AN668 Datasheet, PDF (16/19 Pages) Silicon Laboratories – PRECISION32™ SOFTWARE DEVELOPMENT KIT CODE

AN668 APPLICATION NOTE
by the copper slug becomes very evident (Figure 25).
Due to the thermal capacitance of the slug, the tran-
sient thermal impedance of PowerSO-20/36 after 1
sec pulse duration is 4 times lower.
A summary of the thermal performance of PowerSO-
20/36 considering the different mounting approaches
is given in Table 3, assuming that the maximum rated
temperature of the device is 150°C and that the ∆Tj
is 50°C.
Table 3. Thermal performance (∆Tj: 50°C)
PowerSO-20/36 packages
mounted on
R_th(j-a)
Pdiss
1. FR4 using the rec-
comended pad lay-out
50°C/W
1.0W
2. FR4 with heat sink on
board (6 cm2)
35°C/W
1.4 W
3. FR4 with heatsink (6
cm2) and ground
20°C/W 2.5W
4. FR4 with 16 via holes
and ground layer
15°C/W
3.3W
5. FR4 with 16 via holes
and external heatsink
12°C/W
4.2W
6. IMS floating in air
(40cm²)
7°C/W
7W
7. IMS with external heat- 2.5°C/W
sink applied
20W
Figure 24. Thermal impedance of different
packages
Figure 25.
4. SOLDERING INFORMATION
The soldering process causes considerable thermal
stress to a semiconductor component. This has to be
minimized to ensure a reliable and extended lifetime
of the device.
PowerSO-20/36 are certified as MSL3 according to
Jedec STD 020A
As eflow techniques are the most common in surface
mounting, typical heating profiles are given in Figure
26, with a preheating at 130 - 150°C for 80 -100 sec-
onds, either for mounting on FR4 epoxy substrate or
mounting on metal-backed boards (IMS). Peak tem-
perature should be at least 30°C higher than the
melting point of the solder chosen.
Figure 26.
T (˚C)
250
245˚C
200
215˚C
EPOXY FR4
BOARD
150
Soldering
Preheating
100
D94AN017
Cooling
METAL-BACKED
50
BOARD
0
0
40 80 120 160 200 240 280 320 360
Time (sec)
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