IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第47页

T able 6-1 Printed Board Structure Comparison T ype Major Advantages Major Disadvantages Comments Organic Base Substrate Epoxy fiberglass Substrate size, weight, rework- able, dielectric properties, con- ventional board …

in-circuit test. Directions for this should come from test

engineering.

In addition, it is useful to identify the test vias and lands on

an assembly drawing in event of the need to modify the

circuit topology. Changes made without moving test lands

avoid ﬁxture modiﬁcation, saving cost and time.

Care should be taken when mounting components on the

secondary side to avoid covering a via hole that is a desig-

nated test land. Also, if a via hole is too close to any com-

ponent, damage may result to the component or ﬁxture

during probing (see Figure 5-3).

6 PRINTED BOARD STRUCTURE TYPES

The selection of a packaging and interconnecting structure

for surface mounting applications is important for optimum

thermal, mechanical and electrical systems reliability. Each

candidate structure has a set of properties with particular

advantages and disadvantages when compared to others

(see Table 6-1).

It is probable that no one packaging and interconnecting

structure or printed board will satisfy all of the needs of the

application. Therefore, a compromise of properties should

be sought that offers the best ‘‘tailoring’’ for component

attachment and circuit reliability.

IPC-7351-5-03

Figure 5-3 Test Probe Feature Distance from Component

Component

Height

▼

Free

Area

6.5 mm [0.256 in] height

> 5.0 mm

[0.20 in] Min.

Test

Pad

5.0 mm [0.20 in]

Tall Component

Free area

▼

Test Pad

IPC-7351 February 2005

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Table 6-1 Printed Board Structure Comparison

Type Major Advantages Major Disadvantages Comments

Organic Base Substrate

Epoxy fiberglass Substrate size, weight, rework-

able, dielectric properties, con-

ventional board processing.

Thermal conductivity, X, Y and Z

axis CTE.

Because of its high X-Y plane

CTE, it should be limited to

environments and applications

with small changes in tempera-

tures and/or small packages.

Polyimide ﬁberglass Same as epoxy ﬁberglass plus

high temperatures X-Y axis

CTE, substrate size, weight,

reworkable, dielectric properties,

high T

Thermal conductivity, Z-axis

CTE, moisture absorption.

Same as epoxy ﬁberglass.

Epoxy aramid ﬁber Same as epoxy ﬁberglass,

X-axis CTE, substrate size,

lightest weight, reworkable,

dielectric properties.

Thermal conductivity, Z-axis

CTE, resin microcracking, Z

axis CTE, water absorption.

Volume fraction of ﬁber can be

controlled to tailor X-Y CTE.

Resin selection critical to

reducing resin micro-cracks.

Polyimide aramid ﬁber Same as epoxy aramid ﬁber,

X-axis CTE, substrate size,

weight, reworkable, dielectric

properties.

Thermal conductivity, Z-axis

CTE, resin microcracking, water

absorption.

Same as epoxy aramid ﬁber.

Polyimide quartz (fused

silica)

Same as polyimide aramid ﬁber,

X-Y axis CTE, substrate size,

weight, reworkable, dielectric

properties.

Thermal conductivity, Z-axis

CTE, drilling, availability, cost,

low resin content required.

Volume fraction of ﬁber can

be controlled to tailor X-Y CTE,

drill wear-out higher than with

ﬁberglass.

Fiberglass/aramid

composite ﬁber

Same as polyimide aramid ﬁber,

no surface microcracks, Z axis

CTE, substrate size, weight,

reworkable, dielectric properties.

Thermal conductivity, X and Y

axis CTE, water absorption,

process solution entrapment.

Resin microcracks are conﬁned

to internal layers and cannot

damage external circuitry.

Fiberglass/PTFE®

laminates

Dielectric constant, high

temperature.

Same as epoxy ﬁberglass, low-

temperature stability, thermal

conductivity, X and Y axis CTE.

Suitable for high-speed logic

applications. Same as epoxy

ﬁberglass.

Flexible dielectric Light weight, minimal concern to

CTE, conﬁguration ﬂexibility.

Size, cost, Z-axis expansion. Rigid-ﬂexible boards offer

trade-off compromises.

Thermoplastic 3-D conﬁgurations, low high-

volume cost.

High injection-moulding setup

costs.

Relatively new for these

applications.

Nonorganic Base

Alumina (ceramic) CTE, thermal conductivity,

conventional thick ﬁlm or thin

ﬁlm processing, integrated

resistors.

Substrate size, rework limita-

tions, weight, cost, brittle,

dielectric constant.

Most widely used for hybrid

circuit technology.

Supporting Plane

Printed board bonded to

plane support (metal or

nonmetal)

Substrate size, reworkability,

dielectric properties, conven-

tional board processing, X-Y

axis CTE, stiffness, shielding,

cooling.

Weight. The thickness/CTE of the metal

core can be varied along with

the board thickness, to tailor the

overall CTE of the composite.

Sequential processed

board with supporting

plane core

Same as board bonded to

supporting plane.

Weight. Same as board bonded to

supporting plane

Discrete wire High-speed interconnections,

good thermal and electrical

features.

Licensed process, requires

special equipment.

Same as board bonded to low-

expansion metal support plane.

Constraining Core

Porcelainized copper-clad

invar

Same as alumina. Reworkability, compatible thick

ﬁlm materials.

Thick ﬁlm materials are still

under development.

Printed board bonded

with constraining metal

core

Same as board bonded to low

expansion metal cores, stiff-

ness, thermal conductivity, low

weight.

Cost, microcracking. The thickness of the graphite

and board can be varied to

tailor the overall CTE of the

composite.

Compliant layer sructures Substrate size, dielectric

properties, X-Y axis, CTE.

Z-axis CTE, thermal

conductivity.

Compliant layer absorbs

difference in CTE between

ceramic package and substrate.

February 2005 IPC-7351

6.1 General Considerations Printed board structures

vary from basic printed wiring boards to very sophisticated

supporting-core structures. However, some selection crite-

ria are common to all structures. To aid in the selection

process, Table 6-2 lists design parameters and material

properties which affect system performance, regardless of

PCB type. Also, Table 6-3 lists the properties of the mate-

rials most common for these applications.

Table 6-2 PCB Structure Selection Considerations

Design

Parameters

Material Properties

Transition

Temperatures

Coefficient

of Thermal

Expansion

Thermal

Conductivity

Tensile

Modulus

Flexural

Modulus

Dielectric

Constant

Volume

Resistivity

Surface

Resistivity

Moisture

Absorption

Temperatures

and power

cycling

XXXX

Vibration X X

Mechanical

shock

Temperatures

and humidity

X X XXXX

Power

density

Chip carrier

size

Circuit

density

XXX

Circuit speed XXX

Table 6-3 PCB Structure Material Properties

Material

Material Properties

Glass

Transition

Temperature

Coefficient

of Thermal

Expansion

Thermal

Conductivity

XY Tensile

Modulus

Dielectric

Constant

Volume

Resistivity

Surface

Resistivity

Moisture

Absorption

Unit of

Measure

°C PPM/°C

(Note 4)

W/M°C PSI x 10

-6

At 1 MHz Ohms/cm Ohms Percent

Epoxy

ﬁberglass

125 13-18 0.16 2.5 4.8 10

0.10

Polyimide

ﬁberglass

250 12-16 0.35 2.8 4.8 10

0.35

Epoxy aramid

ﬁber

125 6-8 0.12 4.4 3.9 10

0.85

Polyimide

aramid ﬁber

250 3-7 0.15 4.0 3.6 10

1.50

Polyimide

quartz

250 6-8 0.30 4.0 10

0.50

Fiberglass/

Teﬂon®

75 20 0.26 0.2 2.3 10

1.10

Thermoplastic

resin

190 25-30 3-4 10

N/A

Alumina-

beryllia

N/A 5-7 21.0 44.0 8.0 10

Aluminum

(6061 T-6)

N/A 23.6 200 10 N/A 106 N/A

Copper

(CDA101)

N/A 17.3 400 17 N/A 10

Copper-clad

Invar

N/A 3-6 150XY/20Z 17-22 N/A 10

N/A

IPC-7351 February 2005

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