Mobile Processor Packaging

The heat that processors generate has been a concern since the first computer chips were released. In desktop systems, the heat problem is addressed to a great extent by computer case manufacturers. Multiple cooling fans and better internal layout designs can keep air flowing through the system to cool the processor, which is usually equipped with its own fan and heatsink.

For developers of portable systems, however, not as much can be accomplished with the case arrangement. So, it was up to the chip manufacturers to address the problem in the design and packaging of the chip. Although most portable systems use special mobile processors designed specifically for mobile use, some systems use desktop processors for lower cost, at the expense of battery life and heat generation.

Note

Some manufacturers of portable systems use standard desktop processors. Apart from a greatly diminished battery life, systems such as these can sometimes be too hot to touch comfortably. For this reason, before purchasing a portable system, you should determine whether it uses a mobile or desktop processor and understand the ramifications of each.

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 staggered pin grid array (SPGA) 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.

Figure 4.1. Pentium MMX processor in TCP Mobile Package. (Photograph used by permission of Intel Corporation.)

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 heatsink or physical container is directly attached to the processor, the motherboard itself becomes the conduit to a heatsink 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 save a significant amount of size and weight.  Intel Mobile Pentium tape carrier package pinout