IPC-CM-770D-1996 - 第49页

IPC-CM-770

Januaty

1996

with semiautomatic insertion equipment.

Flatpacks must be insulated when placed over conductive

traces. Unless the user has control over the particular pack-

age material, it may be appropriate to provide insulation

between the bottom of the flatpack and any conductive

traces, regardless of the case material being used.

Users of flatpacks have had problems from non-standard

shipping containers. Another flatpack problem is that the

automated assembly cost is very high compared to other

packages, owing to the fragile leads and the special care

required in assembly.

Flatpacks devices are usually of the outline depicted in

Figure

11-1.

Dimensions of standard flat pack devices are

listed in MIL-M-38510 and in JEDEC publication 95-83.

E

IPC-I-O0212

Figure 11-1 Flatpack Outline Drawing

A.

Flat Pack Device Type

One of the smallest multiple lead components is the flat

pack. The body of these components can be as small as

3.18 mm wide, 6.35 mm long, and 0.79 mm thick. The

component leads are normally flat ribbons

0.5

x 0.25 mm

or smaller, and are located on 1.27 mm centers.

Flat packs are secured several ways:

Form leads to fit a staggered hole-land pattern, using pot

or wave soldering.

Weld or solder leads to lands or tabs.

Form leads with stress relief bends to protect the glass

seals, then weld or solder to lands or tabs that are 0.79

mm wide x 1.9 mm long. In an alternate method, all leads

are simultaneously solder-coated and then reflow-

soldered with semiautomatic insertion equipment. As with

all metal cased components, flat packs must be insulated

when placed over the particular package material, it may

be appropriate to provide insulation between the bottom

of the flat pack and any conductive traces, regardless

of

the case material being used.

Non-standard shipping containers for flat packs have

caused problems. Another flat pack problem is the very

high automated assembly cost, compared to other pack-

ages, due to the fragile leads and the special care required

in assembly.

Flat pack devices are usually similar to the outline depicted

in Figure

11-1.

Dimensions of standard flat pack devices

are listed in MIL-M-38510 and in JEDEC publication 95.

B.

Quad Pack Device Type

Four-sided flat packs, i.e., quad packs (Figure 11-2), are

used to increase the

Il0

capacity of the device without sig-

nificantly increasing the component size. In this respect,

they closely resemble leaded chip carriers.

C. Discrete Device Type

Ribbon leaded discrete devices are also very small leaded

discrete devices. The case is normally no larger than nec-

essary to provide mechanical support for the leads. These

devices were originally developed for use on stripline

boards, where the part body is recessed into the board such

that the part leads are aligned with the stripline circuit

traces.

The special characteristics of these devices (small lead

inductances, low capacitance between leads, and small

physical size) have made these devices useful in non-

stripline applications also. Figure 11-3 is typical of the rib-

bon leaded transistors available. JEDEC publication 95

lists the dimensions of ribbon leaded discrete devices.

t-

-1

Figure 11-2 Quad Pack Configuration

11.2 Through-Hole Mounting

11 2.1 Component Preparation

Through-hole mounting

of flatpacks is accomplished as illustrated in Figure

11-4

.

Lead bending is normally accomplished using fixed dies,

which will bend all leads simultaneously.

Hand held tooling is also available, which will bend all

leads on one side of the device at one time. All tooling

should protect the device seals by supporting the lead

3-12

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

-”

IPC-I-O0213

2t

but

not

less

than

L

Mar

-

Maxlrnurn

Body

length

or

wldth (including

extensions

such

as

lead fillets (both sides),

glass

seals,

etc.]

W

-

Nominal lead mdth

t

-

Nominal lead lhlcknew

I

IPC-1-00216

Figure 11-3 Typical Ribbon Leaded Discrete Device

Outline Drawing

Figure 11-5 Through-hole Mounting. “MO” Flatpack

Outline Drawing

pack mounting method with unclinched leads the flat leads

are formed at a

90”

angle and inserted in mounting holes

in the printed board (see Figure

11-6).

SUGGESTED

SIZES

(IN

i

I

Figure 11-4 Staggered Hole Pattern Mounting. “MO”

Flatpack OutlineDrawing (Only Inches Shown)

between the body and the bend. Lead bending and forming

requirements for typical packages are shown in Figure

11-5.

11.2.2 Land Patterns

Typical land patterns for through-

hole mounting are included in Figure

11-4.

The inline

mounting pattern is much more restrictive regarding toler-

ances and position because of the limited space between

the leads.

The staggered lead arrangement of Figure

11-4

permits

hole sizes of

0.75

mm, which will accommodate

“F”

out-

line devices under all combinations of hole diameter toler-

ance and lead size tolerances.

11.2.3 Lead Configuration After Assembly

11.2.3.1 Unclinched Leads

In the through-the-board flat-

PLATED-THROUGH

HOLE

/

Figure 11-6 Through-the-board board Mounting with

Unclinched Leads

As

can be seen from Figure

11-3,

the in-line mounting

restricts the hole diameter to about

0.5

mm which, in turn,

restricts the maximum width of the lead to less than

0.5

mm. Under these conditions, selection of devices with

small leads may be required.

A. Circumscribing Land

In the through-the-board,

clinched lead with full circumscribing land flatpack mount-

ing method, an additional lead clinching operation is per-

formed (see Figure

11-7).

11.2.3.2 Clinched Leads

A. Circumscribing Land

through-the-board, clinched lead

with full circumscribing land flatpack mounting method, an

additional lead clinching operation is performed (see Fig-

ure

11-7).

The advantages of using this type of flatpack mounting are:

The flatpack is positioned to withstand the forces exerted

upon it during the mass soldering operations.

The hole-to-lead clearance is not as critical.

Solder connections may be more reliable than unclinched

mounting due to additional mechanical contact.

Supported (plated-through) mounting holes need not be

3-13

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Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

used; components can be more readily desoldered and

removed.

a

IPC-I-O0218

Figure 11-7 Through-the-board Mounting with Clinched

Leads and Circumscribing Land

B. Offset Land

A common variation of the through-the-

board, clinched lead flatpack mounting method just

described is that in which the land is offset, instead of

circum- scribing the hole (Figure 11-8).

IPC-I-O0219

Figure 11-8 Through-the-board Mounting with Offset

Land

11.2.4 Mounted Component Configuration

11.2.4.1 lnline Leads

The mounting pattern shown in

Figure 11-3 employs “inline” leads and lands for through-

the-board mount devices. Although such inline lead

arrangements simplify lead forming requirements, they

result in very closely spaced lands (approximately 0.8 mm

clearance) and therefore require the use of close tolerance

manufacturing processes for fabrication and assembly, par-

ticularly for through-the-board mounting.

Another disadvantage of the inline arrangement is the lim-

ited space available for conductor routing between terminal

areas.

11.2.4.2 Staggered Leads

Some of the disadvantages

associated with inline patterns can be overcome by the use

of “staggered” lead arrangements (see Figure

11-4).

In

these mounting patterns the lead hole and lands for adja-

cent leads on the same side of the flatpack are offset by

some convenient distance from the inline axis. Although a

staggered lead arrangement requires somewhat more board

area per device than the inline arrangement, it provides

several advantages:

Tolerances are less critical.

Larger lands can be used.

More space is available for routing conductors between

adjacent lands.

Larger component lead holes can be used to simplify

component insertion.

In the staggered lead arrangement a good compromise

between loss of available board area and the increase in the

number of through conductors can be achieved by the use

of an

2.5

mm offset between adjacent land area. With this

arrangement conventional manu- facturing tolerance are

applicable, and a

2.5

mm annular ring (a practicable mini-

mum) is possible. The maximum offset that can be

achieved with flatpack leads of

6.4

mm length is 3.8 mm.

When this maximum offset is used, only the through-the-

board type of mounting is practicable.

11.3 Surface Mounting

11.3.1 Component Preparation

Component preparation

for surface mounting requires only than an off- set be

formed in the leads to provide contact with the mounting

surface and prevent stress on the component seals. Lead

bending requirements are illustrated in Figure

11-9.

As

with the through-hole mounting configura- tions, lead

forming for surface mounting can be accomplished using

either fixed tooling which forms all leads at the same time,

or hand held tooling which will form all leads on one side

at a time.

21

but

not

less

than

0.4

mm

[0.16”]

7

I

112

W

-

1/2

W

Land Spacing

+

L

Max

=

Maxlmurn Body length

or

wldth [includmg

extensions such as lead fillets (both sides),

glass seals, etc

]

W

=

Normnal lead width

t

=

NomInal lead thickness

IPC-I-O0221

Figure 11-9 Lead Bending Requirements for Surface

Mounting

Two major considerations must be addressed when select-

ing tooling: the extent of the offset, and coplanarity. The

offset in the leads must account for the distance from the

lead exit from the package to the bottom of the package,

the thickness of any insulation under the flatpack, and the

thickness of any adhesive or other spacers under the

device. Coplanarity is important to assure that stresses are

equalized on each lead. A value of

k0.05

mm is recom-

mended.

11.3.2 Land Patterns

With the surface mounted method

of flatpack attachment, the connections to the device on the

component side of the printed board assembly can be

3-14

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services