LG G4 Circuit Schematic PDF Download and Hardware Repair Guide
For technicians repairing the LG G4, the power management IC (PMIC) is the first critical component to examine. Located near the micro-USB port on the PCB, this chip (marked PM8916) fails frequently due to liquid damage or overheating. Check for corrosion under the IC–clean with isopropyl alcohol (99% purity) and reflow with a hot air station at 350°C for 30–45 seconds. If the device powers on but fails to charge, replace the PMIC or surrounding capacitors (common values: 0.1µF, 1µF, 10µF).
The CPU (MSM8992) sits at the heart of the board, connected to the PMIC via banded copper traces. If the phone boots but reboots randomly, inspect these traces for micro-cracks–use a multimeter in continuity mode to verify connections. A common failure point is the BGA solder balls under the CPU; reballing requires a stencil template (0.4mm pitch) and flux (AMTECH NC-559). Avoid excessive heat–limit reflow to 380°C for 60 seconds to prevent PCB delamination.
Power delivery issues often stem from the charge IC (SMB1357), positioned adjacent to the battery connector. Measure voltage at the input (VBAT)–it should read 4.2V ± 0.1V. If voltage drops below 3.8V, replace the IC or check the QFET (AO8810) for shorts. For no-display errors, test the LDO regulators supplying the LCD (outputs: 1.8V, 2.8V)–faulty regulators cause black screens even with working backlight.
Memory failures manifest as boot loops or corrupted storage. The eMMC (Samsung KLMBG4WEAC-B031) is soldered near the CPU. If the device enters Qualcomm CrashDump mode, the eMMC is likely dead–replace it with an identical model (part number must match). For soldering, use a preheater at 180°C to avoid thermal shock. Verify trace connections post-replacement with a USB jig–incorrect alignment causes black screen of death.
Wi-Fi/Bluetooth issues often trace to the WCN36x0 combo chip. Check the antennas (ANT1, ANT2) for physical damage or loose connections. If the signal drops intermittently, resolder the chip with flux core solder (Sn63Pb37)–lead-free alternatives cause cold joints. Measure impedance at the antenna ports (50Ω ± 5Ω); deviations indicate a faulty chip or broken traces.
Always document modifications using PCB photos before disassembly–mark tested components with a heat-resistant label to avoid confusion. For advanced diagnostics, flash a stock firmware (LG-H81820a_00) via QFIL Tool to rule out software corruption. Keep a backup of the EFS partition–losing it bricks IMEI and network connectivity.
LG G4 Circuit Layout: Step-by-Step Diagnostic Walkthrough
Locate power management IC (PMIC) U1001 on the board’s reverse side–its output pins (VSW, VOUT_LDO) must measure 3.8V (±0.1V) when probed with a multimeter in DC mode. If readings drop below 3.5V, check surrounding caps C1002–C1007 for micro-shorts; replace all six if ESR exceeds 0.2Ω. For charging issues, bridge TP4001 (charging coil output) to ground–expected resistance: 220–280Ω. Values outside this range confirm coil damage or corroded pads under connector J1100.
Critical Test Points & Expected Values
| Component | Test Point | Expected Reading | Fault Indication |
|---|---|---|---|
| PMIC (RT5058) | Pin 5 (VOUT) | 3.8V DC | 3.2V or lower = failed boost circuit |
| Baseband (MSM8992) | TP9001 | 1.8V DC | No voltage = corrupt firmware, reflash |
| Flash Memory (H9TQ63ABJT) | Pin 29 (CLK) | Square wave 50MHz | Flatline = shorted data line, replace memory |
| Charging Port (J1100) | Pin 1 (VBUS) | 5V DC | Below 4.5V = liquid ingress, clean with IPA 99% |
Backlight driver IC (LM3630) sits near the left speaker–pins 17–18 should output 12V when screen brightness is set to max. If voltage is absent, probe inductor L2001; resistance above 1Ω signals an open coil. For touchscreen failures, measure resistance between connector J7001 pins 4–5 and 8–9–values should match within 5kΩ. Discrepancies above 10kΩ require flex cable replacement. Use a preheater set to 120°C to avoid pad delamination when reballing BGAs.
Identifying Key Components in LG G4 Motherboard Layout
Begin by locating the MSM8992 processor near the center of the board–marked “U0” in reference files. Use a 10x magnifier to trace power lines (VDD_CORE, VDD_CX) fanning out to surrounding capacitors (C1xxx series). These components cluster within a 5mm radius and fail most frequently due to thermal stress; replace burned-out capacitors with 0402-sized 1µF 6.3V X5R variants before testing.
Critical Interfaces and Their Markers
- PMIC (PM8916): Check the 4x4mm chip labeled “U200” for oxidation on BGA balls–reball if resistance exceeds 10Ω on PP1_5V lines.
- eMCP (K4F8E304HB): Verify continuity on the 12×16 ball grid; shorted pins (especially DQ0-DQ7) correlate with 90% of boot failures.
- RF Section: QFE1100 (L400) amplifies GSM bands–test with spectrum analyzer starting at 824MHz; peaks below -70dBm indicate damaged filters.
Power sequencing requires exact voltage ramp times: PP3V0 init (1.2ms), PP1V8 (0.8ms), PP_GPU (2.1ms). Deviations over ±15% trigger emergency shutdown–measure with oscilloscope probe tips clipped to TP901 (VDD_CORE) and TP902 (PMIC_OUT), using ground spring on the USB shield. Replace the APT1608 choke (L301) if DC resistance surpasses 0.3Ω, as this disrupts current delivery to the CPU core during wake-from-sleep cycles.
Step-by-Step Tracing of Power Delivery Circuits on the LG G4 Board Layout
Locate the battery connector first–marked as J1–on the upper-right section of the reference. Follow pin 1 (VBATT) to the main power switch, typically labeled PMIC_USB_SW. Verify continuity with a multimeter set to diode mode, ensuring no voltage drop exceeds 0.2V from the battery terminal to the PMIC input. If resistance reads open, inspect the coil L9 and capacitor C178 for solder cracks or oxidation.
Trace the output of the primary buck converter–check the inductor labeled L28–which feeds the VDD_MAIN rail at 3.8V. Probe the test point near R342 to confirm stable voltage; fluctuations above ±5% indicate a failing QFN32 PMIC or damaged C52 input cap. Replace C52 if ESR exceeds 20mΩ; use a 10µF 6.3V X5R ceramic for direct substitution.
Move to secondary rails: identify LDO1 via the silkscreen near U12. Its output VDD_1P8 powers the application processor. Probe TP903 while booting; a sudden dip below 1.7V signals a defective LDO or excessive load from the CPU. Desolder U12 and test off-board with a known-good 1.8V source–failure confirms internal die damage requiring IC replacement.
Inspect the charging path by backtracking from the USB connector J2. Check the diode D15 thermal pad for overheating; if discolored, replace with an SS14 schottky. Monitor CHG_IN node for ripple above 50mV–excessive noise points to a dried C28 input cap. Confirm charge current via R501 shunt resistor; expect 0.5A at 5V input–lower readings suggest a saturated Q5 MOSFET or degraded BQ24296 charger IC.
Common Fault Points in LG G4 Charging System and Diagnostic Tips
Check the USB charging port first–corrosion on pins or bent connectors are frequent failures. Use a microscope to inspect for micro-fractures or debris blocking contact surfaces. Replace the port if oxidation is visible, as cleaning often provides only temporary relief. Measure resistance between the port’s data lines and ground; values above 1-2 ohms indicate a compromised connection.
Test the battery’s thermistor resistance with a multimeter in diode mode–normal readings should be 50-150kΩ at room temperature. Values outside this range suggest a faulty battery or damaged flex cable. LG G4 batteries often fail due to swelling; always inspect for bulging or venting before proceeding. If the thermistor checks out, probe the PMIC (power management IC) for stable 3.7-4.2V output during charging–fluctuations point to regulator failure.
Examine the charging IC (typically a Qualcomm PM660 or similar) for cold-solder joints or burn marks. Use a thermal camera to detect abnormal heat signatures during charging; excessive warmth near the IC indicates internal shorting. Reflow or replace the IC if resistance tests show inconsistent readings across its pins. Check adjacent capacitors–especially the 10µF and 22µF components–for leakage or short circuits, as these often fail silently.
Trace the power path from the USB port to the battery connector using a reference layout. Common drop-off points include the fuse near the port (resistance should be near 0 ohms) and the inductor coil (typical 1-3 ohms). If charging stalls at 5%, the problem often lies in the PMIC’s over-voltage protection circuit; bypassing this temporarily can confirm the diagnosis. For intermittent charging, flex the PCB near the charging IC while monitoring voltage–micro-cracks in traces are a frequent issue.
Use a known-good charging module to isolate faults. If the device charges with a substitute part, focus on the original component’s vias and traces. Measure voltage at test points TP4001 (USB 5V input) and TP4002 (battery connector) simultaneously–the difference reveals where power is lost. LG G4 PCBs commonly develop broken traces near the EMI filters; rework these with 38AWG jumper wire for a permanent fix.
Log charging behavior with a USB power meter. Abnormal current draws (e.g., 0mA, >3A, or rapid cycling) suggest faulty battery authentication or damaged fuel gauge data. Erase and reflash the battery EEPROM if the device fails to recognize power inputs. For no-power symptoms, check the power button flex–shorts here can mimic charging failures by cutting off the PMIC’s enable signal.