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What are the Elements of a SDRAM Module?

"PCBs" is the abbreviation for "Printed Circuit Board" and is actually the board on which the whole module is built. For standard DIMMs the PCB is an epoxy resin that is dyed green and consists of several (e.g., four) insulating layers which are separating the conductive layers (consisting of copper) from each other. The individual copper conductive layers are connected with each other through contact holes, also called "VIA's. From several conductive layers only some of them are made of structured conducting paths, e.g. the front and posterior outer layer (these conducting paths are then visible) and the middle layer. Between the layers with structured conducting paths, two continuous/bulk (without structured conducting paths) conducting layers are present, one of them being connected to the power supply VCC and the other one is connected to ground. By means of these two continuous copper layers a good distribution of the supply voltage over the entire module is provided and in addition they are playing the role of capacities (charge reservoirs) able to “catch” undesired big current peaks. The DIMM pins are usually made of gold, deposited on the boards by means of electroplating.

PCB for unbuffered modules

Figure 1: PCB example for unbuffered modules

Figure 1 shows the image of a PCB without SDRAM chips on it. It is clearly visible that each chip-DQ pin is connected through a resistor with a corresponding module-DQ pin. On the other hand the pins for addresses and control signals are common for all chips on the board. The shape of the conducting paths has changed over the PCB Development. On old PCBs the conducting paths from the module to chip pins were fairly straight lines. Since the pins of the chip are at different distances from the DIMM/module pins, time differences with respect to the data transfer occurred: for example a chip will receive the data on different DQ's at different times. In order to solve this problem meanders have been implemented for the conducting paths, which are actually compensating the different lengths in the conducting paths (see Figure 2). However, the meanders should be as short as possible due to capacity reasons which can influence the write and read processes on the module.

Conducting paths on a PCB

Figure 2: Conducting paths on PCBs: with and without meanders

While unbuffered modules contain less components on the PCBs (resistors, capacitors, an SPD-EEPROM and the DRAM chips), for the registered modules registers and the PLLs have to are added to the PCB. The function of the individual components will be described in the following.

Resistors (see Figure 3): All DIMM-DQ's are connected to the chip-DQs via resistors. The resistances of conducting paths on the PCB and the input resistances of DRAM-chips are not always perfectly adapted to each other, so that the signals at the junctions can be partly reflected. More than that, a SDRAM DIMM/module is interacting also with the chipset of the computer and here the fit between the input resistance of the chipset and the resistance of the conducting paths connecting the SDRAM module is also not perfect. The main function of the resistors on the PCB is the so called dampening effect, i.e. to minimize signal reflection on the module. Without damping would therefore appear more reflective signals that can be so large that they could be interpreted as input signals by the SDRAM-chip and thus the desired function of the chips can be disturbed. In addition, voltage spikes can be temporarily created by the computer, dampening-resistors together with capacitors will smooth these spikes so that they can no longer lead to the destruction of the chips.

Resistors on a PCB

Figure 3: Example of resistors on a SDRAM PCB

Capacitors (see Figure 4): The capacitors are used to stabilize the voltage supply, i.e. to smooth for short-term pressures arising from VCC variations. Since the relevant time scales here are very short, normally 2 to 3 capacitors in the vicinity of any SDRAM-chip are soldered.

Capacitances on a PCB

Figure 4: Example of capacitances on a PCB

The SPD EEPROM, often referred briefly as SPD or EEPROM, is the "identity card" of a module. SPD has a volume of 256 byte are, however only 128 bytes are used. SPD contains the information about row and column address numbers, burst lengths supported, allowable maximum speeds for tCK etc. A computer reads at boot this EEPROM from the modules and sets accordingly its settings in BIOS. Which bytes are read out there and which are not, is varying from computer to computer.

SPD  EEPROM on a PCB

Figure 4: SPD-EEPROM on a PCB

The function of registers and PLLs was discussed in the article Registered and stacked SDRAM modules.