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| 型號： | 22247J |
| 英文描述： | 3.3V EconoReset |
| 中文描述： | FBGA封裝用戶指南。版本4.2 |
| 文件頁數(shù)： | 81/97頁 |
| 文件大?。?/td> | 2624K |
| 代理商： | 22247J |
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FBGA User’s Guide
80
Version 4.2, November 1, 2002
3
is more likely, but in any case, the external environment plays a crucial role in the thermal
performance of plastic surface mount devices. As an example, if the PCB is relatively low in
thermal conductivity, e.g. no internal planes and minimized metal traces on the surface,
then the shortest path may well be through the bottom of the package. But, if the PCB has
internal voltage and heat spreading planes, the leads may dominate the heat flow.
Secondly, how are these surfaces made to be isothermal as to satisfy the original intent of
the ceramic based measurements It was believed that using a well stirred fluid bath with a
fluorinert liquid would force all surfaces on the plastic package to an isothermal state. Due
to the nature of stirred fluids, the measurement breaks down to a moving fluid measurement
and the package surfaces are not isothermal. Recently methods have been developed to
use jet impingement to create high heat transfer coefficients on package surfaces, very
nearly creating the isothermal specification. Although these methods are useful for creating
specific boundary conditions for conduction models, the measurement is still not useful for
predicting junction temperature from a known package temperature.
Figure 2. Heat Flow in Plastic Packages
Customer Use
Through the years, the real identity of
θ
JC
was diluted, and today most system houses predict
temperature by placing a thermocouple on the package surface and using the
manufacturer's published
θ
JC
values to compute junction temperature. This is a fallacy and is
wholly inaccurate. Today, it is an all too common practice and is accepted as correct. For
hermetic packages, the correct method is to install a heat sink on the surface identified as
the isothermal reference plane, and then use a thermocouple imbedded in the heat sink
and touching the case to properly calculate junction temperature using
θ
JC
. for plastic
packages, there is no equivalent method.
Enter
Ψ
J-T
During discussions with industry leading suppliers, it became obvious that the practices
described above were in use regularly and without question. Users of semiconductors had
grown accustomed to placing the thermocouple and calculating the junction temperature
without really understanding the implications of their actions. To counter this trend and
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