Mobile Processor Packaging

Most mobile processors include a built-in thermal diode that can be used to monitor CPU temperature. The laptop systems use this to control fan operation and also for processor performance control. Utilities are available that can use this sensor to display the processor temperature information onscreen.

Tape Carrier Packaging

An early solution to the size and heat problems for processors was the tape carrier package (TCP), a method of packaging processors for use in portable systems that reduces the size of the chip, its power consumed, and its heat generated. A Pentium mounted on a motherboard using TCP is much smaller and lighter than the standard SPGA ( staggered pin grid array) that Pentiums used in desktop systems. The 49mm square of the SPGA is reduced to 29mm in the TCP processor, the thickness is reduced to approximately 1mm, and the weight is reduced from 55 grams to less than 1 gram.

Instead of using metal pins inserted into a socket on the motherboard, a TCP processor is essentially a raw die encased in an oversize piece of polyamide film. The film is similar to photographic film. The die is attached to the film using a process called tape automated bonding (TAB), the same process used to connect electrical connections to LCD panels. The film, called the tape, is laminated with copper foil that is etched to form the leads that connect the processor to the motherboard. This is similar to the way electrical connections are photographically etched onto a printed circuit board.

After the leads are formed , they are plated with gold to allow bonding to a gold bump on the silicon die and to guard against corrosion. Next, they are bonded to the processor chip itself, and then the whole package is coated with a protective polyamide siloxane resin and mounted on a filmstrip reel for machine assembly. To get a feel for the small size of this processor, look at Figure 4.1, where it is shown next to a standard-size push-pin for comparison.

Reels of TCP chips are loaded into special machines that stamp-solder them directly to the portable system's motherboard. As such, the installation is permanent; a TCP processor can never be removed from the board for repair or upgrade. Because no heat sink or physical container is directly attached to the processor, the motherboard itself becomes the conduit to a heat sink mounted underneath it, thus using the portable system's chassis to pull heat away. Some faster portable systems include thermostatically controlled fans to further aid in heat removal.

Mounting the TCP to the system circuit board requires specific tooling available from all major board assembly equipment vendors . A special tool cuts the tape containing the processor to the proper size and folds the ends containing the leads into a modified gull-wing shape that contacts the circuit board, leaving the processor suspended just above the board. Another tool dispenses a thermally conductive paste to the circuit board before the tape containing the processor is placed. This is done so that the heat can be dissipated through a sink on the underside of the motherboard while it is kept away from the soldered connections.

Finally, a hot bar soldering tool connects the leads on the tape to the circuit board. The completed TCP assembly forms an efficient thermal contact directly from the die to the motherboard, enabling the processor to run within its temperature limits even in such a raw state. Eliminating the package and essentially bonding the die directly to the motherboard saves a significant amount of size and weight.

Figure 4.2 shows the pinout of a typical Pentium processor using TCP packaging.

Mobile Module

Another form of processor packaging is called the mobile module, or MMO (see Figure 4.3).

The mobile module consists of a Pentium or Pentium II processor in its TCP form, mounted on a small daughterboard along with the power supply for the processor's unique voltage requirements, the system's Level 2 cache memory, and the North Bridge part of the motherboard chipset. This is the core logic that connects the processor to standard system buses containing the South Bridge part of the chipset, as shown in the block diagram in Figure 4.4. North Bridge and South Bridge describe what has come to be an accepted method for dividing the functionality of the chipset into halves that are mounted on separate modules. In a typical portable system design, the manufacturer purchases the mobile module (complete with the North Bridge) from Intel and uses a motherboard designed by another company that contains the South Bridge.

A newer version of the MMO is called the MMC, which stands for Mobile Module Connector and is available in MMC-1 and MMC-2 versions. The original Celeron and Pentium II were available in MMC-1 and MMC-2 versions, whereas the Pentium III was released in the MMC-2 version only. The Pentium II and III MMC-2 feature the Intel 440BX chipset's 82443BX host bridge, which connects to the PIIX4E/M PCI/ISA South Bridge built into the portable computer's motherboard.

In many ways, the MMO/MMC is similar to the Pentium II Single Edge Cartridge (SEC) design, but with part of the motherboard included.

The module interfaces electrically to its host system via a 3.3V PCI bus, a 3.3V memory bus, and Intel chipset control signals bridging the half of the chipset on the module to the other half of the chipset on the system motherboard. The Intel mobile module also incorporates a single thermal connection that carries all the module's thermal output to the mobile PC's main cooling mechanisms.

The MMO is mounted with screws and alignment guides to secure the module against the typical shock and vibration associated with mobile PC usage. The MMO is 4 inches (101.6mm) long x 2.5 inches (63.5mm) wide x 0.315 inch (8mm) high (0.39 inch or 10mm high at the connector).

The mobile module greatly simplifies the process of installing a Pentium III processor into a portable system, permits manufacturers to build more standardization into their portable computer designs, and eliminates the need for manufacturers to invest in the special tools needed to mount TCP processors on circuit boards themselves . The module also provides a viable processor upgrade path that was unavailable with a TCP processor permanently soldered to the motherboard. Portable systems that offer a range of processor options use mobile modules because it enables the vendor to use the same motherboard.

MMC-1

The Mobile Module Connector 1 (MMC-1) is an integrated assembly containing a Mobile Pentium II processor, 256KB or 512KB of L2 cache, a 443BX North Bridge, and a voltage regulator supporting input voltages from 5V to 21V. It is essentially most of a laptop motherboard in a single module (see Figure 4.6). The MMC-1 has a 66MHz bus speed and was available in versions running at speeds of up to 233, 266, 300, 333, 366, or 400MHz.

The MMC-1 also includes a thermal transfer plate that is used for heat sink attachment, and it incorporates thermal sensors for internal and external temperature sensing with programmable trip points.

MMC-2

The Mobile Module Connector 2 (MMC-2) is an integrated assembly containing a Mobile Pentium II or III processor, 256KB or 512KB of L2 cache, a 443BX North Bridge, and a voltage regulator supporting input voltages from 5V to 21V. The MMC-2 has a 66MHz bus speed and was available in Pentium II versions running at speeds of up to 400MHz, or Pentium III versions up to 700MHz.

The MMC-2 also includes a thermal transfer plate that is used for heat sink attachment, and it incorporates thermal sensors for internal and external temperature sensing with programmable trip points (see Figure 4.7).

By including the voltage regulator module (VRM) and North Bridge components , the various mobile modules are more like minimotherboards than just processors. Although this made it easier for a manufacturer to design a single base system that could accept a wider variety of processors, it was also an expensive design and essentially forced laptop manufacturers to purchase not only the processor from Intel, but essentially half the motherboard as well. All of the various mobile module formats were used starting with the Pentium systems through the Pentium II and early Pentium III systems. Most of the later Pentium III systems and all of the Pentium 4 and Pentium M systems abandoned the mobile module designs and went back to the basic design of a separate processor in a socket.

Minicartridge

Intel used another package for the Pentium II, called the minicartridge. The minicartridge is designed especially for use in ultralight portable computers in which the height or bulk of the mobile module would interfere with the system design. It contains only the processor core and 256KB or 512KB of L2 cache memory in a stainless- steel case that exposes the connector and processor die (see Figure 4.8). This is the equivalent of the desktop Pentium II cartridge, but in a smaller mobile form factor.

The minicartridge is approximately 51mmx47mm in size and 4.5mm in height. Overall, the package is about one fourth the weight, is one sixth the size, and consumes two thirds of the power of a desktop Pentium II processor. To connect to the socket on the motherboard, the minicartridge has a 240-pin connector at one end of the bottom side, with the pins arranged in an 8x30 array.

BGA and PGA

The newest packaging for mobile processors is called micro-BGA (ball grid array) or micro-PGA (pin grid array). These packages are just processors, without any of the other circuits that were integrated into the previous mobile module designs. BGA packaging is unique in that it uses solder balls (instead of pins) on the bottom of the chip. The BGA design is mechanically stable (no pins to bend) and enables better heat transfer from the device to the board. The micro-PGA uses conventional pins instead of solder balls, allowing a standard socketed connection.

BGA chips can either be permanently soldered in or socketed, whereas PGA versions are almost always socketed. Both Intel and AMD use both BGA and PGA form factors for their newest mobile processors.

Micro-BGA2

The micro-BGA2 package consists of a die placed face down on an organic substrate, with an epoxy material surrounding the die and sealing it to the substrate. Instead of using pins, the packages use solder balls, which are either soldered directly to the motherboard or connected via a special socket.

Figure 4.9 shows a Pentium III processor in the micro-BGA2 package, which contains 495 balls.

Micro-PGA2

The micro-PGA2 package consists of a BGA chip mounted to an interposer with small pins. The pins are 1.25mm long and 0.30mm in diameter.

Figure 4.10 shows a Pentium III processor in the micro-PGA2 package, which contains 495 pins.

Micro-FCBGA

The micro-FCBGA (flip chip ball grid array) package consists of a die placed face down on an organic substrate, with an epoxy material surrounding the die and sealing it to the substrate (see Figure 4.11). The package uses 479 balls, which are 0.78mm in diameter and normally soldered directly to the motherboard. Unlike micro-PGA, micro-FCPGA includes capacitors on the top side of the package.

Micro-FCPGA and FCPGA2

The micro-FCPGA (flip chip pin grid array) and micro-FCPGA2 packages consist of a die placed face down on an organic substrate, with an epoxy material surrounding the die and sealing it to the substrate. The micro-FCPGA2 version includes a heat spreader (metal cap) over the top of the die for additional mechanical strength and thermal management. Micro-FCPGA uses 478 pins, which are 2.03mm long and 0.32mm in diameter. Unlike micro-PGA2, micro-FCPGA and micro-FCPGA2 do not use an interposer board and include capacitors on the bottom side. Even though the package has 478 pins, the socket supports 479 pins.

Figure 4.12 shows a Pentium III processor in the micro-FCPGA package. Note that the mobile Celeron, Pentium 4, and Pentium M use the same packaging and look virtually identical.

Mobile versions of the Pentium III, Pentium 4, and Pentium M processor are all available in micro-FCPGA form. Although they plug into the same micro-479 pin socket (shown in Figure 4.13), the different processors are not pin-compatible or interchangeable. In other words, if your system has a Pentium III, you cannot install a Pentium 4 or Pentium M, even though they would physically fit into the same socket.

Virtually all of the more recent mobile processors have been introduced in Micro-FCPGA and Micro-FCBGA packages. The main reason is cost. These are "flip-chip" packages, which means that the raw processor die sits face down on top of the substrate. It is connected directly to the substrate by very tiny solder balls around the perimeter. This is much less expensive than the old PGA form factors, in which the chip die was inserted into a cavity on the underside of the package and the contacts had to literally be stitched into place with a very expensive wire-bonding process using fine gold wires. The substrate also had to be ceramic, to withstand the tremendous heat that the processor put out, and a metal cap had to be installed on the bottom over the die to seal it. This involved many steps using many parts .

By comparison, a flip-chip is much simpler, less expensive, easier to assemble, and easier to cool. The die is on the top, so the heat can be transferred directly into the heat sink without having to go through the substrate material. No wire bonding is needed because the tiny solder balls connect the die to the substrate. A fillet of epoxy seals the die to the substrate, eliminating the need for a metal seal. In short, all of the modern desktop and mobile processors from Intel and AMD have gone to the flip-chip design, mainly in the interest of lower manufacturing costs.