IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第48页

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. T o aid in the …

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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6.1.1 Categories In general, a PCB structure will ﬁt into

one of four basic categories of construction: organic base

material, nonorganic base material, supporting plane, and

constraining core.

6.1.2 Thermal Expansion Mismatch A primary concern

when using low expansion surface mount parts is the ther-

mal expansion mismatch between the leadless part and the

PCB structure. This mismatch will fracture solder joint

interconnections if the assembly is subjected to thermal

shock, thermal cycling, power cycling and high operating

temperatures. The number of fatigue cycles before solder

joint failure depends on the thermal expansion mismatch

between the part and the PCB structure, the temperature

range over which the assembly must operate, the solder

joint thickness, the size of the part and the power cycling.

For example, power cycling may cause an undesirable ther-

mal expansion mismatch if a signiﬁcant temperature differ-

ence exists between a device or package and the PB struc-

ture.

6.2 Organic-Base Material Organic-base materials work

best with leaded chip carriers. With leadless chip carriers

and some BGA packaging, the thermal expansion mis-

match between package and substrate can cause problems.

Also, ﬂatness, rigidity, and thermal conductivity require-

ments may limit their use. Finally, attention should be paid

to package size, I/O count, thermal cycling stability, maxi-

mum operating temperature and solder joint compliance.

6.3 Nonorganic Base Materials Nonorganic ceramic

base materials typically used with thick- or thin-ﬁlm tech-

nology, although more costly, are suited for leaded and

leadless chip carrier designs. Suppliers can incorporate

thick- or thin-ﬁlm resistors directly on the ceramic struc-

ture and buried capacitor layers that increase density and

improve reliability. However, repairability of the PCB

structure is limited. Ceramic materials, usually alumina,

appear ideal for PCB structure with leadless ceramic chip

carriers because of their relatively high thermal conductiv-

ity. Unfortunately, the structure is limited to approximately

100 mm square. Ceramic PCB structures have three pri-

mary applications: ceramic hybrid circuits, ceramic multi-

chip modules (MCM-L) and ceramic printed boards.

6.4 Alternative PCB Structures

6.4.1 Supporting-Plane PCB Structures

Supporting

metallic or nonmetallic planes can be used with conven-

tional printed boards or with custom processing to enhance

PCB properties. Depending on the results desired, the sup-

porting plane can be electrically functional or not and can

also serve as a structure stiffener, heatsink and/or CTE con-

straint.

6.4.2 High-Density PCB Technology High-density,

sequentially processed, multilayer PCB structures are avail-

able in a wide variety of organic dielectrics. Using thinner

copper foils for fabrication the board manufacturer can pro-

vide very narrow conductor and spacing features and by

implementing smaller mechanical drills, laser ablation,

photo-lithography or plasma processes, smaller blind

and/or buried vias can be provided for layer-to-layer inter-

connections.

The major advantage of this system is that the vias can be

as small as 0.10 mm [0.00394 in] or less and conductor

widths can range from below 0.12 mm [0.00472 in] for

high interconnection density. Thus, some applications can

be satisﬁed with fewer signal layers while providing addi-

tional layers for power and ground. Refer to IPC-2226 for

more detailed design guidelines for high density PCB.

6.4.3 Constraining Core Structures As with supporting

plane, one or more supporting metallic or nonmetallic

planes can serve as a stiffener, heatsink, and/or CTE con-

straint in constraining core structures.

6.4.4 Porcelainized Metal (Metal Core) Structures An

integral core of low-expansion metal (for example, copper-

clad Invar) can reduce the CTE of porcelainized metal

structures so that it closely matches the CTE of the ceramic

chip carrier. Also, the structure size is virtually unlimited.

However, the low melting point of the porcelain requires

low-ﬁring-temperature conductor, dielectric and resistor

inks.

7 ASSEMBLY CONSIDERATION FOR SURFACE MOUNT

TECHNOLOGY (SMT)

The smaller size of surface mount components and the

option of mounting them on one or both sides of the pack-

aging and interconnecting structure reduces board real

estate signiﬁcantly. The type of SMT assembly is basically

determined by the type of surface mount components to be

used; see 7.1 for a description of types and classes.

7.1 SMT Assembly Process Sequence The SMT assem-

blies are soldered by reﬂow (infrared, hot air convection,

laser, conduction, vapor phase, and/or wave soldering pro-

cesses) depending upon the mix of surface mount and

through-hole mount components. The process sequence for

one-sided SMT is shown in Figure 7-1. Solder paste is

applied, components are placed, the assembly is reﬂow sol-

dered and cleaned. For two-sided SMT assemblies, the

board is turned over and the process sequence just

described is repeated. The assembly process for two-sided

SMT is simply a sequential combination of SMT processes,

however, component weight vs. surface tension should be

calculated to determine if heavy components will require

additional reinforcement prior to the second reﬂow solder-

ing process.

February 2005 IPC-7351