IPC-7095C-2013.pdf - 第86页

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 greas…

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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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

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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previous case, the printed circuit board supports the weight

of the heat sink when the clips are actuated, but some stress

can still be transferred on to the BGA solder balls during

mechanical shock and vibration. In addition, the heat sink

may create a cantilevered load on the solder joint when the

assembly is in a vertical orientation. This load may lead to

premature solder joint failure.

Figure 6-36 shows the case of a heat sink attached to a

BGA package with clips that hook on stakes that are wave

soldered into holes in the printed circuit board. This

method of attachment transfers even less stress on the BGA

solder balls during mechanical shock and vibration than the

previous case. However, the solder joints of the stakes will

bear most of this stress.

Figure 6-37 illustrates the attachment of the heat sink to a

BGA by directly soldering the heat sink into the board dur-

ing the wave soldering process step. The heat sink design

has four or more pins that insert into holes in the board

prior to the wave solder process. As opposed to the previ-

ous cases above, this method does not need any post

assembly processing to attach the heat sink.

The three methods in Figures 6-30, 6-31 and 6-32 have one

drawback not present in the ﬁrst two. These methods

require holes in the printed circuit to be designed in. These

holes may reduce the trace routing real estate on all the

board layers. For highly dense board designs, this could

impact the ﬁnal layer count of the board.

6.8 Documentation and Electronic Data Transfer The

documentation package for describing BGA components

usually consists of a master drawing, master pattern draw-

ing, copies of artwork (ﬁlm or paper), mounting structure

assembly drawing, parts list, and schematic/logic diagram.

The documentation package may be provided in either hard

copy or electronic data. All information about documenta-

tion is also appropriate for electronic data transmission.

Since many CAD systems have their own native database,

everyone is promoting some form of unique format that has

a neutral concept, thus avoiding sending the native data-

base to the suppliers.

The lowest common denominator for years has been a

machine language. This may eventually be replaced by

such formats as the IPC-D-356, or IPC-2581. Archiving

electronic data should be in accordance with these docu-

ments. Delivery of computer generated data as a part of the

documentation package should meet the requirements

stated in those packages. With automated techniques, the

database shall detail all information that will be needed to

produce the printed board or mounting structure for the

bare die. This includes all notes, plating requirements,

IPC-7095c-6-36

Figure 6-36 Heat Sink Attached to a BGA with a Clip that Hooks onto a Stake Soldered in the Printed Circuit Board

Grease/

Gel/PCM

Board

BGA

Heat Sink

Clip

IPC-7095c-6-37

Figure 6-37 Heat Sink Attached to a BGA by Wave Soldering Its Pins in a Through-Hole in the Printed Circuit Board

Grease/

Gel/PCM

BGA

Heat

Sink

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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