IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第49页

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 Mi…

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

标准分享网 www.bzfxw.com 免费下载

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

The process sequence for surface mount with through-hole

or pin-in-hole (PIH) component technology is shown in

Figure 7-2. Adhesive is applied and the surface mounted

components placed. The adhesive is then cured, and the

board is inverted to receive the through-hole component

leads automatically or by hand insertion. After lead clinch-

ing (if required), and with the through-hole components on

top and the surface mount components beneath, the board

is typically wave soldered. An alternative sequence is to

reverse the initial stages i.e., insert (and clinch) the

through-hole components before attaching the surface

mounted components and then wave soldering.

Finally the assembly may be cleaned, inspected, repaired if

necessary, and tested, though not necessarily in that order.

7.2 Substrate Preparation

7.2.1 Adhesive Application In wave soldering surface

mount components, selection and application of adhesive

plays a critical role. With too much adhesive, the adhesive

ﬂows onto lands resulting in poor solder ﬁllets. Too little

adhesive will fail to accomplish its objective of holding

parts to the bottom of the board during wave soldering.

7.2.2 Conductive Adhesive Some applications for SMT

attachment use conductive adhesive as the attachment

material. Unlike solder paste which is redistributed when

reﬂowed, conductive adhesives must be properly controlled

to ensure joint strength. Also, component placement must

be controlled in order to prevent excessive adhesive

squeeze-out, and possible shorts to adjacent lands.

7.2.3 Solder Paste Application Solder paste plays an

important role in reﬂow soldering. The paste tacks the

component before reﬂow. It contains ﬂux, solvent, sus-

pending agent, and alloy of the desired composition. Sol-

der paste is applied on the lands before component place-

ment either by screening, stenciling, or syringe. Screens are

made from stainless steel or polyester wire mesh, and sten-

cils are etched stainless steel, brass, and other stable alloys.

Stencils are preferred for high-volume applications. They

are more durable than screens, easier to align, and can be

used to apply a thicker layer of solder paste, and, where

narrow, point apertures are required for example, for ﬁne-

pitch lands. Electroformed stencils may be required for

very small components such as 0201 capacitors and resis-

tors.

The goal of the technology that’s employed to make the

stencil is to ensure that this transfer is as efficient and com-

plete as possible. There are several post processes that

enhance the stencil’s performance, including electropolish-

ing and trapezoidal section apertures that are created with

laser cut technology

7.2.3.1 Laser Cut Stencil Development Laser cut sten-

cils are produced directly from the customer’s original Ger-

ber ﬁle or GenCAM or IPC-2581 data. Eliminating the

IPC-7351-7-01

Figure 7-1 Typical Process Flow for Full Surface Mount Type 1b and 2b Surface Mount Technology

Basic Assembly Process for SMT

Solder

Printing

Assembly

Test

Package

and Ship

Inspection

Reflow

Solder Process

Clean

and

Inspection

Device

Placement

When cleaning is required.

Test may include functional, in-circuit, burn-in as well as post assembly programming.

IPC-7351 February 2005

标准分享网 www.bzfxw.com 免费下载