Socket 478
Socket 478, formally known as the mPGA478B, is a 478-pin micro pin grid array (mPGA) CPU socket developed by Intel for its NetBurst microarchitecture-based processors.[1] It was introduced in August 2001 as a replacement for the short-lived Socket 423, providing compatibility with desktop and mobile variants of the Pentium 4 and Celeron processor families. The socket employs a zero insertion force (ZIF) mechanism with a 50-mil pin pitch in a 26x26 array (including a 14x14 center cavity), supporting front-side bus (FSB) speeds of 400 MT/s, 533 MT/s, and 800 MT/s, along with processor clock rates ranging from 1.4 GHz to 3.4 GHz.[1][2] It remained in use until mid-2004, when Intel transitioned to the LGA 775 socket for subsequent generations.[3] The socket's design facilitated the integration of Intel's early 2000s processors, starting with the Willamette-core Pentium 4 at 1.9 GHz and 2.0 GHz models, followed by the more efficient Northwood cores that doubled L2 cache to 512 KB. Later compatibility extended to Prescott-core Pentium 4 processors (including the 3.4 GHz Extreme Edition variant) and corresponding Celeron models, all utilizing the FC-PGA2 package with voltages between 1.25 V and 1.525 V.[4][5] Key features included support for Hyper-Threading Technology (HT) on select higher-end models from 2002 onward, AGTL+ signaling protocol, and heatsink attachment via static load without external mechanisms.[1][6] Socket 478 motherboards typically paired with chipsets such as Intel's 845, 850, 865, and 875 series, enabling DDR or RDRAM memory configurations and features like AC'97 audio and USB 2.0.[7] Despite its prevalence in consumer systems during the early NetBurst era, the socket faced criticism for the underlying Pentium 4 architecture's power inefficiency and performance limitations relative to AMD contemporaries, contributing to its relatively brief lifecycle. By 2008, Intel fully discontinued support for Socket 478-compatible processors as part of the broader Pentium 4 end-of-life.[8]Introduction
Development and Release
Intel announced Socket 478 in August 2001 as the new packaging for its Pentium 4 processors, with initial support for later Willamette-core models and the subsequent Northwood core introduced in early 2002 based on the 0.13-micron process. This socket was designed to support the enhanced NetBurst microarchitecture, enabling higher clock speeds and improved performance over prior implementations.[9] The transition from the earlier Socket 423, used briefly for the initial Willamette-core Pentium 4 processors, was driven by the need for a more robust interface to handle evolving processor demands. Socket 423 proved inadequate for sustaining higher power requirements and signal integrity as clock speeds increased beyond 2 GHz.[10] The pin count rose to 478 to accommodate additional power and ground connections—specifically 85 VCC and 181 VSS pins—for better voltage regulation and distribution, alongside signaling enhancements like Assisted Gunning Transceiver Logic (AGTL+) with on-die termination to minimize noise and ringing.[9] First motherboards supporting Socket 478 became available in the first quarter of 2002, with widespread adoption occurring by mid-2002 as Intel phased out Socket 423 production.[11] Targeted primarily at desktop PCs in the mainstream and enthusiast segments but also supporting compatible mobile variants, the socket facilitated competitive performance against AMD's Athlon XP processors in consumer systems.Purpose and Design
Socket 478 was engineered as a surface-mount, Zero Insertion Force (ZIF) socket specifically for Pin Grid Array (PGA) processors based on Intel's NetBurst microarchitecture, which prioritized achieving high clock speeds through a deeply pipelined design rather than maximizing instructions per cycle (IPC).[1][12] The NetBurst architecture aimed to deliver industry-leading frequencies, targeting over 1.6 times the clock rate of the prior P6 family on equivalent process technology, enabling scalable performance for desktop computing tasks.[12] Key design features include a 478-pin layout arranged in a 26x26 grid with a 14x14 central cavity, facilitating improved trace routing on motherboards and enhanced distribution of power and ground connections via 85 VCC and 181 VSS pins.[1][13] The ZIF lever mechanism allows for tool-free processor insertion and removal with minimal force (under 4.5 kg), reducing wear and simplifying upgrades while supporting the 50-mil pin pitch for reliable electrical contact.[1] This configuration offered advantages over the short-lived predecessor Socket 423 by providing greater pin density and better electrical performance, such as lower contact resistance (≤25 mΩ for Kovar pins), to accommodate evolving processor requirements.[1] The socket's design incorporated an Integrated Heat Spreader (IHS) interface on compatible processors for efficient thermal coupling with heatsinks, addressing the heat dissipation needs of high-frequency operations in the NetBurst lineup.[13] It demonstrated scalability by supporting revisions like the Northwood core and select early Prescott variants on the 90 nm process, though later Prescott models transitioned to Socket 775 due to increased power demands.[13] Intended primarily for high-frequency desktop CPUs, Socket 478 included robust electrical specifications, such as low inductance (<3.3 nH) and capacitance (≤1.1 pF), that provisioned for overclocking through base clock (BCLK) adjustments in early unlocked models.[1][13]Technical Specifications
Physical Characteristics
Socket 478, also known as mPGA478, is a zero insertion force (ZIF) socket designed to mate with 478-pin pin grid array (PGA) processors, where the socket provides land grid array (LGA)-style contacts to receive the CPU's pins.[1] The socket features a rectangular grid measuring approximately 45 mm in length by 37 mm in width, excluding the actuation lever, with overall dimensions extending to 45 mm by 41 mm when including the lever. It accommodates 478 pins arranged in a 26 by 26 array pattern, featuring a 14 by 14 central cavity to facilitate the mechanical interface and signal routing. The pins operate on a 1.27 mm (50 mil) pitch, with two pin holes plugged in one corner to achieve the exact 478-contact count, and additional holes integrated for the retention mechanism to secure the processor.[1] The socket body is constructed from high-temperature thermoplastic material rated UL 94V-0 for flame retardancy and reflow compatibility during manufacturing. Contacts are made of high-strength copper alloy, plated with 0.762 µm of gold over 1.27 µm of nickel for enhanced conductivity and corrosion resistance, while the underside includes tin/lead solder balls (63/37 or 60/40 alloy) for surface-mount attachment to the motherboard. The socket height post-reflow ranges from 3.8 mm to 4.2 mm, ensuring compatibility with standard motherboard layouts and cooling solutions.[1]Electrical and Signaling
Socket 478 employs a variable core voltage supply ranging from 1.15 V to 1.75 V, determined by the processor's VID (Voltage Identification) pins, which communicate the required voltage to the motherboard's voltage regulator module (VRM) based on the specific CPU stepping and model.[14] This range accommodates the diverse power needs of Pentium 4 processors across generations, such as the higher voltages of ~1.75 V for early Willamette cores and lower ones of 1.25–1.40 V for efficient Prescott variants, ensuring stable operation without exceeding thermal limits. Additionally, the signaling termination voltage (VTT) is set at 1.05 V to support the front-side bus (FSB) interface, providing the necessary reference for I/O buffer circuits and minimizing signal reflections.[15] Power delivery in Socket 478 is facilitated through dedicated Vcc (core power) and Vss (ground) pins, totaling 85 Vcc pins and 179 Vss pins for a combined 264 power and ground connections out of the 478 total pins.[16] This extensive allocation enables robust current handling, with later Pentium 4 processors exhibiting a thermal design power (TDP) of up to 115 W, as seen in models like the 3.2 GHz and 3.4 GHz variants with Hyper-Threading.[4] The VRM design, adhering to Intel's VRD 10.0 guidelines, supports maximum currents up to 91 A in certain configurations to meet these demands while maintaining voltage tolerance bands of ±19 mV to ±25 mV.[14] Supported processors operate within a case temperature range of 0 °C to 70 °C, with maximum Tjunction up to 100 °C for later models.[16] The socket utilizes AGTL+ (Advanced Gunning Transceiver Logic Plus) signaling for its FSB interface, an open-drain protocol with active pull-up transistors that enhances signal integrity for high-speed data transfer.[1] AGTL+ supports FSB rates from 400 MT/s to 800 MT/s, allowing compatibility with processors operating at 100 MHz, 133 MHz, or 200 MHz base clocks quadrupled for effective throughput.[16] This signaling standard requires precise reference voltages (GTLREF at approximately 2/3 of Vcc) and 60 Ω trace impedance on the motherboard to reduce noise and ensure reliable communication between the CPU and chipset.[15] Key electrical limits include a maximum current of 1.0 A per pin to prevent overheating and damage during peak loads.[1] ESD protection is integrated at the socket level, adhering to industry standards such as EIA-364, with dielectric withstand voltage of 360 V RMS (no discharge greater than 0.5 mA).[1] These parameters collectively ensure the socket's reliability in delivering power and signals for Intel's NetBurst architecture processors.Compatibility
Supported Processors
Socket 478 supported Intel's NetBurst-based desktop and mobile processors, including variants of the Pentium 4 and Celeron families.[9] These processors utilized the mPGA478B pin grid array packaging and required compatible motherboards with appropriate BIOS support for full functionality.[16] The earliest compatible processors were based on the Willamette core, fabricated on a 180 nm process with clock speeds ranging from 1.4 GHz to 2.0 GHz and a 400 MT/s front-side bus (FSB). These featured 256 KB of on-die L2 cache and were introduced as Intel transitioned from Socket 423 to Socket 478 for improved scalability. Mobile variants of the Willamette-core Pentium 4 operated at similar speeds (1.4–2.0 GHz) but with lower power envelopes for laptops. Succeeding the Willamette were Northwood-core processors on a 130 nm process, offering clock speeds from 1.6 GHz to 3.4 GHz, with FSB options of 400 MT/s, 533 MT/s, or 800 MT/s, and 512 KB of L2 cache. Hyper-Threading Technology was supported on models starting at 3.06 GHz with 533 MT/s FSB and all 800 MT/s variants, enabling simultaneous multithreading for enhanced performance in threaded workloads. The Pentium 4 Extreme Edition, based on the Gallatin core (a Northwood derivative), provided limited high-end options at 3.2 GHz and 3.4 GHz with an 800 MT/s FSB, 512 KB L2 cache, and a distinctive 2 MB L3 cache for improved latency-sensitive tasks.[9][17] Northwood-based Celeron processors served as entry-level options, clocked from 1.8 GHz to 2.8 GHz with a 400 MT/s FSB and 128 KB L2 cache compared to Pentium 4 models, targeting budget desktop systems. An earlier Willamette-based Celeron at 1.7 GHz with 128 KB L2 cache (180 nm) was also compatible. Mobile Celeron variants on Northwood cores ranged from 1.5 GHz to 2.5 GHz with similar FSB and cache specifications.[18] Later in the socket's lifecycle, select Pentium 4 processors with the Prescott core on a 90 nm process received partial support via Socket 478, limited to models clocked at 2.4 GHz to 3.4 GHz with an 800 MT/s FSB and 1 MB L2 cache. These required BIOS updates and specific chipsets for compatibility, as the architecture introduced enhancements like improved branch prediction but higher power draw, marking a transitional phase before the shift to LGA 775.[16] Stepping changes across all core revisions often necessitated BIOS firmware updates to ensure stability and feature enablement, such as Hyper-Threading or extended FSB speeds.| Processor Family | Core | Process Node | Clock Speeds (GHz) | FSB (MT/s) | L2 Cache | Notes |
|---|---|---|---|---|---|---|
| Pentium 4 | Willamette | 180 nm | 1.4–2.0 | 400 | 256 KB | Initial Socket 478 adoption; no HT. |
| Pentium 4 | Northwood | 130 nm | 1.6–3.4 | 400/533/800 | 512 KB | HT on select models; standard desktop.[9] |
| Pentium 4 Extreme Edition | Gallatin | 130 nm | 3.2–3.4 | 800 | 512 KB + 2 MB L3 | Limited availability; high-performance variant.[17] |
| Celeron | Willamette | 180 nm | 1.7 | 400 | 128 KB | Budget-oriented; no HT. |
| Celeron | Northwood | 130 nm | 1.8–2.8 | 400 | 128 KB | Budget-oriented; no HT. |
| Pentium 4 | Prescott | 90 nm | 2.4–3.4 | 800 | 1 MB | Partial support; BIOS/chipset dependent.[16] |