IPC-D-279 EN.pdf - 第77页

system enclosure. It represents the temperature of the ulti- mate heat sink towards which the entire ener gy of the sys- tem is evacuated, and it represents the lowest temperature of reference. The ambient temperature is…

Appendix D

Thermal Considerations

D-1.0 GENERAL

The designer of an SM PWA will be constrained by several

external thermal factors:

A. The preference for cost and noise reasons is to cool

by free convection or by conduction of heat to the

outside atmosphere through thermally conducting

copper planes, metallic cardguides and thermally

conducting structural ribs in the system chassis.

B. The preference for contamination and corrosion con-

trol is to seal the PWA away from external air.

C. The heat densities and hotspot temperatures may

require active (forced) cooling with media such as air

or heat control ﬂuids moved by fans or pumps.

D. The spacing between PWAs, the component spacings

and geometries, as well as air velocities may result in

either laminar or turbulent ﬂows, affecting both heat

removal from heat dissipaters and heat transfer to

thermally sensitive components.

The primary thermal parameter which the designer must

address is the temperature of the junction or active ﬁlm of

the component; both the absolute maximum or peak tem-

perature and the steady state operating temperature limits

imposed by the component manufacturer (as modiﬁed by

derating protocols) must be observed. The secondary ther-

mal parameter is the solder joint temperature since large

temperature swings in service will subject the joint to con-

ditions leading to cyclic fatigue, and long service times at

high temperatures will grow copper-tin intermetallic com-

pounds. See also IPC-SM-785, IPC-D-275, and IPC-SM-

782.

For near-eutectic tin/lead solders - the most commonly

used solders for surface mounted assemblies - good engi-

neering practices result in typical operating temperatures of

about 70°C. Higher levels of temperature are associated

with faster aging of the solder joints due to metallurgical

changes of the material structure and composition. Cyclic

variations of the joint temperature are associated with

fatigue degradation, solder being the compliant part of the

system which has to accommodate relatively large differen-

tial expansions of the different materials. The fatigue life of

the system is dictated primarily by the temperature swing,

dwell time and lead compliancy and it is less dependent on

the absolute value of temperature and the rise time neces-

sary to reach the extreme values, providing that the varia-

tion in temperature is slower than 30°C per minute. These

factors show the importance of determining the accurate

thermal conditions − both maximum static temperature and

cycling parameters − at the solder joint layer.

Variation of the external (outside of the equipment enclo-

sure) ambient temperature is one of the multitude of factors

that will determine the actual temperature cycle a speciﬁc

surface mounted device will see in operation. Very simple

equipment, powered continuously at constant power will

see the same temperature swings as the external ambient.

In some cases, the system designer introduces built-in

means of reducing the temperature swing inside the cabi-

net, such as fans activated when the inlet air temperature

exceeds certain limits or inlet air heaters which are

activated when inlet air temperature drops below certain

limits.

In many applications, the variation of the temperature

inside the electronic enclosure is generated by variations of

the power dissipated by the electronics itself. Examples of

this type of behavior are on/off periods for the system or

ﬂuctuations in the power dissipation as in telecommunica-

tion equipment due to variations in the number of simulta-

neous calls passing through the system. Relatively large

temperature variations could be generated inside the system

between the periods with high traffic, mainly during the

working hours, and the periods with low traffic, usually

evening or night hours, even though the system is main-

tained inside an air conditioned room with practically no

ambient temperature variations. A device mounted down-

stream of a high power dissipator sees a temperature varia-

tion related to the variation of the temperature of the ther-

mal wake produced by the power dissipator even though

the temperature inside the enclosure is maintained constant.

In most applications, the temperature variations at a par-

ticular component in a system results from a combination

of system-external and system-internal temperature varia-

tions combined with power dissipation ﬂuctuations within

the component. It follows that different devices inside the

same system might be subjected to very different tempera-

ture cycles. In order to assess the reliability of the solder

joints, the designer must perform a complete thermal

analysis at the device level.

D-2.0 THERMAL ANALYSIS AT THE DEVICE LEVEL

The best way of understanding the different factors that

impact the device temperature and implicitly the system

reliability, is to express the junction temperature on the

silicon, T

, as a summation of temperatures rise at different

levels of integration in the system

+ ∆T

These factors are discussed in D-2.1 through D-2.5 below.

D-2.1 The Ambient Temperature of an Electronic Sys-

tem (T

) The ambient temperature of an electronic system

is deﬁned as the average temperature monitored outside the

July 1996 IPC-D-279

system enclosure. It represents the temperature of the ulti-

mate heat sink towards which the entire energy of the sys-

tem is evacuated, and it represents the lowest temperature

of reference. The ambient temperature is different for dif-

ferent types of applications. It can be a near constant as for

implanted medical equipment, or it can vary over a wide

range of temperatures as for automotive, military or space

applications. For any speciﬁc case, this temperature is dic-

tated by the application and cannot be modiﬁed by design.

D-2.2 The Temperature Rise of the Cooling Agent at the

Device Level (T

) Except for some space applications,

the heat dissipated by device is transferred ﬁrst to a ﬂuid

cooling agent which transports the energy outside the sys-

tem to the ultimate heat sink. For most of the applications

the cooling agent is air which comes in direct contact with

the device. The mechanism of heat transfer is conjugate

convection conduction: part of the heat is transferred direct

to the ﬂuid, part is spread ﬁrst into the solids (board, enclo-

sure) and from there to the ﬂuid. In most cases the tem-

perature of the ﬂuid reaching the device is higher than the

ambient temperature (the temperature of the ultimate heat

sink) due to the fact that the ﬂuid already absorbed some

energy from other devices mounted upstream in the system.

The designer has the power to modify this term by modi-

fying the device position in the system or by introducing

additional cooling/heating elements in order to moderate

dangerous high/low absolute temperature values or strong

temperature swings. The temperature rise of the cooling

agent is also reduced if the mass (volume) ﬂow of the fluid

is increased.

D-2.3 The Temperature Rise Inside the Device Bound-

ary Layer (T

) The temperature rise inside the device

thermal boundary layer is related to the case-to-ambient

thermal resistance of the package. A certain gradient of

temperature is necessary in order to maintain the heat ﬂow

from the package into the ﬂuid acting as a cooling agent.

For devices dissipating less than 300 mW, this gradient of

temperature (the difference in temperature between the

package to the surrounding ﬂuid) is less than 5°C. This is

probably the reason that most designers have the tendency

of ignoring this term, focusing on the temperature rise of

the cooling agent at the device level as the critical factor of

ensuring the device functionability and reliability. As the

power per device increases, the temperature rise inside the

device thermal boundary layer becomes a critical factor.

For devices dissipating more than 1 W, this term alone

could be higher than 30°C and is certainly one of the domi-

nant terms in predicting the silicon junction temperature.

The designer has the capability of controlling the tempera-

ture rise inside the device thermal boundary layer by

increasing the local ﬂuid (air) velocity, by using heat

spreaders or increasing the substrate thermal conductivity.

D-2.4 The Temperature Rise Inside the Device Package

) Sometimes called junction-to-case temperature rise,

this term represents the temperature difference between the

silicon chip and the external temperature of the device

package. Being related to the thermal conductivity of the

materials used for packaging, this term can be controlled

by proper selection of materials and use of materials with

enhanced thermal properties. For devices dissipating high

powers (more than 0.5 W), the resistance of the interfaces

between different layers of materials (contact resistances)

becomes signiﬁcant and discontinuities, delaminations and

voids could add up to signiﬁcant thermal resistances.

D-2.5 Thermal Wake (T

) The thermal wake factor

should be considered when devices are mounted close

together and their thermal boundary layers intersect with

each other. This is particularly important when a sensitive

device is mounted downstream from a high power dissipa-

tor. Under such conditions the device is engulfed inside the

thermal plume of the upstream device. Even though this

term is generally low (lower than 5°C) it can produce sig-

niﬁcant problems if the layout of the board is not carefully

controlled.

D-3.0 DETERMINING THE SOLDER JOINTS TEMPERA-

TURE SWINGS

Considering the multitude of factors which determine the

device temperature, it can easily be seen that variation of

the ambient temperature alone is an indication of the actual

temperature variations the solder joint will see in the ﬁeld

conditions. Assuming the temperature of the solder joint is

almost identical with the external temperature of the pack-

age (case temperature), the temperature cycling is gener-

ated by two factors:

a) the variation of the local environmental conditions

for the considered device (sometimes called adiabatic

temperature or the temperature of the device with

power off)

b) the variation of the device power, which in turn pro-

duces variations of the temperature rise inside the

device thermal boundary layer.

The device adiabatic temperature is the equivalent of the

local ambient temperature for a speciﬁc device inside the

system. It is deﬁned as the temperature that an element

achieves when the convective heat from the element to the

ﬂuid goes to zero. When conduction and radiation from

other elements is negligible, the device adiabatic tempera-

ture is the temperature achieved when no power is applied

to the device and the rest of the system is activated.

The variation of the device adiabatic temperature is the real

temperature cycle that a device operating at a constant

power will see inside the system. The temperature cycle

could be very different for different devices inside the same

system, even though the external ambient is the same.

IPC-D-279 July 1996

When variations of dissipated power are involved, the tem-

perature swing related to the power cycling must be added

to the variations of the device adiabatic temperature. When

the difference in temperature between the device case and

the board underneath the device is small (not larger than

5°C), the simple superposition of the two cycles (local

environment and power cycling) is quite adequate for esti-

mating the real temperature cycle the solder joint of a spe-

ciﬁc device will see in different environments and under

different operational conditions.

D-4.0 COOLING OF ELECTRONIC EQUIPMENT

D-4.1 Radiation

Cooling PWAs by radiation is not prac-

tical as a primary heat transfer mechanism because of the

large temperature differences required for appreciable heat

transfer. In addition, control of heat paths is difficult due to

scattering of radiant energy. In fact, radiant heat transfer is

potentially objectionable because of the possible heating of

near-by temperature sensitive parts by hotter, less sensitive

components. The positive effects of radiation from SMT

printed boards can usually be neglected.

D-4.2 Free Convection There are some commercial

SMT applications which could effectively utilize free con-

vection, although even with free ﬂowing ambient air, the

packaging density would need to be decreased, with at least

5 mm, and preferably up to 20 mm spacings between ver-

tically oriented boards, with heat concentrations of no more

than 12,000 W/m

correspondingly, about 300 W/m

board area dissipation, based on the component side of the

board only; for boards with components on both sides, a

rough guideline would be 230 W/m

per side, maintaining

12,000 W/m

maximum.

These numbers for free convection printed board dissipa-

tions are relatively insensitive to the variations of printed

board size and calculated heat transfer coefficient and hold

for an air-to-board ∆T of 40°C. For sufficiently cooler air

or hotter board temperatures, greater dissipations can be

accommodated. For example, if a 60°C ∆T was acceptable,

then 60/40 = 1.5 times as much dissipation could be

allowed. A detailed design utilizing conduction spreading

of heat within the board should be performed around hot

spots in free convection designs to insure maximum junc-

tion temperature limits are maintained.

For SMT boards in military applications, free convection is

typically a second order effect in removing heat directly

from the board. This is especially true when, as would be

expected for SMT, that the boards are densely packaged in

a structure.

D-4.3 Direct Forced Convection Forced ambient air

directly over the board and components, is the most widely

utilized commercial method for cooling SMT (and high

density) printed boards. Forced conditioned or closed loop

air is also common in many military applications. The pen-

alties for this effective cooling mechanism is expenditure

of energy, added heat, weight, and volume, taken at the

enclosure or system level. The SMT printed board designer

will work to the requirements deﬁning air ﬂow rate and

temperature over the module, as well as possible variations

for altitude, in airborne applications.

Typically, 4 to 8 times as much heat can be removed with

forced convection compared to free convection. As a result,

heat dissipations in the range of 1500 to 3000 W/m

component side printed board area can be achieved with an

average module-to-air ∆T of 40°C.

D-4.4 Conduction Cooling SMT printed boards built

with conductive substrates or ‘‘cores’’ such as beryllium

oxide, aluminum, or one of several materials designed to

match the expansion rate of ceramic components (about 6

ppm/°C), can and should be conductively coupled to the

supporting structure by clamping the edges.

The allowable heat densities are signiﬁcantly greater than

those of free convection, and are a strong function of sub-

strate material, thickness, location of heat sensitive parts

and conduction paths of these parts to the substrate. The

boundary condition may be the temperature of the printed

board module substrate edge, or possibly the structural sur-

face to which this edge mates. In the latter case, the printed

board designer is dependent on the design of the structure

for the interface resistances, a key parameter in the conduc-

tion path to the ultimate heat sink. For a 40°C device

junction-to-module edge ∆T, dissipations on the order of

250,000 W/m

, or for a speciﬁc module, on the order of

4,500 W/m

can be maintained.

D-4.5 Heat Pipes The heat pipe is a thermal conductor

of very high conductivity. It is essentially a closed evacu-

ated chamber lined with a capillary structure or wick. Heat

is transported by evaporation of a suitable volatile ﬂuid,

which is condensed at the cold end and returned by capil-

lary force to the hot end. The vapor passes through the

cavity. Heat pipes can be constructed in practically an end-

less number of conﬁgurations, but always consist of three

zones or sections, namely: the evaporator, condenser, and

the adiabatic section connecting the other two. In some

designs, the adiabatic section may be very short.

Thermal resistances of 1°C/kW can be obtained and heat in

excess of 50 kilowatts can be transferred. Heat pipes offer

important advantages. First, a heat pipe has several thou-

sand times the heat transfer capacity of the best heat con-

ducting materials on a weight and size basis. Second, heat

pipes exhibit an essentially uniform temperature at the heat

input end. Third, the areas and conﬁgurations of contact at

each end of a heat pipe are independent and can be

designed separately to suit the application.

July 1996 IPC-D-279