ADC12D1800 Datasheet, PDF (75/84 Pages) National Semiconductor (TI) – 12-Bit, Single 3.6 GSPS ADC

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ADC12D1800
SNAS500P – MAY 2010 – REVISED JULY 2015
8.3 Thermal Management
The Heat Slug Ball Grid Array (HSBGA) package is a modified version of the industry standard plastic
BGA (Ball Grid Array) package. Inside the package, a copper heat spreader cap is attached to the
substrate top with exposed metal in the center top area of the package. This results in a 20%
improvement (typical) in thermal performance over the standard plastic BGA package.
Mold Compound
"_{JC_1}
Copper Heat Slug
Cross Section Line
IC Die
Not to Scale
"_{JC_2}
Substrate
Figure 8-5. HSBGA Conceptual Drawing
The center balls are connected to the bottom of the die by vias in the package substrate, Figure 8-5. This
gives a low thermal resistance between the die and these balls. Connecting these balls to the PCB ground
planes with a low thermal resistance path is the best way dissipate the heat from the ADC. These pins
should also be connected to the ground plane via a low impedance path for electrical purposes. The direct
connection to the ground planes is an easy method to spread heat away from the ADC. Along with the
ground plane, the parallel power planes will provide additional thermal dissipation.
The center ground balls should be soldered down to the recommended ball pads (See AN-1126
[SNOA021]). These balls will have wide traces which in turn have vias which connect to the internal
ground planes, and a bottom ground pad/pour if possible. This ensures a good ground is provided for
these balls, and that the optimal heat transfer will occur between these balls and the PCB ground planes.
In spite of these package enhancements, analysis using the standard JEDEC JESD51-7 four-layer PCB
thermal model shows that ambient temperatures must be limited to a max of 65°C to ensure a safe
operating junction temperature for the ADC12D1800. However, most applications using the ADC12D1800
will have a printed circuit board which is more complex than that used in JESD51-7. Typical circuit boards
will have more layers than the JESD51-7 (eight or more), several of which will be used for ground and
power planes. In those applications, the thermal resistance parameters of the ADC12D1800 and the circuit
board can be used to determine the actual safe ambient operating temperature up to a maximum of 85°C.
Three key parameters are provided to allow for modeling and calculations. Because there are two main
thermal paths between the ADC die and external environment, the thermal resistance for each of these
paths is provided. θ_JC1 represents the thermal resistance between the die and the exposed metal area on
the top of the HSBGA package. θ_JC2 represents the thermal resistance between the die and the center
group of balls on the bottom of the HSBGA package. The final parameter is the allowed maximum junction
temperature, which is T_J.
In other applications, a heat sink or other thermally conductive path can be added to the top of the
HSBGA package to remove heat. In those cases, θ_JC1 can be used along with the thermal parameters for
the heat sink or other thermal coupling added. Representative heat sinks which might be used with the
ADC12D1800 include the Cool Innovations p/n 3-1212XXG and similar products from other vendors. In
many applications, the printed circuit board will provide the primary thermal path conducting heat away
from the ADC package. In those cases, θ_JC2 can be used in conjunction with printed circuit board thermal
modeling software to determine the allowed operating conditions that will maintain the die temperature
below the maximum allowable limit. Additional dissipation can be achieved by coupling a heat sink to the
copper pour area on the bottom side of the printed circuit board.
Typically, dissipation will occur through one predominant thermal path. In these cases, the following
calculations can be used to determine the maximum safe ambient operating temperature:
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