PCB Troubleshooting Guide

[

1. Visual Inspection (Always First)

Use your eyes and magnification tools: • Magnifying glass / Microscope: Look for: ◦ Burn marks ◦ Cracked components ◦ Corrosion (especially from liquid damage) ◦ Missing or misplaced components ◦ Bad solder joints or bridges Tip: Shine a flashlight at an angle to reveal subtle soldering issues.

2. Continuity Mode (Multimeter)

Use your multimeter in continuity or resistance mode. • Test for shorts to ground: Common places to check: ◦ Power rails (e.g., VCC to GND should not beep) ◦ Capacitors (both legs shouldn't short unless it's a filter cap on a power rail) ◦ Between adjacent pins of ICs (especially voltage regulators, FETs, and microcontrollers) Tip: Always discharge caps and unplug power before checking.

3. Thermal Detection (Power it Briefly)

• Use your finger carefully (just a few seconds of power) • Use alcohol mist or freeze spray — the shorted component will dry faster • Use a thermal camera (if available) Warning: Do not overheat the board; power only briefly and with current-limited supply if possible.

4. Power Rail Resistance Check

Use your multimeter to check the resistance between power and ground. • Low resistance (like <10 ohms) on something like 3.3V or 5V is suspicious. • Normal values vary by board, but <1 ohm is usually a sign of a shorted cap or chip.

5. Inject Voltage with a Lab Power Supply

• Isolate the shorted power rail. • Inject a low voltage (1-2V) and current-limit it (~1A). • Monitor where heat builds up — this is often the faulty component. Example: If 3.3V rail is shorted, inject 1V and look for heat with your finger, thermal cam, or alcohol evaporation.

6. Compare to a Working Board (If Available)

• Use diode or resistance mode to compare readings on identical points • Helps identify anomalies like reversed diodes, bad caps, etc.

7. Trace the PCB

Even without a schematic: • Follow traces visually or with continuity mode. • Look at via patterns, IC part numbers, and silkscreen labels for clues. • Search for datasheets of ICs to identify expected pin functions (VCC, GND, IN, OUT, etc.)

8. Use Diode Mode

With power off: • Use diode mode on your multimeter to test: ◦ Diodes (forward = ~0.5–0.7V drop, reverse = OL) ◦ MOSFETs (check for shorts across drain/source/gate) ◦ IC pins (compare to known good board if possible)

9. Test Passive Components

• Test resistors, caps, and inductors in-circuit if possible. • Look for: ◦ Shorted caps (very common cause of power shorts) ◦ Burnt or cracked resistors ◦ Open inductors

10. Research Component Functions

Even without a schematic: • Use part numbers and silkscreen labels to find datasheets. • IC pinouts often reveal the power rails and signal flow. You can build a partial understanding of the circuit just by reading datasheets.

Tools That Help (Optional but Useful)

• Thermal camera (FLIR One, Seek Thermal) • Bench power supply with current limiting • ESR meter (to check electrolytic caps) • LCR meter (for more accurate passive testing) • USB current meter (for seeing if the board draws excessive current)

11. Use a "Short Hunter" Method (Divide and Conquer)

If a rail is shorted: • Cut or desolder sections of the power rail (carefully!) to isolate sections. • Keep checking resistance after each cut — this helps narrow down which part of the board has the fault. Example: Cut a trace between two sections, check both sides to see where the short persists.

12. Use Tweezers to Wiggle Test Components

• Slight movement of a damaged or cracked MLCC capacitor (multi-layer ceramic) may cause continuity to flicker. • Great for detecting hairline fractures or internal shorts.

13. Reverse Engineering Tools

• Use KiCad, EasyEDA, or Fritzing to sketch out what you trace manually. • Helps you see patterns or build a partial schematic.

14. Look for Clusters of MLCC Capacitors

• MLCC caps are infamous for shorting, especially on power rails. • They are often grouped near: ◦ Voltage regulators ◦ Main ICs ◦ Power input connectors Try lifting one leg of each capacitor in the group to isolate the short.

15. Inductive Ring Testers

• Tools like the "Blue Ring Tester" can detect shorts in transformer windings or inductors without desoldering. • Useful for power supply troubleshooting (flyback, buck converters, etc.)

16. Fuse & Protection Circuit Testing

• Look for tiny fuses (marked as "F") and TVS diodes that may be shorted or blown. • Check: ◦ USB protection chips ◦ Input fuses or PTCs ◦ ESD diodes near connectors Shorted TVS diodes = common cause of USB shorts

17. Check External Connectors for Damage

• Shorts in USB, HDMI, or power connectors can reflect as board-level shorts. • Try removing connectors (or their attached wires) to isolate the fault.

18. Check for Liquid Damage or Carbonized Board

• A spill may cause carbon traces under chips or layers. • Look for darkened spots or sticky residue. • Clean suspect areas with isopropyl alcohol and a toothbrush.

19. Use Controlled Reflow for Suspected ICs

• If you strongly suspect a short is under a BGA or QFN chip: ◦ Use hot air station to lift or reflow it. ◦ Shorts under chips are often from flux residue, bad rework, or thermal expansion damage. Only try this on non-critical boards unless you have experience.

20. Use "Beep and Think" Logic

• If you find a short with continuity mode, ask: ◦ What is this pin supposed to do? ◦ Is it a power rail, ground, signal line, etc.? ◦ What does the adjacent component do? Follow the trace mentally or physically — for example, a short between an LDO output and GND could mean a bad cap or a dead regulator.

BONUS: Label What You Learn

• Use sticky notes or a notebook to log: ◦ Pins you've identified ◦ Voltages expected ◦ Shorts discovered ◦ Components tested You're slowly building your own schematic — and it speeds up future diagnosis.

21. Use a Current Tracer (Signal Injector + Audio Probe)

• Inject a low AC signal onto the shorted line using a signal generator (e.g., 10kHz tone). • Use a sensitive probe (oscilloscope or audio amp) to trace where the signal dies. • The drop-off point = short location. Think of it like using sonar — signal goes until it hits the block.

22. Magnetic Field Detection for Shorts

When you inject current into a shorted rail: • Use a magnetometer, a sensitive audio coil, or a cheap DIY solution (e.g., a magnetic needle or coil + op-amp) to detect the magnetic field generated by current. Stronger field = closer to the short.

23. Suspect Internal Layer Shorts

On multi-layer PCBs: • Shorts may exist between inner layers — nearly impossible to see. • Look for: ◦ Burn marks near vias ◦ Cracks from flexing ◦ Heat damage in a concentrated area You may have to sacrifice a board and Dremel it open if all else fails.

24. Use Bright Light Behind the PCB (Transillumination)

• Shine a strong LED or flashlight from behind the board. • You can sometimes trace inner-layer routes, vias, or power planes. • Great for reverse-engineering or mapping ground/power layers.

25. Use IC Identification Websites

• Search for markings on unknown chips at: ◦ https://www.s-manuals.com ◦ https://octopart.com ◦ https://www.findchips.com ◦ https://icmarking.com (great for decoding laser markings) Often reveals exact pinout, which tells you what's GND, VCC, IN, OUT, etc.

26. Check for Screws or Shielding Shorts

• Sometimes a screw is too long or misplaced and causes a short to a ground plane. • Check metal standoffs, shields, and screw holes. If you've recently removed or reassembled the board, double-check all mechanical fits.

27. Wiggle Connectors and Flex Cables

• Shorts can come from flex cables with cracked traces or connectors with: ◦ Bent pins ◦ Oxidation ◦ Delamination Unplug, clean, and re-seat flexes one at a time. Try board with some disconnected to isolate problems.

28. Use Resistance/Diode Mode Mapping

• Create a resistance "map" from every pin of a major IC to GND. • Do the same on a working board if available. • Comparing the maps = find damaged pins, shorts, and blown inputs. Example: One pin reads 1.2kΩ to GND normally, but reads 12Ω now? That's a problem.

29. Check for Stuck GPIO or Pull-Up Issues

• If a board is partially working: ◦ A shorted GPIO can stop boot. ◦ Missing or shorted pull-up resistors can break I2C, SPI, or UART lines. Scope or logic analyzer is useful here to see dead data lines.

30. Using an Oscilloscope or Logic Analyzer

If power is okay but behavior is weird: • Probe key lines (like clock, reset, data) and look for: ◦ Missing or stuck-low/high signals ◦ Clock lines that don't oscillate ◦ Data lines that chatter once and stop A silent clock often means a bad crystal, dead oscillator, or bad power to the IC.

MINDSET TIPS:

Understand Component Roles: • Large cap near connector? Likely a filter cap — test it. • Three-pin part with a big pad? Likely a regulator or MOSFET. • Identical SMD chips? Often IO expanders, drivers, or RAM. Keep a "Component Zoo": • Salvage and label parts from dead boards to get used to markings. • Study pinouts — learn patterns across manufacturers (like 1117, AMS1117, etc.)

Final Tip: Build a "Test Once" Flow

• Visual check • Diode + resistance sweep • Inject voltage & feel for heat • Remove/disconnect sections • Trace & map ICs • Reflow suspected BGA/QFN • Use thermal/IR/alcohol method

I. CORE MINDSET & THEORY

1. Understand Signal Flow • Inputs → Processing → Outputs • Power always comes in and branches out via regulators, inductors, and traces. • Chips are fed power, clocks, and I/O — if any one is missing, it may not work or short. 2. Everything is Either: • A power path • A data path • A clock/timing path • A protection path • Or supportive passives (filtering, biasing, etc.) Understanding the role helps predict failure modes.

II. DETECTION TECHNIQUES

1. Visual Inspection – Advanced • Use UV light: Reveals flux residue or corrosion • Use oblique lighting: Helps see lifted pins or bent components • Polish pads with alcohol to uncover burnt marks 2. Multimeter Tricks • Diode mode reverse-engineering: Map IC pins • Measure known-good voltages: ◦ USB = 5V ◦ LDOs = 3.3V / 1.8V ◦ DRAM rails = often 1.1V or 1.2V • Lift one pin of suspected shorted caps to test without interference 3. Ohmmeter Decay Test • Measure ohms from VCC to GND and wait. • If value slowly climbs, there are charging caps = likely no dead short. • If value is static and low, a hard short exists.

III. HEAT-BASED DETECTION

1. Alcohol Evaporation (Isopropyl) • Apply IPA with a brush, inject voltage, and watch where it dries first — that's the heat. 2. Thermal Camera Tips • Set scale low (~20–80°C) to detect small heating • Compare with known good board if available 3. Finger & Knuckle Touch • Use the back of your finger or knuckle — less sensitive to pain but picks up warmth.

IV. POWER INJECTION TECHNIQUES

1. Bench Power Supply • Inject 1–2V at 1–3A into the shorted rail • Monitor: ◦ Heat ◦ Current draw ◦ Behavior changes Inject from large cap pads or inductor legs near the CPU/PMIC. 2. USB Cable With Resistor Hack • Cut USB cable, insert 5W 1Ω resistor in line • Observe voltage drop & heat • Use as a low-cost power injection method

V. SIGNAL & COMPONENT ANALYSIS

1. Oscilloscope Probing • Look for clocks (12MHz, 24MHz, etc.) • Stuck I²C lines • Power ripple (should be low) 2. Logic Analyzer • Capture boot sequences • Compare signal chatter on I2C/SPI/UART lines • Useful for checking reset loops or dead MCUs

VI. FAULT TYPES TO LOOK FOR

Shorts • Cap to GND • BGA solder bridge • Via to ground plane • Bad TVS diode or USB chip Opens • Broken trace under solder mask • Hairline cracks (common in flexed boards) • Cold solder joints Leaky Components • MOSFET with body diode damaged • LDO regulator leaking voltage backward • Electrolytic caps conducting slightly

VII. TOOLBOX (Pro + DIY)

Tool Use Multimeter Basics – continuity, resistance, diode mode Bench PSU Power injection with current limiting Thermal camera Non-contact heat detection Hot air rework station Remove or reflow ICs Oscilloscope Signal quality, power ripple, clocks Logic analyzer Protocol decoding LCR meter Test passives in/out of circuit ESR meter Check electrolytic caps Audio probe Follow signal injection through audio or digital path UV flashlight Spot corrosion and residue Logic analyzer Protocol decoding LCR meter Test passives in/out of circuit ESR meter Check electrolytic caps Audio probe Follow signal injection through audio or digital path UV flashlight Spot corrosion and residue

VIII. COMPONENT BEHAVIOR (KEY SIGNS OF FAILURE)

Component Common Failure Symptom MLCC caps Crack, short Board won't power, gets hot Electrolytics Dry out, bulge High ESR, ripple, boot issues LDO/Regulators Short, no output Missing voltage rail MOSFETs Shorted gate-drain-source Always on/off, heating Crystals/Oscillators Dead, cracked No boot, no clock signal TVS diodes Shorted USB/power shorts Microcontrollers Short inside One hot pin, high current draw

IX. REVERSE ENGINEERING TRICKS

1. Make a Map • Take clear photos • Trace nets with colored lines (in photo editor or marker) • Label known VCC/GND pins from datasheets 2. Use IC Datasheets for Guidance • Pin 1 = VCC, GND, IN, OUT — gives you starting points • Use VCC pin to test rail voltage 3. Check for Pull-ups & Series Resistors • Pull-up resistors common on: ◦ I²C/SPI lines ◦ RESET lines ◦ Enable pins Knowing their presence helps you test logic states.

X. TRAPS & PRO TIPS

• Dead short ≠ component fault — sometimes PCB layer or reflow problem • Heated area may not be the cause — sometimes power path heats from loading • Don't overlook connectors — bent pins or solder bridges inside! • If something gets hot in <1s, it's likely the fault • Compare to known-good boards if possible (resistance map, voltage map)

XI. WHEN YOU'RE STUCK

• Remove power-hungry ICs one at a time (PMIC, RAM, USB IC) • Cut power rails strategically and re-test • Let it sit — come back fresh, re-test, re-measure • Ask online communities: Post clear photos, part numbers, and steps tried

BONUS: Know Your Board Type

Board Type Common Fault Focus Phones/Tablets Game Controllers PMIC, charging ICs, BGA shorts, water damage USB chips, ESD diodes, analog sticks, MCU shorts Motherboards VRMs, RAM rails, PCH/CPU shorts Embedded boards (Arduino etc.) Regulator faults, USB power issues ]