Mobile Phone IC For 77638-21-RF Original
The SKY77638-21 (often referred to by its board marking 77638-21) is a high-performance Multimode Multiband (MMMB) Power Amplifier Module (PAM) from Skyworks Solutions. Part of the SkyLiTE™ family, it is designed for global 3G/4G smartphones, providing the necessary signal amplification for LTE, WCDMA, and CDMA networks.
It is a well-known component in the repair industry, serving as a critical network IC for devices such as the Samsung Galaxy S9 and various Xiaomi Redmi models (e.g., Redmi 6/6A).
Technical Specifications: SKY77638-21
| Feature | Specification Detail |
| Model Number | SKY77638-21 (77638-21) |
| Manufacturer | Skyworks Solutions, Inc. |
| Component Type | Multimode Multiband Power Amplifier Module (PAM) |
| Network Support | Quad-band GSM, WCDMA, HSPA+, FDD-LTE, TDD-LTE |
| Frequency Range | 700 MHz to 2700 MHz (Wideband support) |
| Output Power | 28.5 dBm (Typical for LTE/WCDMA) |
| Voltage Supply ($V_{CC}$) | 3.3V to 4.5V (Directly compatible with Li-ion batteries) |
| Interface | MIPI® RFFE (Radio Frequency Front End) 2.0 |
| Package Type | 42-pad MCM (Multi-Chip Module) |
| Dimensions | 4.0 mm x 6.8 mm x 0.8 mm |
| Input/Output Match | Internally matched to $50\Omega$ |
Core Functional Roles
The SKY77638-21 is a “Smart” power amplifier that adapts to the specific needs of the device’s current connection.
1. Multimode Efficiency
This module integrates several power amplifier paths into a single chip. It includes a dedicated block for Low-Band (LB) signals (like LTE Band 5/8/20) and Mid/High-Band (MB/HB) signals (like LTE Band 1/3/7). By using InGaP GaAs HBT (Indium Gallium Phosphide Gallium Arsenide) technology, it achieves high efficiency, which helps reduce heat and preserve battery life during long data sessions.
2. MIPI® RFFE Control
Unlike older “dumb” amplifiers, the 77638-21 is fully programmable via a MIPI interface. This allows the phone’s modem to send digital commands to the IC to change power modes, adjust bias currents for better linearity, or switch frequency bands on the fly without needing dozens of external control wires.
3. Optimized for APT (Average Power Tracking)
The IC is designed to work in conjunction with an APT DC-DC converter. This system dynamically adjusts the voltage supplied to the amplifier based on the signal’s requirements. If the signal is strong, the voltage is lowered to save energy; if the signal is weak, the voltage is boosted to ensure a clear transmission.
Common Signs of a Defective 77638-21
Since this IC is responsible for the final “push” of the radio signal out to the cell tower, its failure usually results in a complete loss of cellular connectivity.
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“No Service” or “Searching” Permanently: If your device can see the SIM card but never finds a network, the 77638-21 may be unable to transmit the “handshake” signal to the tower.
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Weak Signal Strength: The phone only works very close to a cell tower and loses connection as soon as you step indoors.
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Network IC Overheating: Because it handles high current, a shorted 77638-21 will often become extremely hot, sometimes preventing the phone from turning on at all (VCC_MAIN short).
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Call Dropping: The phone has bars but immediately drops the signal as soon as you try to place a voice call or start a data-heavy download.
Professional Repair & Installation Notes
Replacing the SKY77638-21 is an advanced micro-soldering task due to its 42-pad BGA/MCM design and its proximity to other sensitive components.
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Heat Management: Use a hot air station at $335\degree\text{C}$ to $345\degree\text{C}$. This chip has a large ground pad in the center that requires significant heat to melt, but the surrounding internal die is sensitive to prolonged exposure.
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Clean the Ground Pad: Most failures in replacement are due to “floating” chips. Ensure the large central ground pad on the motherboard is perfectly flat before placing the new IC.
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Orientation: Align the Pin 1 marker (a small laser-etched dot) exactly with the board’s silk-screen markings. An inverted installation will short the battery line and likely kill the IC instantly.
