參數(shù)資料
型號(hào)： FBGA
英文描述： FBGA - FBGA User＇s Guide. Version 4.2
中文描述： FBGA封裝- FBGA封裝用戶(hù)指南。版本4.2
文件頁(yè)數(shù)： 80/97頁(yè)
文件大小： 2624K
代理商： FBGA
FBGA User’s Guide
Version 4.2, November 1, 2002
79
2
quite high when compared to air or other gasses, most of the heat generated (~90%) from
the circuitry on the die surface is conducted through the silicon and into the ceramic
substrate. The heat travels through the ceramic and is dissipated into the air or into a heat
sink. Some spreading occurs in the ceramic (at an approximate 45° angle), so the analysis
can be almost purely one-dimensional. This approach works well in any type of hermetic
package including PGAs, CQFPs, CBGAs, and other ceramic packages.
Die
Lid
Heat Flow
surface
Figure 1. Heat Flow in Hermetic Package
When plastic packages gained popularity, much of the thermal analysis was left intact, such
as the
θ
JA
(junction-to-air thermal resistance) parameter, and
θ
JC
. It was assumed, incorrectly,
that the junction-to-case value could be used in plastic packages to predict junction
temperature the same way it was used for hermetic packages. The problem with
plastic packages is fundamental, and it is easily seen how the physical construction of plastic
packages negates the use of this simple parameter.
θ
JC
for
Figure 2 shows the typical construction and heat flow in a plastic quad flat pack (PQFP).
Heat flow paths are represented by a resistor network analogy in the diagram. As can be
seen from the figure, heat flow in the plastic package is very complex when compared to
the hermetic package. In plastic packages, the die is usually mounted onto a copper alloy
die pad, wire bonded to the lead fingers which radially or orthogonally emanate from the
die area, and is finally encapsulated in plastic moulding compound. Because the die is
contacted on all sides by solid matter, heat can flow easily in a multitude of directions. Due
to the copper alloy's high thermal conductivity, the heat immediately spreads into the die
attach paddle, and subsequently into the lead frame. Some heat also flows into the
moulding compound and is released by convection from the package external surfaces. It
is due to this complex heat flow that
θ
JC
is ill defined for plastic packages.
Problematic
θ
JC
for Plastic
First, the shortest thermal path is difficult to determine. If the package is thin, and the die
paddle is close to the exterior of the package, this may be the shortest path. On the other
hand, if the lead fingers are close to the die paddle, the most direct path may be through
the lead frame and into the printed circuit board(PCB). The latter of these two possible paths
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