IPC-SM-782A-表面贴装焊盘图形设计标准.pdf.pdf - 第60页

machines. This geometry results in a low-proﬁle intercon- nection pattern with excellent high-speed electrical charac- teristics and a density normally associated with thick-ﬁlm technology . 6.5 Constraining Core P&I…

Table 6–1 Packaging and Interconnecting Structure Comparison

Type Major Advantages Major Disadvantages Comments

Organic Base Substrate

Epoxy Fiberglass Substrate size, weight,

reworkable, dielectric

properties, conventional 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

temperature and/or small

packages.

Polyimide Fiberglass Same as Epoxy Fiberglass plus

high temperature X–Y axis

CTE, substrate size, weight,

reworkable, dielectric

properties, high Tg.

Thermal conductivity, Z-axis

CTE, moisture absorption

Same as Epoxy Fiberglass

Epoxy Aramid Fiber Same as Epoxy Fiberglass,

X–Y 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 wearout 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/Teﬂon® 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-molding setup

costs

Relatively new for these

applications

Non-Organic Base

Alumina (Ceramic) CTE, thermal conductivity,

conventional thick ﬁlm or thin

ﬁlm processing, integrated

resistors

Substrate size, rework

limitations, weight, cost, brittle,

dielectric constant

Most widely used for hybrid

circuit technology

Supporting Plane

Printed Board Bonded to Plane

Support (Metal or Non-Metal)

Substrate size, reworkability,

dielectric properties,

conventional 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,

stiffness, 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 Structures Substrate size, dielectric

properties, X–Y axis, CTE.

Z axis CTE, thermal

conductivity.

Compliant layer absorbs

difference in CTE between

ceramic package and

substrate.

December 1999 IPC-SM-782A

machines. This geometry results in a low-proﬁle intercon-

nection pattern with excellent high-speed electrical charac-

teristics and a density normally associated with thick-ﬁlm

technology.

6.5 Constraining Core P&I Structures As with

supporting- plane P&IS, one or more supporting metallic

or non-metallic planes can serve as a stiffener, heatsink,

and/or CTE constraint in constraining core P&IS struc-

tures.

6.5.1 Porcelainized-Metal (Metal Core) Structures An

integral core of low-expansion metal (e.g., copper-clad

Invar), can reduce the CTE of porcelainized-metal P&I

structures so that it closely matches the CTE of the ceramic

chip carrier. Also, the P&I structure size is virtually unlim-

ited. However, the low melting point of the porcelain

requires low-ﬁring-temperature conductor, dielectric and

resistor inks.

A number of composite P&I structures use leadless compo-

nents. An integral material with a lower CTE than that of

the printed boards controls the CTE of these structures.

Table 6–2 P & I Structure Selection Considerations

Design Parameters

Material Properties

Transition

Temperature

Coefficient

of Thermal

Expansion

Thermal

Conductivity

Tensile

Modulus

Flexural

Modulus

Dielectric

Constant

Volume

Resistivity

Surface

Resistivity

Moisture

Absorption

Temperature & Power Cycling XXXX

Vibration X X

Mechanical Shock X X

Temperature & Humidity X X XXXX

Power Density X X

Chip Carrier Size X X

Circuit Density XXX

Circuit Speed XXX

Table 6–3 P & I 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

–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

Alumina–Beryllia NA 5–7 21.0 44.0 8.0 10

Aluminum (6061 T–6) NA 23.6 200 10 NA 10

Copper (CDA101) NA 17.3 400 17 NA 10

Copper-Clad Invar NA 3–6 150XY/20Z 17–22 NA 10

Notes:

1. These materials can be tailored to provide a wide variety of material properties based on resins, core materials, core thickness,

and processing methods.

2. The X and Y expansion is controlled by the core material and only the Z axis is free to expand unrestrained. Where the Tg will be

the same as the reinforced resin system used.

3. When used, a compliant layer will conform to the CTE of the base material and to the ceramic component, therefore reducing the

strain between the component and P&I structure.

4. Figures are below glass transition temperature, are dependent on method of measurement and percentage of resin content.

NA = Not Applicable

IPC-SM-782A December 1999

7.0 ASSEMBLY CONSIDERATIONS FOR SURFACE

MOUNT TECHNOLOGY (SMT)

The smaller size of surface mount components and the

option of mounting them on either or both sides of the

packaging 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 paragraph 3.6.1.5 for a description of types and

classes.

This section will brieﬂy discuss the assembly process

issues for SMT assemblies. For additional information, see

IPC-SM-780 and IPC-CM-770. The reader is also directed

to IPC-9191 for continued process improvement.

7.1 SMT Assembly Process Sequence The SMT assem-

blies are soldered by reﬂow (vapor phase, infra red, hot air,

convection, laser, conduction belt) and/or wave soldering

processes depending upon the mix of surface mount and

through hole mount components.

The process sequence for Type 2c SMT is shown in Figure

7–1. The leaded components are automatically or hand

inserted. The assembly is turned over and adhesive applied.

Then the surface mount components are placed by a pick-

and-place machine, the adhesive is cured, the assembly is

turned over again and the wave soldering process is used to

solder both leaded and surface mount components in a

single operation. Finally the assembly is cleaned,

inspected, repaired if necessary, and tested.

The process sequence for Type 1b SMT is shown in Figure

7–2. Solder paste is applied, components are placed, the

assembly is reﬂow soldered and cleaned. For Type 2b SMT

assemblies, the board is turned over and the process

sequence just described is repeated.

The process sequence for Type 2c Complex SMT, shown in

Figure 7–3, is simply a combination of SMT processes.

7.2 Substrate Preparation Adhesive, Solder Paste

7.2.1 Adhesive Application

In wave soldering of SMT,

selection and application of adhesive plays a critical role.

With too much adhesive, the adhesive gets on lands result-

ing 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.

A good adhesive has desirable properties such as being

single part, colored, long shelf life, ease of application, and

an adequate bond strength with short cure time. In addition,

after curing and soldering, the adhesive should remain

moisture resistant, nonconductive, noncorrosive, and be

reworkable. There are various companies that supply adhe-

sives for SMT. Some of these adhesives require UV cure

followed by IR; others can be cured in IR (infrared) or

conventional ovens.

For adhesive cure, temperature plays a bigger role than the

time. Only 10°C temperature variation from the recom-

mended cure temperature may cause loss of chips in the

wave (under cure) or difficult repair problems (overcure).

7.2.2 Conductive Epoxy Some applications for SMT

attachment use conductive epoxy as the attachment mate-

rial. Adequate conductive epoxy volume is essential at the

location of the epoxy on the land.

Unlike solder paste which is redistributed when reﬂowed,

conductive epoxies must be properly controlled to ensure

adequate joint strength. Also, component placement must

be controlled in order to prevent epoxy squeeze-out, and

possible shorts to adjacent lands.

7.2.3 Solder Application, Paste, and Preform Solder

paste plays an important role in reﬂow soldering. The paste

acts as an adhesive before reﬂow. It contains ﬂux, solvent,

suspending agent, and alloy of the desired composition. In

selecting a particular paste, rheological characteristics such

as viscosity, dispensing, screening or stencil, ﬂow, and

spread are very important. Susceptibility of the paste to

solder ball formation and wetting characteristics are also

important.

Solder paste is applied on the lands before component

placement either by screening, stenciling, or syringe.

Screens are made from stainless steel or polyester wire

mesh and stencils are etched stainless steel, brass sheets

and other stable alloy

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.

Solder preforms are sometimes used for through-board-

mounted devices. They come in required size and compo-

sition, with ﬂux either inside the preforms, or as a coating

or without ﬂux. They may be cost effective to avoid wave

solder processes if there are only a few leaded components

on the board.

7.2.4 Solid Solder Deposition

7.3 Component Placement

The accuracy requirements

almost make it mandatory to use autoplacement machines

for placing surface mount components on the board. Selec-

tion of the appropriate autoplacement machine is dictated

by the type of parts to be placed and their volume. There

are basically four types of autoplacement machines

available.

• In-line placement

• Simultaneous placement

• Sequential placement

• Sequential/simultaneous placement

December 1999 IPC-SM-782A