IPC-CM-770D-1996.pdf - 第134页

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 …

January

1996

IPC-CM-770

Figure 25-9

(A)

TWO

Solder

Passes

MOUNT COMPONENTS

INSERT

THROUGH

HOLE

CLEAN ASSEMBLY

(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,

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,

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

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

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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

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

PLACE SURFACE MOUNT COMPONENTS

DRY SOLDER PASTE

SOLDER REFLOW

CLEAN (OPTIONAL)

MACHINE INSERT THROUGH-HOLE

COMPONENTS

INVERT BOARD

APPLY ADHESIVE

PLACE SURFACE MOUNT COMPONENTS

CURE ADHESIVE

INVERT BOARD

HAND INSERT THROUGH-HOLE

COMPONENTS

WAVE SOLDER

CLEAN

IPC-I-

Figure 25-10 Class C-2 Process Sequence

5-32

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January

1996

IPC-CM-770

IPc-I-00050

Figure 25-11 Panel Assembly Tooling Holes

IPC-1-00361

Figure 25-12 Positive Symbol Machine Correction

PCB

Layout:

Spacing

Wave Solder

-7

&%?I

IPC-1-00362

Figure 25-13 Chip Placement

5-33

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