IPC-CM-770D-1996 - 第133页

IPC-CM-770

Januaty

1996

A. All

through hole

B.

All

surface mounting

IPC-1-00357

I

the total quantities of assemblies do not justify the set up

time required for automated component mounting.

A.

Mixed assembly

B.

Mixed assembly

IPC-I-

Figure

25-6

Single-sided Board Assemblies

Figure 25-8 Mixed Technology Assemblies

A. All

through hole

B.

All

surface mounting

IPC-1-00354

Figure

25-7

Double-sidedlmultilayer Board Assembly

In an intermixed assembly (Figure 25-S), components

mounted through-the-board are usually positioned on one

side; whereas, components mounted on the underside of

the board are usually those of the surface mounted variety.

In this technique, the surface mounted components are usu-

ally attached to the board using an appropriate adhesive

after which the through-the-board components are auto-

matically inserted, and then the entire assembly is passed

through a solder wave.

The technique for the assembly operation can vary,

depending on the type of component, the quantity of differ-

ent components being mounted, and the techniques being

used for mounting. It should be understood that if a single

part is surface mounted amidst a large variety of through-

the-board mounted components, the surface mounted part

becomes a minor part of the assembly operation. The same

holds true if the through- the-board parts are in the minor-

ity. In these instances, manual techniques or semi-

automated techniques are usually employed, especially

if

When the intermix of parts is such that there is an equal

number of through- the-board and surface mounted compo-

nents, the tooling concept for automated component

mounting must take into account the need for special fix-

turing or attachment techniques.

Double-sided or multilayer printed board structures have

similar characteristics for the mounting of parts. Figure

25-7A shows a double-sided or multilayer board with

through-hole mounted components. Major differences

between the single-sided board and double-sidedmultilayer

assembly is that leads of through-the-board components are

usually in plated-through-holes. The tolerances associated

with plated-through- holes must be more liberal to allow

for the plating build-up; therefore, the component mount-

ing process is sometimes more restricted, based on the

manufacturing allowances incorporated into the plated-

through-hole printed board. The attachment techniques for

through-the-board components are usually some form of

wave soldering.

When parts are surface mounted to double-sided or multi-

layer printed boards, the surface mounted parts may be

J

mounted all on one side, or mounted on both sides, as

shown in Figure 25-7B. The attachment techniques for all

surface mounted boards are usually some form of reflow

soldering.

When components are intermixed on double-sided or mul-

tilayer printed boards or printed board structures; the inter-

mixing may take several forms. Figure 25-8 shows a few

examples of mixed assemblies where the components are

mounted. Figure 25-SA shows a few examples of mixed

assemblies where the components are mounted on only one

side (type

1).

Figure 25-SB shows the mixed assembly with

components on both sides (type 2).

5-30

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

-

Figure 25-9

1

(A)

TWO

Solder

Passes

1

MOUNT COMPONENTS

J

1

INSERT

THROUGH

HOLE

1

CLEAN ASSEMBLY

1

(B)

One Solder

Pass

IPC-I-

Sample Process Sequence

The attachment sequence for the intermixed assembly

might consist of combinations of reflow or wave soldering.

One typical approach to accomplishing the mixed assembly

shown in Figure 25-SA is shown in Figure 25-9A.

It should be noted that sequences shown in Figures 25-9

and 25-10 have tried to incorporate as much automation as

possible. Semi-automated or manual techniques can also be

used and therefore, would impact the sequence

dramatically.

25.2

Manual Assembly

Manual techniques in an inter-

mixed assembly play an increased role due to the necessity

for secondary component placement and attachment opera-

tions. Lack of automated assembly head clearances, special

part types, heat sensitive parts, or unsealed parts all require

special handling and are, therefore, prime candidates for

manual component mounting techniques.

With the ever increasing trend toward miniaturization and

thus, small parts, the manual techniques usually require

magnification and special tool dexterity in order to insure

that the parts are properly mounted, placed or positioned.

Special equipments are available that provide a semi-

automated technique of taking one component at a time

and positioning it by having the machine assist the opera-

tor through special enhanced movement. Even with manual

or semi-automated techniques, desigh should provide for

adequate clearance around components,

so

that placement

heads or tools do not interfere with previously placed

components.

25.3 Automated Assembly

Automated techniques used

for intermixing of components are similar in nature to

those used for boards that have only through-the-board

mounted components or all surface mounted components.

Major considerations are given in automated techniques for

special fixturing which reduce shock and provide clearance

for components that have previously been mounted.

Because of the special cost of tooling and set up time

required for printed board assemblies that have an intermix

of parts, many designs are assembled in a panel form rather

than in an individual board form. This is especially helpful

when a manufacturer can set up his equipment for a stan-

dard size panel and the equipment is, therefore, impervious

to the fact that the panel contains one or many boards.

Panel assembly requires special tooling and registration

which must be incorporated into the panel during the

manufacturing operation. Tooling holes are located as

shown in Figure 25-11. These require special close toler-

ance considerations to insure that the automated equipment

is able to locate and position components accurately to the

patterns required.

With the ever increasing trend toward miniaturization and,

thus, small parts, the manual techniques usually require

magnification and special tool dexterity in order to insure

that the parts are properly mounted, placed, or positioned.

Special equipments are available that provide a semi-

automated technique of taking one component at a time

and positioning it by having the machine assist the opera-

tor through special enhanced movement. Even with manual

or semi-automated techniques, design should provide for

adequate clearance around components,

so

that placement

heads or tools do not interfere with previously placed

components.

Some assembly equipments use special sensing symbols

which have been incorporated into the design. In this fash-

ion, the equipment senses the location of the symbol and

can zero in on a particular board, or even pattern. Material

movement or shifting of patterns can, therefore, be com-

pensated for by the assembly equipment. With special tool-

ing, features, or holes, the automated assembly can, there-

fore, accommodate panels of

18

to 24 inches without any

loss of accuracy of the component placement task.

Designs should consider whether the assembly will be

accomplished using board assembly or panel assembly.

This becomes increasingly more important, depending on

the technique used for attachment, in that board to compo-

nent orientation is critical in some instances where the

parts pass through the wave and are, thus, attached to the

land patterns. Placement of parts and orientation of parts

5-3

1

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

are critical to insure that the part body does not “shadow”

the solder joint and, depending on how the component

boards are oriented on a panel, will play a significant role

in the type of solder joint when the boards are to be wave

soldered (see Figure 25-13).

Some assembly equipments use special sensing symbols

which have been incorporated into the design. In this fash-

ion, the equipment senses the location of the symbol and

can zero in on a particular board, or even pattern. Material

movement or shifting of patterns can, therefore, be com-

pensated for by the assembly equipment. With special tool-

ing, features, or holes, the automated assembly can, there-

fore, accommodate panels of

18

to 24 inches without any

loss of accuracy of the component placement task (see Fig-

ure 25-12).

Designs should consider whether the assembly will be

accomplished using board assembly or panel assembly.

This becomes increasingly more important, depending on

the technique used for attachment, in that board to compo-

nent orientation is critical in some instances where the

parts pass through the wave and are, thus, attached to the

land patterns. Placement of parts and orientation of parts

are critical to insure that the part body does not “shadow”

the solder joint and, depending on how the component

boards are oriented on a panel, will play a significant role

in the type of solder joint when the boards are to be wave

soldered (see Figure 25-13).

25.4 Quality Assurance

The quality assurance aspects

are the same for intermixed assemblies as they are on

through-the-board and surface mounted assemblies. There

is however, a greater need for process control in that there

are several sequences of assembly prior to completing the

task.

Inspection levels are instituted to ensure that quality assur-

ance aspects of electronic asemblies are inspected for con-

formance of acceptability criteria of IPC-A-6

10.

APPLY SOLDER PASTE

1

PLACE SURFACE MOUNT COMPONENTS

1

DRY SOLDER PASTE

1

I

SOLDER REFLOW

I

1

CLEAN (OPTIONAL)

I

MACHINE INSERT THROUGH-HOLE

COMPONENTS

1

INVERT BOARD

APPLY ADHESIVE

1

PLACE SURFACE MOUNT COMPONENTS

1

I

CURE ADHESIVE

1

I

INVERT BOARD

I

1

HAND INSERT THROUGH-HOLE

COMPONENTS

1

WAVE SOLDER

1

CLEAN

IPC-I-

Figure 25-10 Class C-2 Process Sequence

5-32

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Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services