippers with an adjustable stop help strip precise lengths.

Needle Nose Pliers

  • Helpful for bending, shaping, and holding wires in place while soldering. Choose pliers with insulated handles.

Safety Glasses

  • Protect your eyes from flying bits of debris and splatter while soldering.

Isopropyl Alcohol and Clean Rags

  • For cleaning the PCB and wire ends before soldering to remove oils and residue.

Heat Shrink Tubing

  • Used to insulate soldered wire connections if needed. Cut small pieces to fit each joint after soldering.

Wire

  • Stranded 22-28AWG hook-up wire works well for soldering to PCBs. Stock various colors.

Flux

  • (Optional) Helps solder flow and adhere to metal surfaces. Often not needed for electronics soldering.

Prepping Wires and PCB

Before soldering, you need to prepare the wires and PCB pads:

PCB Pads

  • Clean pads thoroughly with isopropyl alcohol to remove any grease, oil or residue. These can prevent solder adhesion.
  • Carefully scrape away any oxidation or old solder from pads using a knife. pads must be shiny and clean metal for soldering.
  • Apply a small amount of liquid flux to freshly cleaned pads if needed to aid solder flow.

Wires

  • Cut wires to appropriate length, leaving extra length for strain relief.
  • Strip wires to recommended length for the pad size:
    • For small PCB pads, strip 1/8″ (3mm) of insulation from wire ends.
    • For larger pads, 3/16″-1/4″ (5-6mm) wire stripping length works best.
  • Twist stripped wire strands together tightly using pliers. This keeps them together when soldering.
  • Bend stripped wire end into a hook shape. This increases surface area for soldering.
  • Clean wires with isopropyl alcohol to remove enamel, grease and other residue.

Tinning Wires and Pads

Tinning wires and pads applies a thin layer of solder prior to joining them permanently. This improves solderability and heat transfer during the final solder joint.

Tinning Wires

  • Hold stripped wire end with pliers or helping hands tool.
  • Heat wire for 1-2 seconds with soldering iron tip until it starts to melt solder.
  • Feed a small amount of solder onto wire end, allowing it to fully melt and coat wire.
  • Remove solder and continue heating wire for 1-2 seconds to allow excess solder to run off, leaving a thin coating.
  • Repeat process until wire is fully tinned with a smooth silvery coating of solder.

Tinning Pads

  • Scrub pad vigorously with soldering iron tip to heat thoroughly.
  • Feed a small amount of solder onto pad, allowing it to flow fully.
  • Continue heating for a few seconds to let excess solder run off pad, leaving it shiny and tinned.
  • Flux helps aid solder flow and wetting when tinning pads. Use sparingly.

Soldering Techniques

With your wires and pads prepped and tinned, you’re ready to solder them together. Follow these steps:

Prepare Joint

  • Bend tinned wire into a hook shape and insert into pad. This maximizes contact area.
  • Bend wire so it is pressed firmly against pad surface. Proper contact ensures good joint.
  • If needed, apply small amount of flux to wire and pad to aid solder flow in difficult joints.
  • Secure and stabilize wires with tape or helping hands tool so they don’t move.

Heat Joint

  • Place conical soldering iron tip in contact with both wire and pad simultaneously.
  • Heat joint for 2-3 seconds until pad and wire are hot enough to melt solder.
  • Wire and pad should heat up almost instantly when iron contacts them if properly tinned.

Feed Solder

  • With iron still in contact, feed a small amount of solder wire onto joint, opposite the iron.
  • Solder should melt and flow smoothly onto joint within 1-2 seconds.
  • Allow fresh solder to fully wet and be drawn into joint by capillary action.
  • Continue feeding just enough solder for full joint fillet formation. Avoid excessive solder blob.

Allow Cooling

  • Keep soldering iron tip pressed against joint for 1-2 seconds after feeding solder to allow it to fully melt and wet to surfaces.
  • Remove iron tip and allow joint to cool undisturbed for a few seconds until solder solidifies shiny and smooth.
  • Cooling time is important for proper intermetallic bonding and crystal formation.

Inspect Joint

  • Joint should be slightly concave with solder fully wicked and clinging to wire and pad.
  • Solder should have a shiny, smooth appearance when solidified, not crumbly or gritty.
  • There should not be any gaps, pits, cracks or movement in the solder joint.

Inspecting Solder Joints

Once a joint is completed, be sure to inspect it closely for quality. Check for these characteristics of a good solder joint:

  • Smooth and shiny solder surface – Matte, gritty solder indicates a cold joint
  • Concave fillet shape – Shows proper wetting and capillary action
  • Solder adhered evenly around joint – No gaps, holes or pitting
  • Wire and pad not moving separately – Should be fused as one unit

Poor joints may need to be reworked by heating with the iron and feeding in fresh solder. In some cases, bad joints need to be desoldered and redone entirely.

Troubleshooting Soldering Issues

Despite best efforts, problems can arise when soldering wires to PCBs. Here are some troubleshooting tips for the most common issues:

Dry Joint

  • Solder doesn’t properly wet or bond to surfaces
  • Cause – Dirty/oxidized surfaces or insufficient heat
  • Solution – Clean and re-tin pad and wire. Heat joint thoroughly before applying solder.

Cold Joint

  • Weak bonding in crystalline solder joint
  • Cause – Solder cooled too quickly without proper wetting
  • Solution – Reheat joint and apply additional solder for proper wetting and bonding

Excessive Solder

  • Too much solder, causing blobbing and bridges
  • Cause – Applying too much solder or overheating joint
  • Solution – Use solder wick or vacuum desoldering tool to remove excess solder

Solder Bridges

  • Solder blobs that bridge between neighboring pads
  • Cause – Too much solder or insufficient spacing between pads
  • Solution – Carefully remove bridge with desoldering braid then restore pad separation

Lifted Pad

  • Pad separates from PCB due to overheating
  • Cause – Excessive heat transfer to pad for too long
  • Solution – Try to re-adhere pad with low heat. Otherwise, jumper wire to repl

Questions and Answers

Here are some common questions and answers about soldering wires to PCBs:

Q: What temperature should a soldering iron be set to for electronics work?

A: A good target temperature for electronics soldering is between 600-700°F (315-370°C). This allows the solder to melt and properly wet components without overheating.

Q: Is lead-free solder suitable for soldering wires to PCBs?

A: Lead-free solders requiring higher melting temps can work, but require more precision. Leaded solders are easier for beginners to use successfully.

Q: What size solder wire should be used for PCB rework and modifications?

A: A 0.031″ diameter solder works well for soldering component leads and wires 22-28AWG onto standard PCBs. Larger wire may be needed for thicker wires or ground planes.

Q: Should wires be tinned before soldering them to PCB pads?

A: Tinning wires beforehand isn’t mandatory, but helps ensure the solder fully wets and adheres to the wires when joined to the pad.

Q: What’s the best technique for desoldering a wire from a PCB?

A: Apply desoldering braid or a solder sucker while heating the joint to thoroughly remove solder, then gently pull the wire free from the melted pad using pliers.

Conclusion

Soldering wires onto PCBs is a straightforward process once you understand the basic techniques. Following the steps outlined in this guide will help you achieve strong, reliable solder joints time after time. With some practice building your skills, you’ll be soldering like a pro.

The key things to remember are:

  • Use the right soldering iron and supplies
  • Clean and tin pads and wires before joining
  • Allow plenty of heat for good solder flow and bonding
  • Inspect joints closely for defects and rework if needed

Mastering wire-to-PCB soldering makes you more capable modifying, hacking and fixing your electronics projects. So grab your soldering iron and let’s start making circuits!