IPC-7095C-2013.pdf - 第87页

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 add…

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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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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board thickness, etc. The test plots should be employed to

verify that the data matches the requirements.

Other documentation may include numerical control data

for drilling, routing, libraries, tests, artwork, and special

tooling. There are design and documentation features/

requirements that apply to the basic layout, the production

master (artwork), the mounting structure itself, and the end

item component or printed board assembly. All must be

taken into consideration during the design of the mounting

structures for the bare die, or the mounting structure for the

BGA. Documentation shall meet the requirements of IPC-

2611. In order to provide the best documentation package

possible, it is important to review IPC-2611 and identify all

the criteria that are affected by the design process.

6.8.1 Drawing Requirements During the formal design

review prior to layout, special tools that can be generated

by the design area in the form of artwork or numerical

control data shall be considered. This tooling may be

needed by fabrication, assembly, or testing. Examples of

such tooling are drilling data ﬁles, solder paste stencils,

data for component placement, ICT ﬁxture data, and art-

work in the form of layered graphics, silk screen and sol-

der mask.

When viewing the documentation, it is always viewed from

the primary side. All phototool generation is viewed from

that same direction. The deﬁnition of layers of the product

shall be viewed looking through the particular part from

the primary side.

Accuracy and skill must be sufficient to eliminate misinter-

pretation during the artwork generation process. This

requirement can be minimized by adhering to a grid or

pitch of the component terminations, which drive so many

features on the board or the bare die.

Layout notes should be as complete as possible with the

addition of appropriate notations. Marking requirements

and revision status level deﬁnition are key to maintaining

conﬁguration management conditions. It is especially

important for the engineering review cycle, a quoting

effort, and when the document is used by someone other

than the originator.

6.8.2 Equipment Messaging Protocols Throughout the

electronics industry manufacturing consists of numerous

steps, each of which often centers around one supplier’s

equipment. Although the steps are generally well-

automated within themselves, they are only peripherally

connected to one another. Proprietary data formats and

communications protocols prevent the islands of automa-

tion from talking to one another. This condition also pre-

vents factory managers from monitoring, understanding,

and possibly correcting the manufacturing process to

improve throughput and product quality.

In the last few years, building on an International Electron-

ics Manufacturing Initiative (iNEMI) funded project, the

IPC has standardized data syntax and semantics in elec-

tronics assembly, establishing rules for data exchange both

on a single factory ﬂoor and between that ﬂoor and the rest

of a manufacturing organization. The results have been

published as the IPC CAMX (computer-aided manufactur-

ing using the extensible markup language [XML]) stan-

dards: The standards are:

• IPC-2541, Generic Requirements for Electronics Manu-

facturing Shop-Floor Equipment Communication

• IPC-2546, Sectional Requirements for Shop-Floor Equip-

ment Communication Messages (CAMX) for Printed Cir-

cuit Board Assembly

• IPC-2547, Sectional Requirements for Shop Floor Equip-

ment Communication Messages (CAMX) for Printed Cir-

cuit Board Test, Inspection and Rework

At the heart of the standard is a framework with an inter-

mediary. This is the ‘‘message broker’’ that handles infor-

mation exchange and complies with the IPC-2501 Standard

(‘‘Deﬁnition for Web-based Exchange of XML Data’’). The

message broker can be thought of as a post office or a mail

server. Messages are sent to the server and, when the infor-

mation is needed, it is asked for by the equipment or the

manager who wants the data.

In a factory, several lines of equipment and several appli-

cations may connect to the message broker at the same

time. Individual elements need not know any details about

the nature, conﬁguration, or format of the others. They

communicate directly only with the broker. When people

and equipment need speciﬁc information, the broker pro-

vides it in the correct format.

6.8.2.1 Implementation It is relatively easy to set up the

infrastructure and web-based tools to monitor several sets

of manufacturing equipment, including in-circuit testers,

and several suppliers’ pick-and-place machines. The setup

can be used to gathered performance and functional feed-

back data on the message broker. The message broker can

usually be up and running in two days, being able to con-

nect reasonably easily with both legacy equipment and any

XML-ready equipment using an infrastructure that is in

place at the manufacturing site. In a careful installation, it

is also possible to avoid compromising network security

requirements.

Using internet standards like HTTP and XML ensures the

interoperability between different platforms. Although an

application has never been run against a running message

broker before, it can be adapted to the existing communi-

cation framework on site. One important goal of the IPC-

CAMX standards is to lower the technological barrier to

ease the integration of sophisticated equipment like pick-

and-place machines and test equipment, as well as simpler

equipment like printed board handlers. The CAMX stan-

dards provide data about the products under manufacture,

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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