IPC-7095C-2013.pdf - 第84页

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 …

physical evaluations that the process and the recipe used to

make the part stay in the control that is necessary to meet

requirements. This concept is essential for BGAs since one

cannot see the lands or the solder joints once the assembly

has taken place. Test coupons or specimens should reﬂect

the speciﬁc board or panel characteristics. The data derived

from panels should be used to establish the requirements

for vias and lands, conductors, spaces, etc. When speci-

mens are used to establish process control parameters, they

shall consistently use single hole size or land conﬁguration

which reﬂects the process. Process characteristics and gen-

eral board characteristics should be matched.

IPC-2221 provides excellent coupons that are used to

evaluate those board and assembly characterizations. They

include:

• Hole Solderability

• Solder Resist Tenting

• Thermal Stress Plating

• Thickness and Bond Strength

• Plating Adhesion

• Surface Solderability

• Solder Resist

• Surface Mount Solderability

• Surface Bond Strength

• Surface Insulation Resistance

• Moisture Insulation Resistance

• Registration

• Interconnect Resistance

Figure 6-32 shows alternate coupons that can be used to

evaluate the cleanliness of a board after ball attachment has

been completed. These comb patterns are used on the board

in order to make certain that ﬂux or ﬂux residue does not

impair the electrical properties of the product.

6.7 Thermal Management The primary objective of

thermal management is to ensure that all circuit compo-

nents, especially the BGAs, are maintained within both

functional and maximum allowable limits. The functional

temperature limits provide the ambient or component pack-

age (case) temperature range within which the electronic

circuits can be allowed to properly perform.

IPC-7095c-6-31

Figure 6-31 Board Panelization

X.XX

X.XXX

X.XX

X.XXX

DATUM

X.XXX

X.XX

8 BOARD PANELIZATION

PANEL TOOLING HOLE

(3 PLACES)

INDIVIDUAL BOARD TOOLING HOLES

(3 PLACES PER BOARD)

January 2013 IPC-7095C

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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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

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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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

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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