UPMSP High School Mobile-Phone Repair - Code 980 cheat sheet

Electric shock: 50 V AC or 120 V DC is the usual danger threshold; dry skin resistance is ~100 kΩ but drops to ~1 kΩ when wet, so current (not voltage alone) causes injury.

Li-ion cells: nominal 3.7 V, charge to 4.2 V, cut-off 3.0 V; puncture, crush or overcharge causes thermal runaway - store swollen batteries in a metal container away from combustibles.

Soldering-iron tip reaches 300–400 °C; use a stand, never leave the iron on the bench, and unplug before changing tips.

Lead-free solder (SAC305) fumes are flux fumes, not lead vapour; extract with a fume fan and wash hands after handling leaded solder.

ESD control: wrist strap through 1 MΩ to earth, conductive mat, ioniser for plastics; a charged human body can reach 15 kV and destroy MOSFET gates.

Short-circuit current of a phone battery can exceed 5 A; remove the battery before probing the board and never bridge B+ and B− with a meter lead.

Fire response: CO2 or dry powder for electrical fires; never water on live equipment. Class D not needed for electronics.

First aid: isolate supply before touching a casualty; begin CPR if breathing has stopped and call emergency services.

Essential kit: digital multimeter, DC power supply with current limit, hot-air rework station, temperature-controlled iron, tweezers, ultrasonic cleaner, magnifier or microscope.

Multimeter in continuity mode reads <1 Ω as a beep; a short on a power rail typically reads under a few ohms, an open circuit reads OL.

DC supply current limit for first power-up: set 0.3–0.5 A so a shorted board does not burn traces; a healthy idle phone draws 5–50 mA before boot.

Hot-air station: 300–380 °C at 3–6 air flow for SMD removal; preheat board to ~120 °C to reduce thermal shock.

Tweezers must be non-magnetic and ESD-safe; ceramic or plastic tips for coil and shield work to avoid shorting adjacent pads.

Ultrasonic cleaner: use 40–60 kHz with distilled water or IPA-safe solution for 2–5 minutes; remove camera, LCD and speaker first.

Torque discipline: use the correct pentalobe, Phillips and Torx bits; a stripped screw costs more time than the right driver.

Keep a job card per device: model, IMEI, fault reported, parts used, technician and date - it is the record used in disputes.

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. A 1 kΩ resistor across 5 V dissipates 25 mW.

Series: R_total = R1 + R2; parallel: 1/R_total = 1/R1 + 1/R2. Two equal resistors in parallel give half the value.

Capacitor impedance Xc = 1/(2πfC); it blocks DC and passes AC, which is why a decoupling capacitor sits across each supply rail.

Diode drop: ~0.7 V silicon, ~0.3 V Schottky; a diode reads OL one way and a junction voltage the other in diode-test mode.

BJT: base-emitter ~0.7 V forward; MOSFET: gate is insulated, so a gate-source short or open is found by resistance, not diode drop.

Inductor impedance XL = 2πfL; in a boost converter it stores energy that is dumped into the output capacitor to raise voltage.

Battery capacity: mAh ÷ 1000 = Ah; runtime ≈ capacity (Ah) ÷ average load current (A). A 3000 mAh cell at 0.5 A lasts about 6 h.

Ground reference: measure all rails against the main ground pad, not the shield can, which may be isolated by an inductor.

Block chain: battery → PMIC → CPU/SoC → RAM and eMMC/UFS → display, camera, modem, audio codec, sensors.

PMIC generates the rails: VBAT input, then buck converters (VDD_CPU, VDD_GPU) and LDOs (VDD_IO, camera rails).

Charging path: USB/VBUS → charger IC → battery; the charger IC also reports battery temperature via an NTC thermistor.

Display interface: MIPI DSI; touch is MIPI I2C or SPI. No display but working touch (or vice versa) points to a separate flex or connector.

Camera: MIPI CSI lanes plus I2C control; a dead camera on one module only usually means the module or its connector, not the SoC.

RF front end: antenna → switch/duplexer → PA → transceiver; a missing band usually traces to the PA or antenna switch, not the baseband.

Sensors: accelerometer, gyroscope, magnetometer, proximity and ambient light; they share an I2C bus, so one dead sensor can pull the bus down.

Water-damage indicator turns red on contact with liquid; corrosion under shields is the usual cause of intermittent faults after a drop or spill.

Boot chain: boot ROM → bootloader (fastboot/download mode) → kernel → Android/Linux init → system services; a boot loop can stop at any stage.

Fastboot commands run from a PC over USB: fastboot devices, fastboot flash, fastboot reboot; download mode is the vendor equivalent on many chipsets.

Flashing wipes user data when the firmware package includes userdata; back up before flashing and record the exact firmware build.

FRP (Factory Reset Protection) locks a device to the previous Google account after a reset; only the account credentials or an authorised unlock remove it.

Bootloader unlock voids warranty on most brands and trips a hardware fuse or e-fuse; check the vendor policy before unlocking.

OTA update failure is usually low storage or an interrupted download; clearing cache or re-downloading the package resolves most cases.

Baseband and IMEI live in a protected partition; flashing a full firmware that overwrites it without a backup can leave the phone with no network.

Custom recovery (TWRP-type) allows partition backup and restore; verify the recovery matches the exact model and chipset before flashing.

Start with the complaint, then reproduce it; check charger, cable, SIM and SD before opening the phone - many 'faults' are accessories.

Divide-and-conquer: measure VBAT at the battery connector, then each PMIC rail; the first missing rail localises the fault block.

Current-draw signature on a DC supply: 0 mA = open path; a few mA and no boot = standby fault; >0.5 A with heat = short.

No power: check battery voltage, then the charger IC and PMIC input; a swollen or deeply discharged cell (below 3.0 V) may need a slow pre-charge.

No charging: test cable, port, charger IC and battery NTC; a port with bent pins or a broken data line fails both charge and USB.

No display: test with a known-good LCD, then check the display connector, backlight boost and DSI lines; a torch-light test separates backlight from panel.

No network: check antenna spring contacts, SIM reader, then the RF PA; a phone that sees SIM but no signal usually has a PA or antenna fault.

Intermittent faults after a drop: reflow or reball the suspect BGA, check for cracked solder joints under shields and loose connectors.

Iron temperature: 320–360 °C for leaded, 350–380 °C for lead-free; contact time under 3 s per joint to protect pads.

Flux is essential: it removes oxide and improves wetting; clean residue with IPA and a brush after rework.

Hot-air removal: heat the whole component evenly, lift with tweezers only when solder is liquid; never force a cold joint.

BGA rework needs preheat from below (100–150 °C), top heat to ~220–230 °C, and a reball or replacement stencil for the new chip.

Solder wick plus flux removes bridges; add flux, lay wick, press with the iron and lift when the braid turns silver.

Tombstoning on a two-pad part means uneven heating or unequal pad solder; rework with hot air and re-align.

Cold joint looks dull and grainy and reads high resistance; reflow with flux rather than adding more solder.

Underfill and glue around shields must be cut before lifting; pulling a glued shield lifts pads and destroys the board.

Cellular generations: 2G GSM (voice/SMS), 3G UMTS, 4G LTE (all-IP data and VoLTE), 5G NR (sub-6 GHz and mmWave).

SIM is a smart card with ICCID and IMSI; the IMSI identifies the subscriber to the home network, the ICCID identifies the card.

IMEI is a 15-digit device identity: 8-digit TAC, 6-digit serial, 1 check digit (Luhn). It is not the SIM and does not change with the SIM.

Changing or cloning an IMEI is illegal in most countries; a phone with a null or invalid IMEI will be refused by the network.

Network selection: the phone scans, attaches to the strongest permitted cell, then registers; 'emergency calls only' means no home or roaming agreement.

APN settings control mobile data; wrong APN gives voice but no internet - check APN, then data toggle, then SIM provisioning.

Wi-Fi and Bluetooth are 2.4/5 GHz ISM radios separate from cellular; a phone with no Wi-Fi but working cellular has a Wi-Fi/BT combo fault.

Signal bars show RSRP/RSSI, not throughput; a strong signal with slow data points to backhaul congestion or an APN/provisioning issue.

Written estimate before work: fault, parts, labour, warranty period and expected completion date, signed by the customer.

Warranty terms must be explicit: parts warranty (often 1–3 months) versus labour warranty; water damage and physical damage are usually excluded.

Data privacy: never browse customer photos or messages; wipe test data and record that the device was returned with data intact.

Pricing: cost of part + labour + overhead + margin; a 30–50% gross margin on parts and a fixed labour rate is a common shop model.

Inventory: keep fast-moving parts (screens, batteries, charge ports, flex cables) in stock; track serials for high-value modules.

GST/tax registration thresholds and invoicing rules depend on jurisdiction; issue a numbered invoice for every paid repair.

Record every device with IMEI at intake; an IMEI log protects the shop if a stolen handset is presented.

Marketing: Google Business profile, reviews and local referrals outperform paid ads for a single-location repair shop.

Practical exam routine: safety check, tool check, fault confirmation, repair, functional test, documentation - each step is marked.

A viva answer must name the tool and the measurement: 'I measured 3.8 V at the battery connector with the multimeter on DC 20 V range.'

Project report structure: title, objective, tools, procedure, observations, result, conclusion, and safety precautions.

Common viva questions: function of PMIC, difference between eMMC and UFS, why flux is used, what FRP is, how IMEI is structured.

Demonstrate soldering on a practice board: a good joint is shiny, concave and wets both pad and lead; a cold joint is dull and balled.

Test after every repair: power on, charge, call, camera, speaker, mic, Wi-Fi and touch - record pass/fail per function.

Keep a lab notebook with dated entries; examiners ask for evidence of practice hours and fault cases solved.

Clean the bench and return tools before the examiner signs off; housekeeping is part of the practical mark.

SoC integrates CPU, GPU, modem, ISP and memory controller; separate RAM and storage chips sit beside it on the board.

eMMC is a managed NAND with a controller; UFS is faster and full-duplex - a slow phone with full storage may have a failing eMMC.

Power rails are labelled on schematics: VBAT, VPH_PWR, VREG_L* for LDOs and VREG_B* for bucks; measure against ground.

Speaker and earpiece are driven by an audio codec plus amplifier; a dead earpiece with working speaker is usually the earpiece or its flex.

Vibrator motor is driven by a small driver IC from the PMIC; no vibration with working everything else points to the motor or driver.

USB port carries VBUS, D+/D−, ID and ground; a broken D+/D− allows charging but blocks data and flashing.

NFC antenna is a coil in the back cover or battery; a replacement cover without the coil disables NFC.

Thermal design: the SoC throttles above ~45 °C skin temperature; overheating during charging often means a failing battery or charger IC.

Recovery mode offers wipe data/factory reset, wipe cache and apply update from ADB or SD card; it does not need a PC.

Safe mode disables third-party apps; a phone that works in safe mode has a rogue app, not a hardware fault.

ADB (Android Debug Bridge) needs USB debugging enabled; adb devices, adb logcat and adb shell are the core diagnostic commands.

Logcat shows kernel and app errors; a repeating 'FATAL EXCEPTION' names the crashing app or service.

A hard brick shows no boot, no fastboot and no charging icon; recovery needs an eMMC/UFS programmer or a vendor service tool.

Firmware must match the exact model code (for example, a regional variant); a mismatched package causes a boot loop or dead radio.

Verify the firmware checksum before flashing; a corrupted download is a common cause of a failed flash.

After flashing, the first boot can take 5–15 minutes while caches rebuild; do not power off during this window.

Thermal diagnosis: touch or use a thermal camera after power-up; a hot spot within seconds identifies the shorted component or rail.

Voltage-injection method: feed a current-limited supply into the shorted rail and find the warming component; keep the limit low (0.5–1 A).

Diode-mode test on rails: a healthy rail reads a few hundred mV to ground; 0 mV means a hard short, OL means an open.

Battery calibration: a phone that dies at 30% may have a worn cell; check cycle count and internal resistance if the tool supports it.

Audio faults: test mic, earpiece, loudspeaker and headset separately; the audio codec and its flex are the usual suspects.

Touch faults: a ghost-touch screen is often a bad digitizer, a swollen battery pressing the panel, or a poor ground on the frame.

Charging-port faults: wiggle test the cable; intermittent charge points to worn port pins or cracked solder on the port.

Document the fault tree you followed; the examiner and the customer both need the reasoning, not just the result.

SMD sizes: 0402 is 1.0 × 0.5 mm, 0201 is 0.6 × 0.3 mm; use a microscope and fine tweezers below 0402.

Drag soldering with flux and a hoof tip works for fine-pitch ICs; clean bridges with wick afterwards.

Hot-air nozzle size should match the component; too large a nozzle blows nearby parts off the board.

Preheat the board to 100–150 °C before hot air to reduce warping and delamination of the PCB.

BGA reball: clean pads, apply flux, align stencil, spread solder paste or spheres, reflow, then inspect for uniform spheres.

Do not reuse a BGA chip without reballing; leftover solder balls cause bridges and cold joints.

Measure a reworked joint with continuity to the next test point; a joint that looks good can still be open.

Conformal coating and underfill must be removed before rework and reapplied after for water resistance.

Frequency bands: LTE bands are numbered (B1 2100 MHz, B3 1800 MHz, B5 850 MHz, B40 2300 MHz); a phone must support the carrier's band.

VoLTE carries voice over LTE as IP packets; without VoLTE the phone falls back to 2G/3G for calls, which matters as 2G/3G are shut down.

SIM card sizes: standard, micro and nano; the nano SIM is 12.3 × 8.8 mm and is the current standard.

eSIM is soldered into the device and provisioned by a carrier profile download; it cannot be swapped between phones like a plastic SIM.

Roaming requires a roaming agreement; 'no service' abroad with a valid SIM usually means roaming is disabled or no agreement exists.

IMEI check digit uses the Luhn algorithm; a failed check digit means the IMEI was mistyped or altered.

Wi-Fi channels: 2.4 GHz uses channels 1–13 (1, 6 and 11 are non-overlapping); 5 GHz has many more non-overlapping channels.

Bluetooth pairing failure is often a stale pairing list; remove the device on both sides and re-pair.

Intake checklist: device model, IMEI, accessories received, visible damage, passcode policy and customer signature.

Turnaround promise: quote a realistic time; a missed promise costs more reputation than a higher price.

Refurbished versus new parts: state which is fitted and price accordingly; customers accept refurbished if told in advance.

Record parts serial numbers for screens and batteries; this supports warranty claims against the supplier.

Cash-flow rule: keep at least one month of rent and wages as reserve; parts inventory ties up capital.

Staff training: one certified technician per shift plus a front-desk person who can triage faults and quote prices.

Data handling: ask the customer to back up and sign a data-loss disclaimer before any board-level repair.

Feedback loop: log every comeback repair and its root cause; repeat faults point to a supplier or a process gap.

Practical marks usually split into safety, tool use, fault finding, repair quality, testing and documentation; know the split before the exam.

Demonstrate the multimeter on the correct range: DC volts for rails, continuity for tracks, diode mode for junctions.

Show the DC supply current limit setting before powering a board; examiners check this as a safety item.

Viva: explain why a phone with a shorted rail is powered through a current-limited supply rather than the battery.

Project idea examples: build a fault-diagnosis chart for a model, compare two battery brands, or document a screen replacement.

Report writing: state the observation, not the opinion - 'battery read 3.2 V, phone did not boot' rather than 'battery was bad'.

Keep photographic evidence of before, during and after repair; it supports the report and the viva.

Final check: all functions tested, device cleaned, screws torqued, customer/job card signed and dated.

Battery swelling is a hazard, not a cosmetic issue: isolate the cell, do not charge it, and dispose of it through an authorised e-waste channel.

Solvent safety: IPA is flammable (flash point ~12 °C); keep it away from hot-air and iron, and use it in a ventilated area.

Eye protection is required when clipping leads, using compressed air or cleaning with solvents.

Never work on a powered board with wet hands or on a wet bench; water plus mains-powered tools is the main shock risk.

Lockout: unplug the iron and hot-air station at the end of the shift, not just switch off at the button.

Compressed air can drive metal swarf into eyes and skin; use a nozzle with a safety tip and short bursts.

Class C extinguisher (CO2) is preferred for electronics; a water extinguisher damages the stock and risks shock.

Report every injury and near miss in the shop log; the log is evidence of a safe system of work.

DC power supply leads: use fine probes with a series resistor or current limit to avoid dumping current into a shorted rail.

Multimeter accuracy: a cheap meter may read 0.1–0.2 V low; check against a known source before trusting a rail measurement.

Microscope or loupe at 10–20× is the minimum for inspecting BGA joints and fine-pitch connectors.

Hot-air station airflow is measured in a arbitrary scale (1–8); start low and raise only if the part will not release.

Keep a known-good battery and a known-good screen of each common model as test jigs.

Isopropyl alcohol 99% is the standard cleaner; 70% leaves water and causes corrosion under shields.

Screwdrivers must fit the screw head exactly; cam-out damages the head and the surrounding plastic.

Calibrate the iron tip temperature with a thermocouple once a month; a drifted station causes cold joints.

Kirchhoff's laws: current into a node equals current out; the sum of voltages around a loop is zero.

Voltage divider: Vout = Vin × R2/(R1 + R2); used for battery sensing and thermistor networks.

RC time constant τ = R × C; a 10 kΩ and 1 µF pair gives τ = 10 ms, which sets the reset delay.

Zener diode regulates by reverse breakdown; it holds a fixed voltage when biased above its knee.

Op-amp basics: virtual short between inputs, gain set by feedback resistors; used in sensor signal conditioning.

ESR (equivalent series resistance) rises in aged capacitors; a bulging or high-ESR capacitor causes unstable rails and resets.

Continuity is not the same as low resistance: a beep means under the meter threshold, not necessarily a good connection.

Battery internal resistance rises with age; a cell above ~150 mΩ in a phone pack usually needs replacement.

Display types: LCD needs a backlight; OLED emits per pixel, so a black screen with working touch may be a panel or driver fault.

Touch controller sits on the display flex or a separate IC; a cracked digitizer can show image but no touch.

Charging port flex often carries the microphone and antenna contacts; replacing it can fix mic and signal faults together.

Front camera and proximity sensor share a flex on many models; a dead proximity sensor blanks the screen during calls.

Antenna contacts are spring pins on the board or flex; dirty or bent pins cause weak signal and dropped calls.

NFC and wireless charging coils sit in the back cover; a non-original cover without them disables those features.

Speaker mesh and gasket seal the acoustic chamber; a missing gasket causes muffled or distorted sound.

Board layers: many phones use a multi-layer PCB with blind vias; track routing is not visible, so schematics are essential.

Developer options enable USB debugging, stay-awake and background process limits; they are the first stop for software diagnosis.

Battery stats can be reset from recovery; a wrong stat causes the phone to shut down early or charge slowly.

Google account removal before factory reset avoids FRP lock; a reset without removal leaves the phone locked.

OTA packages are signed; a modified system partition blocks the update and can cause a boot loop.

Fastboot oem unlock is vendor-specific; some brands require an unlock code and a waiting period.

A boot loop after flashing is often a mismatched boot image or vbmeta; reflash the full matching package.

Backup before flashing: contacts, photos and chat backups are the customer's data, and the shop is responsible for it.

Check storage free space before an OTA; most updates need 2–5 GB free and fail silently if short.

Rule out the accessory first: swap charger, cable, SIM, SD card and battery before opening the device.

Compare with a known-good board of the same model when the fault is unclear; substitution isolates the block quickly.

Measure the battery connector voltage with the charger connected; a rise toward 4.2 V shows the charger IC is working.

A phone that charges but shows no battery percentage may have a failed battery NTC or a bad fuel-gauge line.

Overheating in standby with no app running points to a shorted rail or a stuck wake-lock; check current draw first.

No sound on calls but media sound works points to the earpiece or its flex, not the codec.

Camera black screen with working flash points to the camera module or its connector, not the SoC.

Record the fault tree and the measurements in the job card; this is the evidence used in a comeback dispute.

Tip selection: chisel for pads and drag soldering, conical for fine pins, hoof for multi-pin ICs.

Solder wire diameter: 0.3–0.5 mm for phone work; thicker wire floods fine pads.

Wetting angle: a good joint has a low contact angle; a ball sitting on the pad is not wetted.

Desoldering braid must be fluxed; dry braid lifts pads and leaves a rough surface.

Hot-air reflow of a connector: heat the whole connector, not one pin, or the plastic body warps.

Shield cans are soldered on four sides; cut the tabs with a blade or use hot air and low-melt alloy to lift without pad damage.

Low-melt (Wood's metal) alloy can be used to lower reflow temperature on heat-sensitive parts; clean it off completely afterwards.

Inspect rework under magnification and test continuity before reassembling the phone.

SIM reader contacts: VCC, RST, CLK, I/O and GND; a missing clock or reset line gives 'SIM not detected'.

SIM PIN and PUK: three wrong PIN entries lock the SIM; ten wrong PUK entries block it permanently and a replacement is needed.

Network modes: 2G/3G/4G/5G auto versus locked; a phone locked to 3G may show no service after a 3G shutdown.

IMEI is reported to the network at attach; a blacklisted IMEI is refused even with a valid SIM.

Carrier aggregation combines LTE bands for speed; a phone missing a band still works but slower.

Wi-Fi calling (VoWiFi) needs carrier provisioning and a valid address for emergency calls; test it before promising it.

Mobile data roaming is a separate toggle from voice roaming; a customer abroad may have voice but no data.

Signal strength in dBm: −50 dBm is excellent, −100 dBm is marginal, below −110 dBm is usually no service.

Quote in writing and get approval before ordering a part; an unapproved part is a loss the shop carries.

Deposit policy: take a deposit on special-order parts; it protects against a customer who does not return.

Warranty exclusions must be listed on the invoice: liquid damage, physical damage, unauthorised repair and software modification.

Track repair status on a board or app; a customer who calls three times for status is a lost referral.

Supplier management: keep two suppliers per fast-moving part to avoid a single-source delay.

Break-even: fixed costs ÷ (average repair price − average part cost) gives the number of repairs needed per month.

Staff pay: a base plus a share of repair revenue aligns the technician with turnaround and quality.

Local marketing: a Google Business profile with photos, hours and reviews is the highest-return channel for a repair shop.

Practical sequence: identify the model, record the IMEI, confirm the fault, open safely, diagnose, repair, test, close, document.

Opening a phone: heat the adhesive to 60–80 °C, use a suction cup and pick, and never pry near the battery.

Battery replacement: discharge below 25% before removal to reduce fire risk if the cell is punctured.

Screen replacement: test the new screen before sealing the adhesive; a sealed bad screen costs a second teardown.

Water-damage procedure: disconnect battery, strip the board, ultrasonic clean, dry, inspect under magnification, then reassemble and test.

Viva: be ready to explain the difference between a short and an open, and how you would find each on a phone board.

Project report must include the tools used, the measurements taken and the final functional test results.

Sign-off: customer signs the job card, receives the invoice and the warranty terms, and the device is photographed.