Original 45nm Intel® Core™ Microarchitecture

The Technical Challenges of Transitioning Intel® PRO/Wireless Solutions to a Half-Mini Card

DEVELOPING THE PCI EXPRESS HALF-MINI CARD SPECIFICATION

With an increasing market pressure to integrate more and more wireless radio functionality into thinner and lighter notebook designs, the PCI-SIG in early 2004 was asked by some platform OEMs to consider space-saving alternatives to the PCI Express Mini Card specification that had been released only one year earlier and had yet to even have products developed based on it. Platform OEMs were heavily motivated by the need to figure out how to enable getting an increasing number of separate wireless radios into the already tightly-packed base of the notebook and by the need to establish a development roadmap toward a more space-efficient card format that wireless technology suppliers could eventually move to.

Starting in December 2005, the PCI-SIG Mini Working Group (WG) initiated the development of a specification for what would ultimately come to be known as the Half-Mini Card. The primary objective was to define a smaller variant of the PCI Express Mini Card, now to be known as the Full-Mini Card, which would enable notebook designs to accommodate an increased number of wireless cards while keeping the platform base volume associated with wireless applications at parity. The goal was to potentially get two smaller cards in the space of the one larger card while retaining interface and socket connector compatibility across the two card types. In this effort, the Mini WG, consisting of ten voting and seven observing member companies, succeeded in completing an acceptable specification in just over six months.

(Figure 2) overlays color-highlighted card outlines aligned at a top-right origin to visually compare the decreasing planar size progression as the standardized card form factors shrank from the original Mini PCI Card (shown in green) down through to the Full-Mini Card (blue) and toward the Half-Mini Card (red). It should be noted that no change to the z-height and assembly stack-up profile of the Mini Card was made going from the Full-Mini Card to the Half-Mini Card format. If solely based on the outline dimensions of the Half-Mini Card, the format appears to be slightly larger than half the size of the Full-Mini Card, 804mm ² versus 1528.5mm ² , but the practical area for functional circuitry collectively across both top and bottom sides of the card is actually smaller than half, 1220mm ² versus 2670mm ² , or about 45 percent of the useful area.

Figure 2: Comparing standardized card sizes

The most critical factors in determining the size and shape specification of the smaller Mini Card included the area and volume reduction impact for circuit components, thermal density, and cooling impacts, and considerations for supporting a socket configuration that allowed for sharing between the larger and smaller cards.

Even as many wireless circuit technologies are progressing down a size-reduction roadmap, the proposed smaller Half-Mini Card format represented a challenge for both existing and emerging technologies. Initially proposed at being just less than half the size of the existing Mini Card, at one point there was even a proposal on the table for a smaller card on which the usable circuit area volume could have been reduced to as little as 37 percent. To help resolve the debate, each WG member company was asked to perform an independent feasibility review to determine if the smaller proposed sizes would be too constraining. Intel's considerable internal review included analyses of six different wireless technologies (for LAN, WAN, PAN, and digital TV), both singularly and in some likely product combinations, and it took into account technology reduction trends over a period of many years. The result was that there was strong evidence that not all wireless applications would be relevant in the smaller card form factors. For a majority of the specification development period, the WG settled on a target of 24.6mm in length while keeping the card width the same as that of the Full-Mini Card. In the end, and after an even more extensive detailed review by Intel, the final dimension was increased to 26.8mm to better accommodate a wider range of applications.

By its very nature, wireless technology can generate considerable thermal dissipation, this proving to be a key technical issue that an earlier concept to integrate wireless technology within notebook lids was unable to resolve. As a general rule, as the card format is reduced in size, the thermal density of a given application, when considered over its volume, is increased, and the cooling solution becomes more important. Unfortunately, reducing the size of a given radio technology doesn’t necessarily imply a reduction in thermal dissipation. However, as it turns out, the most common cooling issues with a radio solution are often localized to the area around the power amplifiers. With Half-Mini Card designs, the concentration of this dissipated heat doesn’t dramatically change. The WG chose to keep the thermal dissipation allowance the same between the two sizes of cards, but the notebook system designer must be cautioned that if two Half-Mini Cards are specifically placed within the platform to fill the previous space of a Full-Mini Card, then the cooling design for that space must take into account the potential doubling of the thermal dissipation.

Finally, the last major consideration was the potential re-configurability of a Mini Card socket, especially if the selection of Mini Card options that are to be offered for a given notebook platform design will include both standardized sizes. The primary factors in managing this include the orientation and placement of the socket connector(s) and the method used for holding the installed card in place (using the defined screw holes located at the corners of the card). (Figure 3) illustrates how a socket can be configured for dual-use, given a second set of hold-down positions, with these hold-down points often being implemented as a boss and screw arrangement.

Figure 3: Dual-use socket concept

In light of this dual-use configuration and another that specified a head-to-head configuration allowing for two opposing Half-Mini Card sockets that also support substituting a single Full-Mini Card, we defined options related to the bottom side keep-out areas of the Mini Cards to promote interoperability in multi-use sockets. In Table 1 we summarize the Mini Card and multi-use socket compatibility options that were defined. Notebook OEMs are allowed to also consider other configurations including just simply isolating and positioning individual sockets in convenient locations throughout the platform.

The remainder of the functional and performance specifications for both Full- and Half-Mini Cards is identical, including the support for both PCI Express and USB as the system I O interfaces, the defined wireless-specific signaling features, and the available power-delivery pins. A recent unrelated change to the Mini Card specifications restructured the power supply interface to align on two voltage sources instead of three and to allocate more pins to power delivery as a means to reduce voltage drop across the interface. All of the changes that we discuss are normatively covered by Revision 1.2 (dated October 27, 2007) of the specification [1].

Intel's role in all of the Mini Card standardization efforts to date has been unique in that we are the only participating technology supplier delivering at both the notebook system chipset and the wireless communications levels. As such, Intel has been able to supply a broad range of technical expertise to review and guide the specification development activities. As the technical editor for PCI-SIG Mini WG, we have also been able to play a leadership role in establishing useful specification requirements across a diverse set of industry participants including three major notebook OEMs, a number of wireless technology suppliers, and a number of connector suppliers.

Back to Top