Kontron COMe Starterkit Eval T2 - Manual (Page 167)

Power and Reset
3.5. Routing Considerations
3.5.1. VCC_12V and GND
The primary consideration for the +12V power input (VCC_12V) to the Module is that the trace be
wide enough to handle the maximum expected load, with plenty of margin. A power plane may
be used for VCC_12V but is not recommended; VCC_12V should not be used as a reference for
high-speed signals, such as PCIe, USB, or even PCI, because there may be switching noise
present on VCC_12V.
A 40W CPU Module can draw over 3.5A on the VCC_12V pins. Sizing the VCC_12V delivery
trace to handle at least twice the expected load is recommended for good design margin. It is
best to keep the Carrier Board VCC_12 trace short, wide, and away from other parts of the
Carrier Board. See the following section for advice on how to size the trace.
If there are layer transitions in the power delivery path, use redundant “power” vias – vias that are
sized with larger holes and pads than default vias.
For the GND return, it is best to use a solid, continuous plane, or multiple planes, using the
heaviest possible copper.
It is very important to connect all available power and ground pins available on the COM Express
Module to the Carrier Board.
3.5.2. Copper Trace Sizing and Current Capacity
The current capacity of a PCB trace is proportional to the trace’s cross-sectional area – the
product of the trace width and thickness. The trace thickness is proportional to the “weight” of
copper used. The copper weight is expressed in ounces per square foot in the United States.
Usually people will omit the “per square foot” and just use “ounce” to describe the copper.
Copper weights of ½ ounce/ 17μm and sometimes 1 ounce/ 35μm are common for inner layer
traces. A copper weight of 1 ounce/ 35μm is common for power planes. A copper weight of ½
ounce/ 17μm results in a thickness of approximately 0.7 mil, and 1 ounce/ 35μm copper yields
approximately 1.4 mil. Outer layer traces are usually built with ½ ounce/ 17μm copper, but then
are “plated up” with additional conductive material, often yielding an effective copper weight of
about 1 ounce/ 35μm. The effective weight of outer layer traces may vary with different PCB
processes. Check with your PCB vendor, or play it safe and make conservative assumptions.
Consult sources such as the IPC-2221 for charts that relate copper weight, trace width and trace-
current capacity at a given temperature rise to the current capability. It is best to assume a
conservative trace temperature rise, such as 10° C maximum, when making trace-width
decisions. Per the IPC charts, external layer traces can carry significantly more current than
internal layer traces, assuming the same base copper weight and the same temperature rise.
Approximate current handling capabilities of selected trace widths read off of the IPC-2221 charts
are shown in Table 56 below.
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
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