PCB Stackup Design Considerations for Intel® FPGAs

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Date 6/28/2017
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1.4.8. Copper Weight

A copper foil is specified in terms of its weight in ounces per square foot (oz/ft2), although it is more commonly referred to in ounces (oz). Typical PCB copper weights are 0.5 oz, 1 oz, and 2 oz. Below table lists the corresponding copper thicknesses for various copper weights.

Table 4.  Copper Weight and Thickness
Copper Weight (oz) Thickness (μm) Thickness (mil)
0.5 17.5 0.7
1 35 1.4
2 70 2.8
Half-ounce copper is typically used for signal traces and one-ounce copper is used for plane layers. Denser copper weights can be used for power delivery if higher DC current is expected. However, thicker copper layers increase the overall board thickness and raise manufacturability concerns related to lamination and drilling. For high frequency signals, increasing the copper weight does not significantly improve the signal loss from trace resistance because of the skin effect. Instead, use wider traces on either half- or one-ounce copper. The current carrying capacity in copper is determined by the allowable temperature rise above ambient resulting from the current flowing through it. Below table list the current rating for different copper weights extrapolated from graphs published in the military standard specification, MIL-STD-275E. You can use these tables to appropriately size your power delivery structures. For example, to pass 10 A of current on 1 oz surface copper layers, you must use at least 200 mil wide copper trace to maintain at most a 20° C rise in temperature above ambient for the conductor.
Table 5.  MIL-STD 275E External Layer Current Capacity Rating
Trace Width (mil) Temperature Rise Above Ambient
10° C 20° C 30° C
Copper Weight (oz/ft2)
0.5 1 2 0.5 1 2 0.5 1 2
Maximum Current (A)
10 0.5 1.0 1.4 0.6 1.2 1.6 0.7 1.5 2.2
15 0.7 1.2 1.6 0.8 1.3 2.4 1.0 1.6 3.0
20 0.7 1.3 2.1 1.0 1.7 3.0 1.2 2.4 3.6
25 0.9 1.7 2.5 1.2 2.2 3.3 1.5 2.8 4.0
30 1.1 1.9 3.0 1.4 2.5 4.0 1.7 3.2 5.0
50 1.5 2.6 4.0 2.0 3.6 6.0 2.6 4.4 7.3
75 2.0 3.5 5.7 2.8 4.5 7.8 3.5 6.0 10.0
100 2.6 4.2 6.9 3.5 6.0 9.9 4.3 7.5 12.5
200 4.2 7.0 11.5 6.0 10.0 11.0 7.5 13.0 20.5
250 5.0 8.3 12.3 7.2 12.3 20.0 9.0 15.0 24.0
300 5.8 9.8 - 8.2 13.8 - 10.1 17.0 -
350 6.4 11.1 - 9.3 15.2 - 11.3 19.6 -
400 7.0 12.0 - 10.0 16.7 - 12.6 21.2 -
Table 6.  MIL-STD 275E Internal Layer Current Capacity Rating
Trace Width (mil) Temperature Rise Above Ambient
10° C 20° C 30° C
Copper Weight (oz/ft2)
0.5 1 2 0.5 1 2 0.5 1 2
Maximum Current (A)
10 0.25 0.4 0.63 0.35 0.54 0.85 0.42 0.64 1.1
15 0.36 0.55 0.88 0.44 0.7 1.2 0.57 0.9 1.6
20 0.41 0.67 1.15 0.55 0.86 1.5 0.65 1.18 1.7
25 0.49 0.75 1.3 0.63 1.1 1.75 0.76 1.38 2.15
30 0.58 0.78 1.5 0.72 1.25 2.0 0.88 1.62 2.5
50 0.80 1.4 2.1 1.18 1.80 3.00 1.25 2.20 3.75
75 1.08 1.9 2.83 1.45 2.75 4.0 1.75 3.5 5.0
100 1.32 2.2 3.5 1.75 3.1 5.0 2.2 3.9 7.2
200 2.2 3.60 5.8 3.0 5.15 8.0 3.8 6.50 10.5
250 2.5 4.20 6.8 3.6 6.30 10.0 4.3 7.50 12.4
300 2.9 4.80 - 4.1 6.90 - 5.2 8.50 -
350 3.3 5.60 - 4.6 7.70 - 5.8 9.80 -
400 3.6 6.00 - 5.2 8.70 - 6.4 10.6 -