Soldering Techniques

Soldering Techniques
Competency	Soldering
Difficulty	Intermediate
Time Required	3-4 hours (practice) + 1 hour (theory)
Prerequisites	Soldering Guide, experience soldering 50+ through-hole joints
Materials Needed	Temperature-controlled iron, solder, flux pen, tweezers, hot air station (optional), SMD practice board
Next Steps	Custom sensor modules, PCB rework and repair, advanced motor driver assembly

Soldering Techniques is the intermediate tutorial for surface-mount device (SMD) soldering, desoldering, and PCB rework. This tutorial covers techniques needed to assemble advanced robot PCBs, add SMD sensors, and repair damaged electronics.

By the end of this tutorial, you'll be able to:

Hand-solder SMD resistors and capacitors (0805, 0603)
Solder multi-pin SMD ICs using drag soldering technique
Use hot air rework station to remove and replace SMD components
Apply flux effectively for improved solder flow
Desolder through-hole and SMD components without damaging PCBs
Repair common PCB damage (lifted pads, broken traces)

This tutorial assumes you've completed Soldering Guide and are comfortable with through-hole soldering.

Part 1: SMD Component Sizes

Surface-mount devices (SMD) are tiny components soldered directly to PCB pads (no through-holes).

Standard SMD Package Sizes

Package	Dimensions (mm)	Difficulty	Applications
1206	3.2 × 1.6	Easy	Beginner practice, power resistors
0805	2.0 × 1.25	Easy	Most common resistors/capacitors
0603	1.6 × 0.8	Moderate	High-density boards, standard SMD
0402	1.0 × 0.5	Hard	Very high density, requires magnification
0201	0.6 × 0.3	Very Hard	Specialized applications, microscope required

Start with 0805 - Large enough to handle with tweezers, small enough to develop technique.

IC Package Types

SOIC (Small Outline IC) - 1.27mm pin pitch, relatively easy
TSSOP (Thin Shrink Small Outline Package) - 0.65mm pitch, moderate difficulty
QFP/TQFP (Quad Flat Pack) - 0.8mm to 0.5mm pitch, pins on all four sides
QFN (Quad Flat No-leads) - Pads on bottom, requires hot air or reflow
BGA (Ball Grid Array) - Solder balls on bottom, requires reflow or specialized rework

Start with SOIC or TSSOP for learning multi-pin IC soldering.

Part 2: Essential SMD Tools

Beyond basic through-hole tools, SMD work requires:

Required Tools

Fine-tip soldering iron - Chisel or conical tip (0.5-1.0mm)
Tweezers - Fine-point or angled, non-magnetic preferred
Flux pen - Essential for SMD work (no-clean or rosin flux)
Magnifying glass or lamp - 2-10× magnification to see SMD components
Fine solder - 0.5mm diameter (0.3mm for fine-pitch work)

Highly Recommended Tools

Hot air rework station - Remove and replace SMD components ($$50-150)
SMD practice board - Various package sizes for skill development ($10-20)
Solder paste - Can be used with hot air instead of wire solder
Anti-static mat and wrist strap - Protect sensitive ICs from ESD
Kapton tape - Heat-resistant tape for masking

Professional Tools (Nice to Have)

Stereo microscope - 10-20× magnification for fine-pitch work ($100-400)
PCB preheater - Preheat board to reduce thermal stress ($50-200)
Chip Quik low-temp solder - Ease removal of multi-pin ICs ($15-20)
Vacuum pickup tool - Place tiny SMD components ($20-50)

Part 3: Flux - The Secret Ingredient

Flux is even more critical for SMD work than through-hole.

What Flux Does

Removes oxidation - Cleans metal surfaces for better wetting
Improves heat transfer - Helps solder flow smoothly to small pads
Prevents bridging - Solder flows to flux, making drag soldering possible
Lowers surface tension - Allows solder to wet small SMD pads

Types of Flux

Rosin flux (R) - Natural tree resin, mildly corrosive, common in solder cores
No-clean flux (NC) - Leaves minimal residue, doesn't require cleaning
Water-soluble flux (OA) - Most active, requires cleaning after soldering
Tacky flux - Gel consistency, holds components in place while soldering

Recommendation: No-clean flux pen for most SMD work (easy to apply, minimal cleanup).

Flux Application

Apply small amount to pads before soldering
Component will "snap" into alignment when heated (flux surface tension)
Excess flux will smoke during soldering (normal)
Clean residue if desired (isopropyl alcohol and brush)

Key principle: When SMD soldering is difficult, add more flux!

Part 4: SMD Resistor and Capacitor Soldering

The fundamental SMD technique applies to all two-terminal components.

The "Tack and Flow" Method

This is the standard approach for 0805/0603 components:

Apply flux to both pads
Tin one pad - Add small amount of solder to one pad
Pick up component with tweezers (hold on sides, not ends)
Position component on pads (alignment doesn't need to be perfect yet)
Tack first side - Heat tinned pad, component will sink into solder
Adjust alignment - Component will self-align if flux is present
Solder second side - Apply iron and solder to opposite end
Re-flow first side (optional) - Ensure good joint on both ends

Step-by-Step: 0805 Resistor

Clean PCB pads with isopropyl alcohol
Apply flux to both pads with flux pen
Tin left pad: touch iron to pad, apply small amount of solder
Pick up 0805 resistor with tweezers
Position resistor on pads (approximately centered)
Touch iron to left end of resistor while holding with tweezers
Solder melts, component sinks into place
Remove iron, hold steady 1-2 seconds
Move iron to right end of resistor
Apply solder wire to joint (iron touching both pad and component)
Solder flows onto pad and component
Remove iron and solder wire

Inspection:

Component aligned with pads (flux self-aligns if not forced)
Solder visible on both ends (small fillet)
No excess solder (shouldn't look like through-hole joint)

Common SMD Mistakes

Tombstoning:

Component stands on end (one side soldered before other)
Prevention: Tack one side first, then solder opposite side
If using hot air or reflow, ensure even heating

Insufficient solder:

Component appears to float above pad
Add more solder - SMD joints are small but need wetting

Excess solder:

Large blob obscures component and pads
Remove excess with solder wick

Part 5: Multi-Pin IC Soldering (SOIC, TSSOP)

ICs with many pins use "drag soldering" technique.

Drag Soldering Method

Drag soldering uses flux and a flood of solder, then drags iron across pins to separate:

Apply flux generously to all IC pads (this is critical!)
Align IC on pads using tweezers
Tack one corner pin - Hold IC with tweezers, solder one pin to secure IC
Check alignment - IC should be centered on pads
Tack opposite corner - Ensure IC doesn't shift
Flood one side with solder - Apply solder liberally along pins (will create bridges)
Drag iron across pins - Draw iron along pins with slight pressure
Solder separates - Flux causes solder to "ball up" on individual pins
Repeat for each side of IC
Inspect for bridges - Look carefully between pins
Remove bridges with solder wick if present

Why Drag Soldering Works

Flux lowers surface tension - Solder prefers to ball up on pads rather than bridge
Solder follows heat - Iron draws excess solder away from pins
Much faster than soldering each pin individually
Requires practice - Don't expect perfection on first attempt

Pin-by-Pin Alternative

For beginners, soldering each pin individually is slower but more controlled:

Align and tack IC (corners first)
Apply flux to all pins
Touch iron to first pin and pad, apply small amount of solder
Move to next pin, repeat
Inspect for bridges after each side

Inspecting IC Solder Joints

Use magnifying glass to check:

All pins have solder fillet (not dry or unsoldered)
No bridges between adjacent pins (continuity test with multimeter if unsure)
IC is aligned with pads (no pins floating off pads)

Part 6: Hot Air Rework

Hot air stations remove and replace SMD components with heated air.

Hot Air Station Basics

Temperature - 300-380°C for lead-free (adjust based on component size)
Air flow - Low for small components (blown away by high flow), medium for ICs
Nozzle - Use smallest nozzle that covers component

Removing SMD Components

Apply flux around component
Set temperature - 350°C for small passives, 380°C for ICs
Set air flow - Low-medium to avoid blowing away components
Heat component - Hold nozzle 10-20mm above component
Watch solder melt - Solder will become shiny when molten
Lift with tweezers - Gently tug component; if not free, heat more
Clean pads - Remove old solder with wick or iron while still hot

Installing SMD Components with Hot Air

Clean and tin pads (small amount of solder on each pad)
Apply flux to pads
Position component with tweezers
Heat with hot air - Hold nozzle above component
Solder melts - Component will self-align (flux surface tension)
Remove heat when component settles into position
Hold steady 2-3 seconds while cooling

Hot Air Safety

Protect adjacent components - Use Kapton tape or aluminum foil shield
Don't overheat - Plastic connectors and components can melt
Burns - Hot air nozzle is extremely hot; never touch during or immediately after use
Airflow - Don't direct at nearby components (can blow them off board)

Part 7: Advanced Desoldering Techniques

Solder Wick Mastery

Solder wick (braid) removes excess solder effectively when used correctly:

Cut fresh wick - Used/saturated wick doesn't absorb well
Apply flux to wick and target solder
Press wick firmly onto solder with iron tip
Wait for solder to melt - Wicks into braid by capillary action
Remove iron and wick together - Solder solidifies in wick
Repeat if needed with fresh wick section

Advanced tip: Pre-tin the wick with solder for better heat transfer, then use to remove solder.

Desoldering Pump Technique

Also called "solder sucker" - mechanical vacuum device:

Heat solder joint until fully molten
Cock pump spring mechanism
Position tip close to molten solder (not touching iron)
Press button - Creates vacuum, sucks solder into pump
Clean pump chamber regularly (dump out collected solder)

Works best for through-hole components; less effective for SMD.

Removing Through-Hole ICs

Multi-pin through-hole ICs require special technique:

Method 1: Chip Quik low-temp solder

Add Chip Quik (low-melt) solder to all pins
Heat all pins with iron - Chip Quik stays molten longer
Gently pull IC while heating multiple pins
IC releases when all pins are molten

Method 2: Pin-by-pin

Heat one pin, push component from other side
Heat next pin, push component
Alternate sides to avoid bending IC
Requires patience - 20-30 pins takes time

Method 3: Cut and remove

Cut IC body with cutters (destroy IC)
Remove pins individually from PCB
Clean holes with desoldering pump or wick
Use when IC is already damaged

Removing Stubborn SMD Components

Some SMD parts are difficult to remove with hot air alone:

Large thermal mass - Ground planes conduct heat away; use preheater or higher temperature
Lead-free solder - Higher melting point; ensure hot air reaches 380°C
Multiple rows of pins - QFP, QFN require flux and patience
Adhesive - Some components glued in addition to soldering; may need force

Part 8: Repairing PCB Damage

Mistakes and accidents cause PCB damage. Here's how to fix common issues:

Lifted Pad

Pad separated from PCB (adhesive failed from heat or force):

Identify trace connection - Where does the pad connect?
Scrape soldermask from trace near damaged pad
Solder component directly to trace - Bypass lifted pad
Reinforce with wire (if needed) - Jumper from component to nearby pad/via
Test continuity - Verify connection with multimeter

Broken Trace

Copper trace cut or damaged:

Expose trace ends - Scrape soldermask on both sides of break
Solder jumper wire across break - 30AWG wire works well
Secure with glue (optional) - CA glue or epoxy to prevent stress
Test continuity - Verify repair with multimeter

Missing Pad

Pad completely removed from PCB:

Find nearest via or pad on same net
Run jumper wire from component to via/pad
Secure component with CA glue (won't have mechanical strength from pad)
Test connection with multimeter

Bridged Pins Under IC

Solder bridge you can't access:

Add flux around IC
Re-flow with hot air - Heat IC until solder melts
Flux should separate bridge (surface tension pulls solder to individual pins)
Alternative: Remove IC, clean pads with wick, reinstall IC

Part 9: Practice Projects

Build skills with these progressive exercises:

Exercise 1: SMD Resistors (0805 → 0603)

Materials: SMD practice board or breakout board, 0805 and 0603 resistors

Solder 10× 0805 resistors using tack-and-flow method
Inspect each joint - proper alignment? Solder on both ends?
Solder 10× 0603 resistors (smaller, requires more precision)
Remove and replace 5× resistors with hot air (if available)

Goal: Consistent SMD joints, confidence with small components.

Exercise 2: SOIC IC (8-pin or 16-pin)

Materials: SOIC practice board, 8-pin or 16-pin SOIC chip

Apply flux to all pads
Align and tack IC (opposite corners)
Drag solder one side of IC
Inspect for bridges
Remove bridges with solder wick if present
Repeat for remaining sides

Goal: Successful drag soldering with no bridges.

Exercise 3: SMD Motor Driver Assembly

Materials: TB6612FNG breakout board kit (SparkFun, Adafruit) or similar

Assemble complete SMD board from kit
Includes various SMD component sizes and IC
Demonstrates real-world application
Test functionality when complete

Goal: Fully functional assembled board.

Exercise 4: Desoldering and Rework

Materials: Old/broken PCB or previous practice boards

Remove 10× SMD components with hot air
Remove 10× SMD components with iron and tweezers
Remove through-hole IC with Chip Quik or pin-by-pin
Clean pads with solder wick
Reinstall components

Goal: Proficiency with removal and replacement.

Part 10: Quality Control

Visual Inspection

Inspect SMD joints under magnification:

☐ Solder visible on all component ends/pins
☐ No bridges between adjacent pins
☐ Components aligned with pads (not crooked)
☐ No excess solder obscuring component markings
☐ No missing joints (all pins soldered)

Electrical Testing

Continuity - Test connections with multimeter
No shorts - Check adjacent pins for bridges
Power rails - Verify no shorts between power and ground
Functional test - Power up board and test operation

Common SMD Defects

Dry joint - Solder didn't wet pad or component
Insufficient solder - Joint looks weak or incomplete
Bridging - Adjacent pins connected by solder
Tombstoning - Component standing on end
Misalignment - Component not centered on pads
Cold joint - Dull, grainy appearance

Part 11: Real-World Applications

Motor Driver Boards

Many advanced motor drivers use SMD components:

TB6612FNG breakout (SparkFun)
DRV8833 (Pololu, Adafruit)
TMC2209 stepper drivers

Skills required:

0805 resistors and capacitors
SOIC or TSSOP motor driver IC
Through-hole terminal blocks

Sensor Modules

Custom sensor breakout boards:

IMU sensors (MPU6050, BNO055)
Time-of-flight sensors (VL53L0X)
Optical encoders