IPC-SM-782A-表面贴装焊盘图形设计标准.pdf.pdf - 第62页

In-line placement equipment (Figure 7–4) employs a series of ﬁxed position placement stations. Each station places its respective component as the printed board moves down the line. Cycle times vary from 1.8 to 4.5 secon…

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

In-line placement equipment (Figure 7–4) employs a series

of ﬁxed position placement stations. Each station places its

respective component as the printed board moves down the

line. Cycle times vary from 1.8 to 4.5 seconds per board.

Simultaneous placement equipment (Figure 7–5) places an

entire array of components on the printed board at the same

time. Typical cycle times vary from 7 to 10 seconds per

board.

IPC-782-7-1

Figure 7–1 Typical process ﬂow for underside attachment type 2c (simple) surface mount technology

Insert/Clinch

Leaded

Thru-Hole

Components

▼

Invert

Board

▼

Apply

Adhesive

Cure

Adhesive

Place

Surface Mt.

Components

Invert

Board

Wave

Solder

Clean*

Test

▼

*Optional depending on flux and cleanliness requirements.

IPC-782-7-2

Figure 7–2 Typical process ﬂow for full surface mount type 1b and 2b surface mount technology

Solder Paste

Side 1

▼

Place

Components

▼

Clean*

Invert

Board

Clean*

Type 2 (Double Sided) SMT

Type 1 (Single Sided) SMT

▼

Solder Paste

Side 2

Place

Components

Dry

Paste**

Test

Reflow

Solder

*Optional depending on flux

and cleanliness requirements

**Typically used for vapor phase soldering

Dry

Paste**

Reflow

Solder

IPC-SM-782A December 1999

Sequential placement equipment (Figure 7–6) typically uti-

lizes a software controlled X-Y moving table system. Com-

ponents are individually placed on the printed board in

succession. Typical cycle times vary from 0.3 to 1.8 sec-

onds per component.

Sequential/simultaneous placement equipment (Figure 7–7)

features a software controlled X-Y moving table system.

Components are individually placed on the printed board

from multiple heads in succession. Simultaneous ﬁring of

heads is possible. Typical cycle times vary around 0.2 sec-

onds per component.

There are many autoplacement machines available in each

of the four categories. One must establish guidelines for

selection of a machine. For example, what kind of parts are

to be handled? Will they come in bulk, magazine, or on a

tape? Can the machine accommodate future changes in

tape sizes?

Selection and evaluation of tapes from various vendors for

compatibility with the selected machine is very important.

The off-line programming, teach mode, and edit capability

along with computer aided design/computer aided manu-

facture (CAD/CAM) compatibility may be very desirable,

especially if a company has already developed a CAD/

CAM data base. Special features such as adhesive applica-

tion, component testing, board handling, and reserve capa-

bility for further expansion in a machine may be of special

interest for many applications. Reliability, accuracy of

placement, and easy maintenance are important to all users.

IPC-782-7-3

Figure 7–3 Typical process ﬂow for mixed technology type 2c (complex) surface mount technology

''' '

''''

'''' '

'''

Solder Paste

Dry

Paste**

Reflow

Solder

Place

Surface Mt.

Components

Clean*

Clinch Leaded

Thru-Hole

Components

Insert

Thru-Hole

Components

Invert

Board

Invert

Board

Wave

Solder

Apply

Adhesive

Cure

Adhesive

Place

Surface Mt.

Components

Clean* Test

*Optional depending on flux and cleanliness requirements. If no flux is used for solder paste

and/or wave soldering, cleaning and cleanliness test may be omitted.

**Typically used for vapor phase soldering.

IPC-782-7-4

Figure 7–4 In-line placement equipment

•

Moving board/fixed head

•

Each head places one component

•

1.8 to 4.5 seconds/board

IPC-782-7-5

Figure 7–5 Simultaneous placement equipment

•

Fixed table/head

•

All components placed simultaneously

•

Seven to ten seconds/board

▼

December 1999 IPC-SM-782A