PCB Assembly Guide

Detailed instructions for assembling the Moto32 PCB, from preparing your workspace to final quality inspection.

Before You Begin

Required Skills

• Basic soldering experience
• SMD soldering skills (recommended)
• Ability to work with small components
• Patience and steady hands
• Attention to detail

Tools & Equipment

Essential Tools

Advanced Tools (Optional)

Workspace Setup

Create a proper electronics workbench:

• Well-lit area (LED task light recommended)
• Static-free surface (ESD mat)
• Organized component storage
• Good ventilation (solder fumes)
• Clean, dust-free environment
• Comfortable seating for precision work

Assembly Methods

Method 1: Solder Paste & Hot Air (Recommended)

Best for: Multiple boards, professional results

Process:

1. Apply solder paste with stencil
2. Place all components with tweezers
3. Reflow with hot air station or oven
4. Inspect and touch up any issues

Advantages:

• Faster for multiple boards
• Professional results
• Easier alignment of small parts
• Less chance of cold joints

Method 2: Hand Soldering with Iron

Best for: Single boards, learning

Process:

1. Apply flux to pads
2. Place component
3. Solder one pin to tack it down
4. Solder remaining pins
5. Inspect and clean

Advantages:

• No special equipment needed
• Full control over each joint
• Easy to rework mistakes
• Good learning experience

Method 3: PCBWay Assembly Service

Best for: First units, no soldering experience

Process:

1. Order PCBs with assembly service
2. Upload BOM and Pick & Place files
3. Wait for assembled boards to arrive
4. Skip directly to testing phase

Advantages:

• No soldering required
• Professionally assembled
• Quality guaranteed
• More expensive but saves time

Assembly Order

Work from smallest to largest components:

1. Small passives (0402): Resistors and capacitors
2. Larger passives (0603): Capacitors
3. Small ICs (SOT-23): Buck converter
4. Medium ICs (SOP-16): CH340C USB-UART
5. MOSFETs (TO-252): Output switches
6. Large modules: ESP32-S3
7. Connectors: USB-C, headers
8. Through-hole: Crystal oscillator (if present)

Step-by-Step Assembly

Step 1: Prepare the PCB

1. Inspect PCB for damage
   • Check for scratches, contamination
   • Verify all pads are intact
   • Confirm no broken traces
2. Clean surface with IPA if needed
3. Secure in PCB holder or vise
4. Verify orientation (check silkscreen markings)

Step 2: 0402 Resistors

Components: R1-R22 (22 resistors total)

Important: 0402 parts are TINY (1mm × 0.5mm). Organization is critical!

1. Organize resistors by value in labeled containers
2. Apply small amount of flux to pads
3. Pre-tin one pad with small solder amount
4. Place resistor with tweezers while re-heating pre-tinned pad
5. Solder other end
6. Inspect with magnification

Common Values:

• 10kΩ: 10 pieces (R1, R6-R8, R10, R13-R17)
• 1kΩ: 4 pieces (R3-R5, R9)
• Others: Various (see BOM)

Tips:

• Work in good lighting
• Use flux generously
• Don't apply too much heat (<3 seconds per joint)
• Check orientation (doesn't matter for resistors)

Step 3: 0402 Capacitors

Components: C1, C3, C5-C9, C13 (8 capacitors)

Same process as resistors:

1. Organize by value (1µF, 100nF)
2. Apply flux to pads
3. Pre-tin one pad
4. Place and solder first side
5. Solder second side
6. Inspect for shorts

Polarity: Ceramic capacitors have no polarity (can go either way)

Step 4: 0603 Capacitors

Components: C2, C4, C10-C12 (5 capacitors)

Larger than 0402, easier to handle:

• Same technique as smaller caps
• 0603 size: 1.6mm × 0.8mm
• Values: 22µF and 10µF
• More forgiving alignment

Step 5: Diodes

Components: D1 (SMA), D2-D3 (SOD-123FL), D4-D5 (SMC)

Identifying Polarity:

• Cathode marking: Line on component body
• PCB marking: Line on silkscreen shows cathode
• Match markings: Component line to PCB line

Soldering:

1. Check datasheet for orientation
2. Apply flux to pads
3. Pre-tin one pad
4. Place diode (verify orientation!)
5. Solder both ends
6. Double-check polarity before proceeding

Step 6: Buck Converter (U3)

Component: AP63203WU-7 (SOT-23-6)

Package: 6-pin SOT-23 (small IC)

1. Apply flux to all 6 pads
2. Check pin 1 marking (dot or line on IC)
3. Align IC carefully (pin 1 to marked pad)
4. Tack down one corner pin
5. Check alignment, adjust if needed
6. Solder remaining 5 pins
7. Inspect for bridges between pins

Pin 1 Identification:

• IC: Dot, dimple, or line near pin 1
• PCB: Dot or different pad shape

Troubleshooting:

• Solder bridges: Use solder wick or flux and drag technique
• Tombstoning: Reduce heat, ensure even pad temperature

Step 7: USB-UART Bridge (U2)

Component: CH340C (SOP-16, 150mil pitch)

Package: 16-pin SOIC, easier than SOT-23

Method 1 - Drag Soldering:

1. Apply flux generously to all pads
2. Align IC carefully (pin 1 to marked pad)
3. Tack opposite corner pins
4. Verify alignment
5. Apply solder to tip and drag across pins
6. Flux prevents bridges from forming
7. Clean with IPA and inspect

Method 2 - Individual Pins:

1. Place and tack IC as above
2. Solder each pin individually
3. Use minimal solder
4. Keep iron time <2 seconds per pin

Step 8: MOSFETs (Q5-Q12)

Components: 8x DMP4015SK3Q-13 (TO-252-2 / DPAK)

Larger components with thermal pad

Soldering:

1. Apply flux to pads
2. Place MOSFET (check orientation!)
3. Tack down tab first (this is the thermal/electrical connection)
4. Solder gate and source pins
5. Add more solder to tab for good thermal contact
6. Inspect alignment and solder joints

Tips:

• Tab needs good solder joint (electrical connection)
• Use higher temperature for thermal pad (large copper area)
• Hold iron longer (5-7 seconds) for tab
• Check for gaps under component

Step 9: ESP32-S3 Module (U1)

Component: ESP32-S3-WROOM-1-N8R2

Large module with many pins

Method 1 - Hot Air (Recommended):

1. Apply solder paste to all pads (use stencil if available)
2. Place module carefully (align castellated edges)
3. Heat from bottom with hot air (320-350°C, medium airflow)
4. Watch solder paste reflow (shiny appearance)
5. Remove heat and let cool naturally

Method 2 - Soldering Iron:

1. Apply flux to ALL pads
2. Place module (verify alignment)
3. Tack down opposite corner pads
4. Solder around perimeter systematically
5. Use drag soldering for pin rows
6. Check each pad with magnification

Verification:

• All pads should have visible solder fillets
• No cold solder joints (grainy appearance)
• No bridges between pads
• Module sits flat on PCB

Step 10: USB-C Connector

Component: TYPE-C-31-M-12

Mixed through-hole and SMD pads

1. Insert connector (pins through holes)
2. Ensure connector sits flush against PCB
3. Solder through-hole pins first (mechanical strength)
4. Solder SMD pads for USB signals
5. Add extra solder to mounting pins for durability

Tips:

• Use more heat for through-hole pins
• Check connector is straight before soldering all pins
• Test USB insertion/removal after assembly

Step 11: Crystal Oscillator (X1)

Component: HC-49S 12MHz

Through-hole component

1. Insert crystal into PCB holes
2. Bend leads slightly to hold in place
3. Solder from bottom side
4. Trim excess leads with flush cutters

Step 12: Optional Components

Headers / Expansion Connectors:

If adding GPIO headers:

1. Insert header pins
2. Use tape to hold straight
3. Solder one pin
4. Check alignment
5. Solder remaining pins

Post-Assembly

Visual Inspection

Check every joint with magnification:

• Good joints: Shiny, smooth fillet, wets both pad and component
• Cold joints: Grainy, dull appearance - reheat!
• Insufficient solder: No fillet visible - add more
• Bridges: Solder connects adjacent pins - remove with wick
• Tombstoning: Component standing on end - desolder and replace

Cleaning

1. Brush IPA over entire PCB with cotton swab
2. Scrub gently to remove flux residue
3. Inspect under magnification again
4. Let dry completely (5-10 minutes)
5. Repeat if needed for stubborn flux

Electrical Testing

Before applying power:

1. Visual check for shorts
   • Use multimeter in continuity mode
   • Check 12V to GND: Should be open circuit (>1MΩ)
   • Check 3.3V rail to GND: Should be open circuit
2. Component orientation
   • All diodes correct polarity
   • All ICs pin 1 in correct position
   • MOSFETs oriented properly

First Power-Up

Using USB-C only (safest):

1. Connect USB-C cable to computer
2. Watch for smoke or unusual smells (if any, disconnect immediately!)
3. Measure voltages:
   • 3.3V rail should be present and stable
   • Check with multimeter at test points
4. Computer should detect CH340 USB device
5. LED indicators should illuminate (if populated)

If 3.3V is present and stable:

1. Disconnect USB
2. Connect 12V supply (use current-limited bench supply if available)
3. Set current limit to 500mA initially
4. Power up and verify:
   • 12V input present
   • 3.3V rail still stable
   • No excessive current draw (<200mA idle)
5. Gradually increase current limit to 2A

Troubleshooting Assembly

Common Issues

No 3.3V output:

• Check buck converter (U3) orientation
• Verify input caps C10, C11 are installed
• Check for shorts on 3.3V rail
• Measure 12V input to U3

ESP32 not detected on USB:

• Check CH340C (U2) orientation
• Verify USB-C connector soldering (all pins)
• Check 12MHz crystal (X1) installation
• Verify 3.3V present on U2 VCC pin

Outputs don't work:

• Check MOSFET orientation (each one!)
• Verify gate resistors are installed
• Test with multimeter: Gate to source voltage when activated
• Check ESP32 GPIO connections

USB connection unstable:

• Reflow USB-C connector solder joints
• Check USB data line resistors (R18-R21)
• Verify TVS diodes (D4, D5) not shorted

Repair Techniques

Removing components:

1. Apply flux liberally
2. Heat all pins simultaneously with hot air
3. Gently lift component with tweezers
4. Clean pads with solder wick
5. Apply fresh solder and place new component

Fixing solder bridges:

1. Add more flux (counterintuitive but works!)
2. Drag soldering iron tip across pins
3. If persistent, use solder wick to remove excess
4. Re-solder with minimal solder amount

Lifted pads:

• If pad lifts from PCB, carefully scrape soldermask to expose trace
• Solder component directly to exposed trace
• Use wire jumper if necessary

Quality Checklist

Before considering assembly complete:

• All components installed per BOM
• All solder joints inspected and acceptable
• No solder bridges between pins
• All ICs in correct orientation (pin 1 markers)
• All diodes in correct polarity
• All MOSFETs in correct orientation
• Board cleaned of flux residue
• No damaged components or PCB traces
• Electrical continuity tests passed
• 3.3V rail present and stable
• USB connection recognized
• 12V input tested (if applicable)
• No shorts between power rails

Next Steps