主板结构
由于主板是电脑中各种设备的连接载体，而这些设备的各不相同的，而且主板本身也有芯片组，各种I/O控制芯片，扩展插槽，扩展接口，电源插座等元器件，因此制定一个标准以协调各种设备的关系是必须的。所谓主板结构就是根据主板上各元器件的布局排列方式，尺寸大小，形状，所使用的电源规格等制定出的通用标准，所有主板厂商都必须遵循。
主板结构分为AT、Baby-AT、ATX、Micro ATX、LPX、NLX、Flex ATX、EATX、WATX以及BTX等结构。其中，AT和Baby-AT是多年前的老主板结构，现在已经淘汰；而LPX、NLX、Flex ATX则是ATX的变种，多见于国外的品牌机，国内尚不多见；EATX和WATX则多用于服务器/工作站主板；ATX是目前市场上最常见的主板结构，扩展插槽较多，PCI插槽数量在4-6个，大多数主板都采用此结构；Micro ATX又称Mini ATX，是ATX结构的简化版，就是常说的“小板”，扩展插槽较少，PCI插槽数量在3个或3个以下，多用于品牌机并配备小型机箱；而BTX则是英特尔制定的最新一代主板结构。
在PC推出后的第三年即1984年，IBM公布了PCAT。AT主板的尺寸为13"×12"，板上集成有控制芯片和8个I/0扩充插槽。由于 AT主板尺寸较大，因此系统单元（机箱）水平方向增加了2英寸，高度增加了1英寸，这一改变也是为了支持新的较大尺寸的AT格式适配卡。将8位数据、20 位地址的XT扩展槽改变到16位数据、24位地址的AT扩展槽。为了保持向下兼容，它保留62脚的XT扩展槽，然后在同列增加36脚的扩展槽。XT扩展卡仍使用62脚扩展槽（每侧31脚），AT扩展卡使用共98脚的的两个同列扩展槽。这种PC AT总线结构演变策略使得它仍能在当今的任何一个PC Pentium/PCI系统上正常运行。
PC AT的初始设计是让扩展总线以微处理器相同的时钟速率来运行，即6MHz 的286，总线也是6MHz；8MHz的微处理器，则总线就是8MHz。随着微处理器速度的增加，增加扩展总线的速度也很简单。后来一些PC AT系统的扩展总线速度达到了10和12MHz。不幸的是，某些适配器不能以这样的速度工作或者能很好得工作。因此，绝大多数的PC AT仍以8或8.33MHz为扩展总线的速率，在此速度下绝大多数适配器都不能稳定工作。
AT主板尺寸较大，板上能放置较多的元件和扩充插槽。但随着电子元件集成化程度的提高，相同功能的主板不再需要全AT的尺寸。因此在1990年推出了Baby/Mini AT主板规范，简称为Baby AT主板。
Baby AT主板是从最早的XT主板继承来的，它的大小为15"×8.5"，比AT主板是略长，而宽度大大窄于AT主板。Baby AT主板沿袭了AT主板的I/0扩展插槽、键盘插座等外设接口及元件的摆放位置，而对内存槽等内部元件结构进行了紧缩，再加上大规模集成电路使内部元件减少，使得Baby AT主板比AT主板布局紧凑而功能不减。
但随着计算机硬件技术的进一步发展，计算机主板上集成功能越来越多，Baby AT主板有点不负重荷，而AT主板又过于庞大，于是很多主板商又采取另一种折衷的方案，即一方面取消主板上使用较少的零部件以压缩空间（如将I/0扩展槽减为7个甚至6个，另一方面将Baby AT主板适当加宽，增加使用面积，这就形成了众多的规格不一的Baby AT主板。当然这些主板对基本I/0插槽、外围设备接口及主板固定孔的位置不加改动，使得即使是最小的Baby AT主板也能在标准机箱上使用。最常见的Baby AT主板尺寸是3/4Baby AT主板（26.5cm×22cm即10.7"×8.7"），采用7个I/0扩展槽。
由于Baby AT主板市场的不规范和AT主板结构过于陈旧，英特尔在95年1月公布了扩展AT主板结构，即ATX（AT extended）主板标准。这一标准得到世界主要主板厂商支持，目前已经成为最广泛的工业标准。97年2月推出了ATX2.01版。
ATX结构主板
Baby AT结构标准的首先表现在主板横向宽度太窄（一般为22cm），使得直接从主板引出接口的空间太小。大大限制了对外接口的数量，这对于功能载来越强、对外接口越来越多的微机来说，是无法克服的缺点。其次，Baby AT主板上CPU和I/0插槽的位置安排不合理。早期的CPU由于性能低、功耗小，散热的要求不高。而今天的CPU性能高、功耗大，为了使其工作稳定，必须要有良好的散热装置，加装散热片或风扇，因而大大增加了CPU的高度。在AT结构标准里CPU位于扩展槽的下方，使得很多全长的扩展卡插不上去或插上去后阻碍CPU风扇运转。内存的位置也不尽合理。早期的计算机内存大小是固定的，对安装位置无特殊要求。Baby AT主板在结构上按习惯把内存插槽安放在机箱电源的下方，安装、更换内存条往往要拆下电源或主板，很不方便。内存条散热条件也不好。此外，由于软硬盘控制器及软硬盘支架没有特定的位置，这造成了软硬盘线缆过长，增加了电脑内部连线的混乱，降低了电脑的中靠性。甚至由于硬盘线缆过长，使很多高速硬盘的转速受到影响。ATX主板针对AT和Baby AT主板的缺点做了以下改进：
- 主板外形在Baby AT的基础上旋转了90度，其几何尺寸改为30.5cm×24.4cm。
- 采用7个I/O插槽，CPU与I/O插槽、内存插槽位置更加合理。
- 优化了软硬盘驱动器接口位置。
- 提高了主板的兼容性与可扩充性。
- 采用了增强的电源管理，真正实现电脑的软件开/关机和绿色节能功能。
Micro ATX保持了ATX标准主板背板上的外设接口位置，与ATX兼容。
MATX结构主板
Micro ATX主板把扩展插槽减少为3-4只，DIMM插槽为2-3个，从横向减小了主板宽度，其总面积减小约0.92平方英寸，比ATX标准主板结构更为紧凑。按照Micro ATX标准，板上还应该集成图形和音频处理功能。目前很多品牌机主板使用了Micro ATX标准，在DIY市场上也常能见到Micro ATX主板。
BTX是英特尔提出的新型主板架构Balanced Technology Extended的简称，是ATX结构的替代者，这类似于前几年ATX取代AT和Baby AT一样。革命性的改变是新的BTX规格能够在不牺牲性能的前提下做到最小的体积。新架构对接口、总线、设备将有新的要求。重要的是目前所有的杂乱无章，接线凌乱，充满噪音的PC机将很快过时。当然，新架构仍然提供某种程度的向后兼容，以便实现技术革命的顺利过渡。
BTX具有如下特点：
- 支持Low-profile，也即窄板设计，系统结构将更加紧凑；
- 针对散热和气流的运动，对主板的线路布局进行了优化设计；
- 主板的安装将更加简便，机械性能也将经过最优化设计。
而且，BTX提供了很好的兼容性。目前已经有数种BTX的派生版本推出，根据板型宽度的不同分为标准BTX （325.12mm）， microBTX （264.16mm）及Low-profile的picoBTX （203.20mm），以及未来针对服务器的Extended BTX。而且，目前流行的新总线和接口，如PCI Express和串行ATA等，也将在BTX架构主板中得到很好的支持。
值得一提的是，新型BTX主板将通过预装的SRM（支持及保持模块）优化散热系统，特别是对CPU而言。另外，散热系统在BTX的术语中也被称为热模块。一般来说，该模块包括散热器和气流通道。目前已经开发的热模块有两种类型，即full-size及low-profile。
得益于新技术的不断应用，将来的BTX主板还将完全取消传统的串口、并口、PS/2等接口。
From AT to BTX: Motherboard Form Factors
By: Vangie Beal
Posted: 04-29-2005 , Last Updated: 09-01-2010
You've probably heard the term motherboard a thousand times, but do you know what it really means and how it relates to the rest of your computer?
The form factor of a motherboard determines the specifications for its general shape and size. It also specifies what type of case and power supply will be supported, the placement of mounting holes, and the physical layout and organization of the board. Form factor is especially important if you build your own computer systems and need to ensure that you purchase the correct case and components.
The Succession of Motherboard Form Factors
AT & Baby AT
Prior to 1997, IBM computers used large motherboards. After that, however, the size of the motherboard was reduced and boards using the AT (Advanced Technology) form factor was released. The AT form factor is found in older computers (386 class or earlier). Some of the problems with this form factor mainly arose from the physical size of the board, which is 12" wide, often causing the board to overlap with space required for the drive bays.
Following the AT form factor, the Baby AT form factor was introduced. With the Baby AT form factor the width of the motherboard was decreased from 12" to 8.5", limiting problems associated with overlapping on the drive bays' turf. Baby AT became popular and was designed for peripheral devices — such as the keyboard, mouse, and video — to be contained on circuit boards that were connected by way of expansion slots on the motherboard.
Baby AT was not without problems however. Computer memory itself advanced, and the Baby AT form factor had memory sockets at the front of the motherboard. As processors became larger, the Baby AT form factor did not allow for space to use a combination of processor, heatsink, and fan. The ATX form factor was then designed to overcome these issues.
ATX
With the need for a more integrated form factor which defined standard locations for the keyboard, mouse, I/O, and video connectors, in the mid 1990's the ATX form factor was introduced. The ATX form factor brought about many chances in the computer. Since the expansion slots were put onto separate riser cards that plugged into the motherboard, the overall size of the computer and its case was reduced. The ATX form factor specified changes to the motherboard, along with the case and power supply. Some of the design specification improvements of the ATX form factor included a single 20-pin connector for the power supply, a power supply to blow air into the case instead of out for better air flow, less overlap between the motherboard and drive bays, and integrated I/O Port connectors soldered directly onto the motherboard. The ATX form factor was an overall better design for upgrading.
micro-ATX
MicroATX followed the ATX form factor and offered the same benefits but improved the overall system design costs through a reduction in the physical size of the motherboard. This was done by reducing the number of I/O slots supported on the board. The microATX form factor also provided more I/O space at the rear and reduced emissions from using integrated I/O connectors.
LPX
White ATX is the most well-known and used form factor, there is also a non-standard proprietary form factor which falls under the name of LPX, and Mini-LPX. The LPX form factor is found in low-profile cases (desktop model as opposed to a tower or mini-tower) with a riser card arrangement for expansion cards where expansion boards run parallel to the motherboard. While this allows for smaller cases it also limits the number of expansion slots available. Most LPX motherboards have sound and video integrated onto the motherboard. While this can make for a low-cost and space saving product they are generally difficult to repair due to a lack of space and overall non-standardization. The LPX form factor is not suited to upgrading and offer poor cooling.
NLX
Boards based on the NLX form factor hit the market in the late 1990's. This "updated LPX" form factor offered support for larger memory modules, tower cases, AGP video support and reduced cable length. In addition, motherboards are easier to remove. The NLX form factor, unlike LPX is an actual standard which means there is more component options for upgrading and repair.
Many systems that were formerly designed to fit the LPX form factor are moving over to NLX. The NLX form factor is well-suited to mass-market retail PCs.
BTX
The BTX, or Balanced Technology Extended form factor, unlike its predecessors is not an evolution of a previous form factor but a total break away from the popular and dominating ATX form factor. BTX was developed to take advantage of technologies such as Serial ATA, USB 2.0, and PCI Express. Changes to the layout with the BTX form factor include better component placement for back panel I/O controllers and it is smaller than microATX systems. The BTX form factor provides the industry push to tower size systems with an increased number of system slots.
One of the most talked about features of the BTX form factor is that it uses in-line airflow. In the BTX form factor the memory slots and expansion slots have switched places, allowing the main components (processor, chipset, and graphics controller) to use the same airflow which reduces the number of fans needed in the system; thereby reducing noise. To assist in noise reduction BTX system level acoustics have been improved by a reduced air turbulence within the in-line airflow system.
Initially there will be three motherboards offered in BTX form factor. The first, picoBTX will offer four mounting holes and one expansion slot, while microBTX will hold seven mounting holes and four expansion slots, and lastly, regularBTX will offer 10 mounting holes and seven expansion slots. The new BTX form factor design is incompatible with ATX, with the exception of being able to use an ATX power supply with BTX boards.
Today the industry accepts the ATX form factor as the standard, however legacy AT systems are still widely in use. Since the BTX form factor design is incompatible with ATX, only time will tell if it will overtake ATX as the industry standard.
Did You Know...
ATX and Baby AT boards are approximately the same size, but the ATX board is rotated 90 degrees within the case to allow for easier access to components.
Key Terms To Understanding Motherboard Form Factors
motherboard
The main circuit board of a microcomputer.
form factor
The physical size and shape of a device. It is often used to describe the size of circuit boards.
AT
Short for advanced technology, the AT is an IBM PC model introduced in 1984.
ATX
The modern-day shape and layout of PC motherboards.
BTX
The BTX specification provides new tools and design space for developers to lay out desktop systems, whether designing small, compact systems or very large, expandable systems.
Baby AT
The form factor used by most PC motherboards prior to 1998.
Based in Nova Scotia, Vangie Beal is has been writing about technology for more than a decade. She is a frequent contributor to EcommerceGuide and managing editor at Webopedia. You can tweet her online @AuroraGG.