MIL-PRF-19500/406L
6.4 Applications data.
6.4.1 PCB (PCB1) mounting with FR4 material for only 0.6 W. For a printed board mounting example with FR4
base material where only 0.6 watts of power is used as shown on figure 6 with 200ºC/W thermal resistance junction
to ambient at a TJ of 175ºC and ambient temperature of 55ºC, the following steps guide the user in what the printed
board copper mounting pad size will need to be in area for each pad with 1 ounce, 2 ounce, and 3 ounce copper. For
axial-leaded, the lead length for mounting will be .187 inch (4.75 mm) or less from body to entry point on PCB
surface.
a. Look up thermal resistance value of the required 200°C/W on the Y-axis using a thermal resistance versus
copper mounting pad area plot on each of the three curves on figure 15 for different weights of copper foil and
then intersect curve horizontally to get the answer. These curves assume still air and horizontal printed board
position.
b. In this example, the copper mounting pad sizes for the different copper foil weights would be as follows:
2
2
(1) .0085 in (5.48 mm ) for 1 ounce copper foil.
*
2
2
(2) .0050 in (3.23 mm ) for 2 ounce copper foil.
*
2
2
(3) .0032 in (2.06 mm ) for 3 ounce copper foil.
*
c. Add a conservative guard-band to the copper mounting pad size (larger) to keep TJ below 175ºC.
6.4.2 PCB (PCB2) mounting with FR4 material for the full 1.5 W. For a printed board mounting example with FR4
base material where the full 1.5 watt power rating is used as shown on figure 7 with 80ºC/W thermal resistance
junction to ambient at a TJ of 175ºC and ambient temperature of 55ºC, the following steps guide the user in what the
printed board copper mounting pad size will need to be in area for each pad with 1 ounce, 2 ounce, and 3 ounce
copper. For axial-leaded, the lead length for mounting will be .187 inch (4.75 mm) or less from body to entry point on
PCB surface.
a. Look up thermal resistance value of the required 80ºC/W on the Y-axis using a thermal resistance versus
copper mounting pad area plot on each of the three curves on figure 15 for different weights of copper foil and
then intersect curve horizontally to get the answer. These curves assume still air and horizontal printed board
position.
b. In this example the copper mounting pad sizes for the different copper foil weights would be as follows:
2
2
(1) .20 in (129 mm ) for 1 ounce copper foil.
*
2
2
(2) .12 in (77.4 mm ) for 2 ounce copper foil.
*
2
2
(3) .08 in (51.6 mm ) for 3 ounce copper foil.
*
c. Add a conservative guard-band to the copper mounting pad size (larger) to keep TJ below 175ºC.
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