March 1999 SDRAM Comparison

If you remember the hype at the introduction of Intel's PC100 Memory Specification, you'll remember that Intel wanted to increase the difficulty of poor quality modules to make their way into the hands of the consumers by implementing a strict identification policy for all PC100 memory modules. While the idea was sound in theory, it lacked the enforcement necessary to make this bill a law, and the industry went on pretty much as it had in the past. Fortunately the market did gain some stability out of the whole PC100 compliance fad of 1997 in that many manufacturers started increasing the standards for the production of their memory modules. Although the improvements were significant, they were still no where near as strict as what Intel had originally hoped to accomplish with the PC100 standard.

As a refresher, the PC100 specification was briefly outlined by the following 11 calls to action:

• Minimum and maximum trace lengths for all signals on the module
• Precise specifications for trace width and spacing
• 6 layer PCB's with unbroken power and ground planes
• Detailed specifications for the distances between each circuit board layer
• Precisely matched clock trace lengths, as well as routing, loading, and termination requirements
• Series termination resistors on the data lines
• Detailed SDRAM component specification
• Detailed EEPROM programming specification
• Special Marking Requirements
• ElectroMagnetic Interference Suppression
• Selectively gold plated printed circuit boards

Interpreting the Specification

Ok, its great that we know what the PC100 Specification includes, however what does that matter if we have no idea why in the world specifications for trace width and spacing must be precise, and what placing series termination resistors on the data lines allows us to do? So let's filter out some of the garbage and come up with a "translated" version of the PC100 specification.

One of the biggest problems with the generic SDRAM modules you can go out to just about any vendor and pick up was that their modules, while they could be using high quality NEC chips, were built on low quality Printed Circuit Boards (PCB's). Think of it as putting a Ford engine in a Porsche frame, don't expect to get the same performance as if you used a genuine Porsche V-8, the same concept applies to SDRAM, although it is an obscure comparison it relates almost seamlessly. For the most part, generic SDRAM "sticks" as they are often called, used high quality chips from companies like NEC, Micron, Hitachi, or Samsung (SEC), however the PCB's were manufactured in a cost effective manner, in most cases rendering the SDRAM modules incapacitated when used in certain combinations. In order to cut costs some manufacturers chose to use 4-layer PCB's compared to the recommended 6 and 8-layer PCB's for SDRAM, as a result 4-layer boards were much susceptible to physical damage, and as you might guess a 4-layer board contains much less electrical insulation compared to 6 and 8-layer boards, this provides for increased levels of noise in the current. For this reason the PC100 specification clearly states that modules must be manufactured on 6-layer PCB's, and nothing less, so say good bye to those cheaper 4-layer boards.

Since the introduction of the PC100 specification, AnandTech has not run into a single SDRAM module that has experienced stability or incompatibility problems during normal operation as the result of a poorly manufactured PCB. The once common 4-layer boards are almost extinct with the remaining few being those belonging to the PC66 era, with the higher quality 6 and 8-layer boards dominating the marketplace. Once again, the user is the one that benefits.