IPC-CM-770D-1996.pdf - 第130页

January

1996

IPC-CM-770

and can be stored at room temperature for up to two years.

After encapsulation, the materials are dried to drive off the

solvent vehicle.

25.0 INTERMIXED TECHNOLOGY

Sections 21, 22, 23 and 24 cover the principles of compo-

nent mounting -where components are mounted through-

the-board and components are surface mounted. Intermix-

ing of the techniques on the same electronic assembly adds

an additional dimension to the aspects of component

mounting. Although the techniques used for through-hole

and surface mounting in intermixed assemblies are similar

to the approach used in “all the same type of mounting”

assemblies, there are many additional concerns facing the

assembler of printed boards who uses both technologies in

the same assembly.

25.1 General Considerations

Because of the dual or

multistep component mounting operation, the designer of

intermixed assemblies must take into account all of the

fabrication and assembly steps necessary to complete the

electronic assembly. These concerns that take place during

the design cycle include:

A. Component Types

B.

The Assembly Processes Used

Single-sided assembly

Double-side assembly

Component securing

C. Joining Techniques

Single-sided joining techniques

Double-sided joining techniques

Care required for heat-sensitive components

Handling of unsealed components

D. Sequence of Events

The last item on the list shown above is usually never con-

sidered during the design cycle; however, in intermixed

assemblies, if the sequencing of component mounting is

not taken into account, components will interfere with one

another or will be removed during a second mounting step.

25.1.1 LeadlLand Relationships

Hole and land require-

ments for intermixed assembly are identical to those

requirements already stipulated in through-the-board and

surface mounted land pattern configurations.

No

special

requirements are necessary and implementation of the

proper land pattern into the design will provide the appro-

priate solder joint after placement.

25.1.2 Component Preparation

All lead extension and

forming requirements on parts to be mounted on printed

board structures are identical to the techniques described

for through-the-board and surface mount component

mounting and positioning.

One major difference to be acknowledged is that some

individuals have chosen to reduce the number of compo-

nent mounting and attachment sequences by taking compo-

nents designed for through- the-board mounting and con-

verting them to surface mount parts; or parts that are

designed for surface mounting are converted to through-

the-board mounted parts.

Figure 25-1 shows the concept of taking a standard flat-

pack which is usually surface mounted and forming the

leads,

so

that the parts may be mounted through-the-board.

This would be a practice used when there are only several

flatpacks on an otherwise all through-mounted board

assembly.

Sockets are also used quite often as a technique for taking

components that are surface mounted and converting them

over to through-the-board mounted parts.

When components that have been designed for through-

the-board mounting are converted to surface mounting,

there are additional steps necessary in lead forming. Axial

leaded parts that are normally mounted through-the-board

would have their leads coined as shown in Figure 25-2A.

These parts would then be surface mounted. In addition,

some manufacturers have started to use the concept of a

dual inline with

I

beam leads. In this technique, dual inline

packages have their leads foreshortened and are surface

mounted to the board in the manner shown in Figure

25-2B. Special care must be taken to insure that the com-

ponents do not move during the soldering operations. Usu-

ally the solder paste is sufficient to hold a dual inline pack-

age in place prior to reflow soldering.

I

IPC-I-O0215

Figure 25-1 Component Modifications for Through-hole

Applications

25.1.3 Component Placement

Sophisticated tooling is

available, both for the placement of axial leaded compo-

nents and the place- ment of surface mounted components.

Figure 25-3 shows an example of the sequencing used for

plac- ing two leaded axial parts and a tape used to place

5-27

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

Straight for

1

D

but not

less

than 0.8mm

"

Flush

to

0.635mm [0.25]

Max

Cqonductor

IPC-1-00363

L

Figure 25-2 Component Modifications for Surface

Mounting Applications

chip components between the parts. Also, machine head

clearances must be considered.

IPC-1-00355

I

Figure 25-3 Placement Machine Considerations

In the example shown, the chip resistors would be placed

first and probably attached through reflow soldering. The

design must provide adequate clearances for the machine

that assembles and clinches the leads for the

axial

leaded

components. Without proper knowledge of machine clear-

ances or considerations included into the design, manufac-

turers would have to insert components semi-automatically

or use manual techniques.

25.1.4 Component Securing

Techniques for shock and

vibration mounting of intermixed assemblies do not vary

dramatically from techniques previously described for

boards that have only through-the-board components or are

all surface mounted.

One aspect of the intermixed assembly is the fact that,

although components can take the shock of the final assem-

bly, certain components and their joints may not take the

shock of a secondary assembly operation. As an example,

if chip components are mounted on the underside of the

board and attached using some form of adhesive, the next

assembly operation should not impart additional shocks to

the assembly

so

that the parts that have been secured to the

underside are dislodged.

The characteristic of shock from secondary assembly

operations is even more important when parts that have

previously been mounted are attached using soldering tech-

niques. Now the shock and vibration is not just on the part

itself, shock and vibration can also affect the reliability of

the soldered joint.

Special fixturing should normally be provided during the

assembly operation to insure that no damage is imparted to

those parts that have been previously mounted or attached.

25.1.5 Assembly Sequence

Although this guideline

deals primarily with the mounting of components and not

the joining process, in intermixed assemblies, the two can-

not be separated. In some sequencing operations, parts

must be secured or attached permanently before the next

level of component mounting can be accomplished. J-STD-

001

covers the requirements for solder joints used to attach

electronic parts.

In considering the design process, each component has its

own unique characteristics for component mounting. These

have already been previously described in this guide in

Sections

5

through

18.

In general, component types that

mount through- the-board are shown in Figure

25-4

and

components that mount on the board surface are shown in

Figure

25-5.

During assembly sequencing another major item to con-

sider is whether the component placement will be done

singularly

(1

component at a time) or whether multiple

components will be placed by the equipment. The primary

concerns in these areas are the tool head clearances for

automatic placement and the set up procedures that are

necessary in order to insure that the placement of one com-

ponent does not dislodge or disturb the placement of a pre-

viously set part.

Previous sections have identified that electronic assemblies

can be divided into two types-those types that have com-

ponents mounted on only one side (type

1)

and those that

have components mounted on both sides (type

2).

As stated

earlier, components that are mounted through-the-board

usually are only of the type

1

variety; whereas surface

mounted components, or intermixed component mounting

techniques, can use both sides of the printed board for

component mounting surfaces.

In addition, the type of board or packaging interconnection

structure used to interconnect the components can also play

a dramatic role in the sequencing of electronic component

5-28

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

January

1996

IPC-CM-770

A. Axial Parts

(2

Leads)

B.

Radial Parts

(2

Leads)

C. Radial Parts

(3/4

Leads)

E. Single ln-Line Packages

F.

Dual ln-Line Packages

G.

Pin Grid Arrays

H.

Sockets and Connectors

D. TO-5 ICs

D.

QE.

F.

H

I.

Others (Transformers, Chokes, Coils, Trim Pots, etc.)

.

..

=El===

A.

W

G.

L.

IPC-1-00353

J.

Discrete-Rectangular, Square

K.

Discrete-Cylindrical Ends Caps

L. Small-Outline Transistors

M.

Small-Outline ICs (SOC)

N.

Plastic Chip Carrier (leaded PCCs)

O.

Ceramic Leaded Chip Carrier (LCC)

P. Ceramic Leadless Chip Carrier (LLCC)

Q.

Flat Packs

R. Quad Packs

S.

Other Connectors (Coils, Sockets)

L.

Q

N.

O.

P.

Q.

c

J.

IPC-1-00354

Figure

25-5

Surface Mount Component Types

mounting. Figure 25-6 shows the various techniques for

that contains the single layer

of

copper circuitry.

If

all

of

component mounting on a single-sided printed board or

the components are surface mounted, the component side

P&I structure. In Figure 25-6A all components are

and solder attachment side become one and the same (Fig-

mounted through- the-board and interconnected on the side

ure 25-6B).

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services