IEI Integration ICE-DB-T6 - Manual (Page 77)

Type 6 Carrier Board Design Guide
Page 67
4.1 Chapter Overview
A brief description of the Printed Circuit Board (PCB) for COM Express based board is
provided in this section. From a cost- effectiveness point of view, a four-layer board is the
target platform for the motherboard design. For better quality, a six-layer or 8-layer board
is preferred. This chapter also provides the ATX/AT power supply design recommendation
for customer’s reference. IEI ICE module carrier board use 4-layer PCB stack.
4.2 Microstrip or Stripline
Either edge-coupled microstrip, edge-coupled stripline, or broad-side striplines are
recommended for designs with differential signals. Microstrip lines have the advantage
that a lower number of layers can be used. With microstrip lines, it may be possible to
route from a connector pad to the device pad without any via. This provides better signal
quality on the signal path that connects devices. Microstrip lines can only be routed on the
two outside layers of the PCB, thus routing channel density is limited.
Stripline may be either edge-coupled or broad-side coupled lines. Stripline designs
provide additional shielding since they are embedded in the board stack and are typically
in between ground and power planes. This reduces radiation and coupling of noise onto
the lines. Striplines offer the disadvantage that they require the use of vias to connect to
them.
4.3 PCB Stackup Example
It is recommended to use PCB's with at least a 4-layer stackup where the impedance
controlled top layer (layer 1) is used for differential signals and bottom layer (layer 4) for
other periodic signals (CMOS/TTL). The dedicated power planes (layer 2 – GND and layer
3 – VCC) are required for high-speed designs. It is necessary for the solid ground plane to
establish a controlled impedance for the transmission line interconnects. A narrow spacing
between ground and power planes will create an excellent high frequency bypass
capacitance additionally. The following example shows a four layer PCB stackup using
microstrip trace routing. A good rule to follow for microstrip designs is to keep S < W and S
< H (“H” = space between differential signal layers and the reference plane). The best
practice is to use the closest spacing, “S,” allowed by the PCB vendor and then adjust
trace widths, “W,” to control differential impedance.