IPC-7095C-2013.pdf - 第85页

or blower . Heat transfer by forced convection can be as much as ten times more ef fective than natural convection. 6.7.4 Thermal Interface Materials Attachment of heat sinks to BGAs is the most common technique today in…

The cooling technique to be used in the printed board

assembly applications must be known in order to ensure

the proper printed board assembly design. For commercial

applications, direct-air cooling (i.e., where cooling air con-

tacts the printed board assembly), is usually used.

For rugged and hostile usage, other cooling techniques

must be used to cool the printed board assembly. In this

application, the assembly is mounted to the cooling struc-

ture that is air or liquid cooled, and the board components

are cooled by the conduction through a heat exchange sur-

face. These designs must use appropriate metal heatsinks

on the printed board assembly. Appropriate component

mounting and bonding may be required. To ensure

adequate design, thermal dissipation maps must be pro-

vided to aid analysis and thermal design of the printed

board assembly.

The dissipation of heat generated within electronic equip-

ment results from the interaction of the three basic modes

of heat transfer: conduction, radiation, and convection.

These heat transfer modes can, and often do, act simulta-

neously. Thus, any thermal management approach should

attempt to maximize their natural interaction.

6.7.1 Conduction Conduction takes place to a varying

degree through all materials. The conduction of heat

through a material is directly proportional to the thermal

conductivity constant (K) of the material, the cross-

sectional area of the conductive path and the temperature

difference across the material. Conduction is inversely pro-

portional to the length of the path and the thickness of the

material (see Table 6-11).

6.7.2 Radiation Thermal radiation is the transfer of heat

by electromagnetic radiation, primarily in the infrared (IR)

wavelengths. It is the only means of heat transfer between

bodies that are separated by a vacuum, as in space environ-

ments.

Heat transfer by radiation is a function of the surface of the

‘‘hot’’ body with respect to its emissivity, its effective sur-

face area and the differential to the fourth power of the

absolute temperatures involved.

The emissivity is the ratio of energy radiated by a material

to energy radiated by a black body with an emissivity of

one (1.0) at the same temperature The optical color of a

body has little to do with it being a ‘‘thermal black body.’’

The emissivity of anodized aluminum is the same if it is

black, red or blue. However, surface ﬁnish is important. A

matte or dull surface will be more radiant than a bright or

glossy surface (see Table 6-12).

Devices, components, etc. close to one another will absorb

each others’ radiant energy. If radiation is to be the prin-

ciple means of heat transfer, ‘‘hot’’ spots must be kept clear

of each other.

6.7.3 Convection The convection heat transfer mode is

the most complex. It involves the movement of molecules

within a ﬂuid, usually air.

The rate of heat ﬂow by convection from a body to a ﬂuid

is a function of the surface area of the body, the tempera-

ture differential, the velocity of the ﬂuid and certain prop-

erties of the ﬂuid.

The contact of any ﬂuid with a hotter surface reduces the

density of the ﬂuid and causes it to rise. The circulation

resulting from this phenomenon is known as ‘‘free’’ or

‘‘natural’’ convection. The air ﬂow can be induced in this

manner or by some external artiﬁcial device, such as a fan

IPC-7095c-6-32

Figure 6-32 Comb Pattern Examples

IPC-7095C January 2013

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or blower. Heat transfer by forced convection can be as

much as ten times more effective than natural convection.

6.7.4 Thermal Interface Materials Attachment of heat

sinks to BGAs is the most common technique today in

cooling the silicon devices packaged within the BGAs.

These heat sinks require a thermal interface material to be

sandwiched between the heat sink and the BGA in order to

conduct the heat from the top of the package surface and

into the bottom of the heat sink surface.

When selecting a thermal interface material, keep in mind

the surface ﬂatness of the BGA body and the heatsink.

Warpage of the BGA package during reﬂow and large tol-

erances on the heatsink contact surface can result in large

gaps that are difficult to ﬁll reliably with some interface

materials. This in turn can cause poor thermal conductivity

and/or a weak heatsink attachment bond.

There are various types of thermal interface materials

(TIM). These are described below.

6.7.4.1 Adhesives Metal ﬁlled epoxies and silicone

adhesives were commonly used as TIMs initially. They

play a dual part of being a TIM as well as a mechanical

attachment method since when cured they become highly

cross-linked and attain high cohesive strength. Hence,

unlike the other TIMs, supplementary mechanical attach-

ment methods are not necessary when using adhesive. Dis-

advantages of adhesives include a thermal cure step being

necessary after the BGA is soldered on the board and the

potential for severe delamination at the interfaces that the

adhesive bonds to due to coefficient of thermal expansion

mismatches between the heat sink and the package.

Another subclass of adhesives are the Pressure Sensitive

Adhesives (PSAs), which do not require a cure to generate

the interfacial bond, but instead require a certain amount of

pressure, typically in the 20 to 30 psi range. Their use for

BGAs is therefore limited since this pressure, if not prop-

erly controlled, may adversely impact the BGA solder

joints.

6.7.4.2 Greases Thermal greases are metal ﬁlled poly-

mers that have the inherent advantage of being a viscous

liquid and conforming to the macroscopic and microscopic

irregularities in the surfaces of the heat sink and the BGA

component. They have excellent thermal performance and,

unlike adhesives, do not require a cure. A major disadvan-

tage of thermal greases is that they tend to migrate out

from between the heat sink and package interfaces over

time. This phenomenon is known as ‘‘pump-out,’’ and is

caused by the thermo-mechanical stresses exerted at the

interfaces during temperature cycling.

6.7.4.3 Phase Change Materials (PCM) Phase change

materials are in a solid state at room temperature but

become liquid at the higher temperatures at which they are

required to conduct heat away from the BGA surface.

Hence, they offer ease of handling and dispensing because

they typically are in a ﬁlm form and do not need to be

cured. However, their thermal conduction properties are

inferior to greases, adhesives and the other TIM alterna-

tives so they are only suitable for use with low wattage

devices.

Table 6-11 Effects of Material Type on Conduction

Materials

Thermal Conductivity (K)

Watts/inch °C Watts/m °C Gram-calorie/cm °C • s

Still Air 0.0007 0.0276 0.000066

Epoxy 0.005 0.200 0.00047

Thermally Conductive Epoxy 0.02 0.787 0.0019

Aluminum Alloy 1100 5.63 222 0.530

Aluminum Alloy 3003 4.88 192 0.459

Aluminum Alloy 5052 3.52 139 0.331

Aluminum Alloy 6061 4.36 172 0.410

Aluminum Alloy 6063 4.88 192 0.459

Copper 4.93 194 0.464

Steel Low Carbon 1.19 46.9 0.112

Table 6-12 Emissivity Ratings for Certain Materials

Material and Finish Emissivity

Aluminum Sheet - Polished 0.040

Aluminum Sheet - Rough 0.055

Anodized Aluminum - any color 0.80

Brass - Commercial 0.040

Copper - Commercial 0.030

Copper - Machined 0.072

Steel - Rolled Sheet 0.55

Steel - Oxided 0.667

Nickel Plate - Dull Finish 0.11

Silver 0.022

Tin 0.043

Oil Paints - Any Color 0.92-0.96

Lacquer - Any Color 0.80-0.95

January 2013 IPC-7095C

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6.7.4.4 Gels Gels are comprised of a lightly cross-linked

silicone polymer ﬁlled with metal or ceramic particles that

impart the thermal conductivity necessary for this TIM.

Gels combine the beneﬁcial properties of greases and cured

adhesives, do not pump-out and do not require a post appli-

cation cure step. Their modulus is low enough to relieve

thermo-mechanical stresses and avoid interfacial delamina-

tions. They have high bulk thermal conductivities and have

been used in cooling BGA components containing high

wattage CPU devices.

6.7.4.5 Thermal Conductive Pressure Sensitive Tape

Thermal conductive pressure sensitive tape may occasion-

ally be used to impart the thermal conductivity necessary to

remove heat from the BGA. This thermal interface material

is becoming more widely used due to the ease of handling

and applying to the surfaces that are used for cooling BGA

components.

6.7.5 Heat Sink Attachment Methods for BGAs There

are quite a few techniques for attaching heat sinks to

BGAs. These are depicted in the following illustrations.

Figure 6-33 shows a heat sink attached to the top of a BGA

package with a thermally conductive adhesive. The adhe-

sive acts as both a thermal conduction medium as well as

a mechanical attachment medium. As mentioned above,

however, this technique requires a post solder thermal cure

step to cross-link the adhesive and harden it.

Figure 6-34 illustrates a heat sink attached to the top of a

BGA package with clips that hook on to a BGA substrate.

However, there is danger of damage to the solder joints

while attaching the clips. The thermal interface material in

this case is a grease, or PCM or gel, i.e., one that does not

provide a strong mechanical bond between the heat sink

and the top of the BGA package. This method has one

drawback. The weight of the heat sink is supported by the

package and, during mechanical shock and vibration, the

solder balls of the BGA have to bear the mechanical

stresses generated by the additional mass of the heat sink.

Figure 6-35 depicts the case of a heat sink attached to the

BGA with clips that hook into holes in the printed circuit

board. These holes do not have to be plated. Unlike the

IPC-7095c-6-33

Figure 6-33 Heat Sink Attached to a BGA with an Adhesive

BGA

Adhesive

Board

Heat Sink

IPC-7095c-6-34

Figure 6-34 Heat Sink Attached to a BGA with a Clip that Hooks onto the Component Substrate

Grease/

Gel/PCM

Board

Clip

BGA

Heat Sink

IPC-7095c-6-35

Figure 6-35 Heat Sink Attached to a BGA with a Clip that Hooks into a Through-Hole on the Printed Circuit Board

Board

Clip

Grease/

Gel/PCM

BGA

Heat Sink

IPC-7095C January 2013

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No reproduction or networking permitted without license from IHS

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