S.E.P package

S.E.P. is short for Single Edge Processor. The S.E.P. package is similar to a S.E.C.C. or S.E.C.C.2 package but it has no covering. In addition, the substrate (circuit board) is visible from the bottom side. The S.E.P. package was used by early Intel Celeron processors, which have 242 contacts.

S.E.C.C package

S.E.C.C. is short for Single Edge Contact Cartridge. To connect to the motherboard, the processor is inserted into a slot. Instead of having pins, it uses goldfinger contacts, which the processor uses to carry its signals back and forth. The S.E.C.C. is covered with a metal shell that covers the top of the entire cartridge assembly. The back of the cartridge is a thermal plate that acts as a heatsink. Inside the S.E.C.C., most processors have a printed circuit board called the substrate that links together the processor, the L2 cache and the bus termination circuits. The S.E.C.C. package was used in the Intel Pentium II processors, which have 242 contacts and the Pentium® II Xeon™ and Pentium III Xeon processors, which have 330 contacts.

Form Factors of Motherboard

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.

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.

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.

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.