IPC-7095C-2013.pdf - 第83页

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 joint…

6.5.4.2 In-Circuit Test Concerns In-circuit testing is

used to ﬁnd shorts, opens, wrong parts, reversed parts, bad

devices, incorrect assembly of printed board assemblies

and other manufacturing defects. In-circuit testing is nei-

ther meant to ﬁnd marginal parts nor to verify critical tim-

ing parameters or other electrical design functions.

In-circuit testing of digital printed board assemblies can

involve a process that is known as backdriving (see IPC-T-

50). Backdriving can also cause devices to oscillate and the

tester can have insufficient drive to bring a device out of

saturation. Backdriving can be performed only for con-

trolled periods of time, or the junction of the device (with

the overdriven output) will overheat.

The two main concerns for designing the printed board and

printed board assembly for in-circuit testability are design

for compatibility with in-circuit test ﬁxturing and electrical

design considerations.

6.5.4.3 Functional Testing Concerns There are several

concerns for designing the printed board assembly for

functional testability. The use of test connectors, problems

with initialization and synchronization, long counter

chains, self-diagnostics, and physical testing are topics

which are discussed in detail in the following subsections

and are not meant to be tutorials on testability but rather

ideas of how to overcome typical functional testing prob-

lems. Fault isolation on conformal coated boards or most

SMT and mixed technology designs can be very difficult

because of the lack of access to the circuitry on the board.

If strategic signals are brought out to a test connector or an

area on the printed board where the signals can be probed

(test points), fault isolation may be much improved. This

lowers the cost of detection, isolation and correction. It is

also possible to design the circuit so that a test connector

can be used to stimulate the circuit (such as taking over a

data bus via the test connector) or disable functions on the

printed board assembly (such as disabling a free running

oscillator and adding single step capability via the test con-

nector).

6.6 Other Design for Manufacturability Issues The lay-

out generation process should include a formal design

review of layout details by as many affected disciplines

within the company as possible, including fabrication,

assembly and testing. The approval of the layout by repre-

sentatives of the affected disciplines will ensure that these

production-related factors have been considered in the

design. The success or failure of an interconnecting struc-

ture design depends on many interrelated considerations.

From an end-product usage standpoint, the impact on the

design by the following typical parameters should be con-

sidered. Other design for manufacturability issues include:

• Equipment environmental conditions, such as ambient

temperature, heat generated by the components, ventila-

tion, shock and vibration

• If an assembly is to be maintainable and repairable, con-

sideration must be given to component/circuit density, the

selection of board/conformal coating materials, and com-

ponent placement for accessibility

• Installation interface that may affect the size and location

of mounting holes, connector locations, lead protrusion

limitations, part placement, and the placement of brackets

and other hardware

• Testing/fault location requirements that might affect com-

ponent placement, conductor routing, connector contact

assignments, etc.

• Process allowances such as etch factor compensation for

conductor widths, spacings, land fabrication, etc.

• Manufacturing limitations such as minimum etched fea-

tures, minimum plating thickness, board shape and size,

etc.

• Coating and marking requirements

• Assembly technology used, such as surface mount

• Through-hole, and mixed

• Board performance class

• Materials selection

• Producibility of the printed board assembly as it pertains

to manufacturing equipment limitations

– Flexibility (Flexural) Requirements

– Electrical/Electronic

– Performance Requirements

• ESD sensitivity considerations

6.6.1 Panel/Pallet Design Panelization of boards and

pallets is a standard process for both test and assembly. A

datum system is required for the fabrication panel, as well

as each individual board or pallet in the panel. To reduce

tolerance buildup, it is important to relate each individual

datum to the panel datum (see Figure 6-31).

Most assembly companies want to build the assembly in

pallet format, similar to that shown in Figure 6-31. The

board manufacturer of these pallets would position them

on the standard fabrication panel, which is usually 460 x

610 mm. Designers are encouraged to work with their

manufacturing suppliers in order to optimize the material

movement, and the manner in which the pallets/boards are

removed from their respective position and how they are

tested.

6.6.2 In-Process/End Product Test Coupons Coupons

have been used by the industry for many years in an effort

to evaluate the product being built. These coupons repre-

sented the features of the board or the features of the

assembly. They are incorporated into the borders of the

panels either used for board manufacturing or the pallets

provided to the assembly company. Most manufacturers

and assemblers have their processes in control, neverthe-

less, coupons are of value to make certain through various

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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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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