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參數(shù)資料
| 型號： | MC74LCX2952SD |
| 廠商： | ON SEMICONDUCTOR |
| 元件分類： | 總線收發(fā)器 |
| 英文描述： | LVC/LCX/Z SERIES, 8-BIT REGISTERED TRANSCEIVER, TRUE OUTPUT, PDSO24 |
| 封裝： | PLASTIC, SSOP-24 |
| 文件頁數(shù)： | 31/43頁 |
| 文件大?。?/td> | 383K |
| 代理商： | MC74LCX2952SD |
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Reliability Information
LCX DATA
BR1339 — REV 3
247
MOTOROLA
Thermal Considerations
Prepared by: Lance K. Packer
LCX Application Engineering
Reliability of Plastic Packages
Although today’s plastic packages are as reliable as
ceramic packages under most environmental conditions, as
the junction temperature increases a failure mode unique to
plastic packages becomes a significant factor in the long
term reliability of the device.
Modern plastic package assembly utilizes gold wire
bonded to aluminum bonding pads throughout the
electronics industry. As the temperature of the silicon
(junction temperature) increases, an intermetallic compound
forms between the gold and aluminum interface. This
intermetallic formation results in a significant increase in the
impedance of the wire bond and can lead to performance
failure of the affected pin. With this relationship between
intermetallic formation and junction temperature established,
it is incumbent on the designer to ensure that the junction
temperature for which a device will operate is consistent with
the long term reliability goals of the system.
Reliability studies were performed at elevated ambient
temperatures (125
°C) from which an Arrhenius Equation
(Eq 1), relating junction temperature to bond failure, was
established. The application of this equation yields the values
in 1. . This table relates the junction temperature of a device
in a plastic package to the continuous operating time before
0.1% bond failure (1 failure per 1000 bonds).
( Eq 1 )
T = 6.376
× 10 –9 e
11554.267
273.15 + TJ
Where:
T = Time to 0.1% bond failure
1. . Tj vs Time to 0.1% Bond Failure
Junction
Temp. (
°C)
Time (hours)
Time (yrs.)
80
1,032,200
117.8
90
419,300
47.9
100
178,700
20.4
110
79,600
9.1
120
37,000
4.2
130
17,800
2.0
140
8,900
1.0
Thermal Management
As in any system, proper thermal management is
essential to establish the appropriate trade–off between
performance, density, reliability and cost. In particular, the
designer should be aware of the reliability implication of
continuously operating semiconductor devices at high
junction temperatures.
The increasing popularity of surface mount devices (SMD)
is putting a greater emphasis on the need for better thermal
management of a system. This is due to the fact that SMD
packages generally require less board space than their
through hole counterparts so that designs incorporating SMD
technologies have a higher thermal density. To optimize the
thermal management of a system it is imperative that the
user understand all of the variables which contribute to the
junction temperature of the device.
The variables involved in determining the junction
temperature of a device are both supplier and user defined.
The supplier, through lead frame design, mold compounds,
die size and die attach, can positively impact the thermal
resistance and the junction temperature of a device. Motorola
continually experiments with new package designs and
assembly techniques in an attempt to further enhance the
thermal performance of its products.
It can be argued that the user has the greatest control of
the variables which commonly impact the thermal
performance of a device. Depending on the environment in
which an IC is placed, the user could control over 75% of the
current that flows through the device. Ambient temperature,
air flow and related cooling techniques are the obvious user
controlled variables, however, PCB substrate material, layout
density, size of the air–gap between the board and the
package, amount of exposed copper interconnect, use of
thermally–conductive epoxies and number of boards in a box
and output loading can all have significant impacts on the
thermal performance of a system.
PCB substrates all have different thermal characteristics,
these characteristics should be considered when exploring
the PCB alternatives. The user should also account for the
different power dissipations of the different devices in his
system and space them on the PCB accordingly. In this way,
the heat load is spread across a larger area and “hot spots”
do not appear in the layout. Copper interconnect traces act
as heat radiators, therefore, significant thermal dissipation
can be achieved through the addition of interconnect traces
on the top layer of the board. Finally, the use of thermally
conductive epoxies can accelerate the transfer of heat from
the device to the PCB where it can more easily be passed to
the ambient.
The advent of SMD packaging and the industry push
towards smaller, denser designs makes it incumbent on the
designer to provide for the removal of thermal energy from
the system. Users should be aware that they control many of
the variables which impact the junction temperatures and,
thus, to some extent, the long term reliability of their designs.
Calculating Junction Temperature
The following equation can be used to estimate the
junction temperature of a device in a given environment:
TJ = TA + PDΘJA
where:
TJ
= Junction Temperature
TA
= Ambient Temperature
PD
= Power Dissipation
ΘJA = Avg Pkg Thermal Resistance (Junction Ambient)
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