IPC-CM-770D-1996.pdf - 第51页

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

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

attached to either (a) a solid metal post, or

(b)

a conven-

tional land as illustrated in Figure

11-10.

(a)

"Post" Method

TINNED

LEAD

(b)

"Surface

Terminal Area

Method"

IPC-1-00381

Figure 11-10 Surface-mounted Flatpacks

The surface land method of flatpack mounting, Figures

11-10b and

11-11,

terminate the flatpack leads to lands

which are an integral part of the conductor pattern formed

on the component side of the structure.

11.3.3 LeadlLand Configuration After Assembly

11.3.3.1 Soldered Terminations

Part placement, fabri-

cation, and assembly processes should permit the final

product to meet the requirements illustrated Figure

11-12,

11-13,

11-14,

and J-STD-001.

Part leads should have a minimum of

0.75

mm for soldered

type type connections and a minimum of

1.14

mm for

welded type connections in contact with the land (Figure

11-12).

Part leads should be axially parallel to land areas

whenever possible. (see Figures

11-13

and

11-14).

11.3.3.2 Welded Terminations

For some applications of

integrated circuits it may be necessary or desirable to use

welded rather than soldered connections to the devices.

Figure

11-15

shows two methods which may be used for

making such welded connections. In general, the mechani-

cal space considerations described in J-STD-O01 for sol-

dered connections (lead bending, tolerances, etc.) apply

equally for welded connections.

11.3.4 Mounted Component Configuration

The surface

I

IPC-I-O0222

Figure 11-11 Typical Surface Land Flatpack Mounting

0.030

rnin

soldered

0.045

rnin

welded

(INCHES SHOWN)

IPC-1-00057

Figure 11-12 Minimum Planar Lead Contact

RECOMMENDED

L

030

mlnlsolder

045

mln

weld

NOT

RECOMMENDED

I

RECOMMENDED NOT RECOMMENDED

Pan lead centered

or

edge

of

lead

does not

Edge

of

lead overhangs land

overhang land

area

by

more

than

005 Inch

(INCHES SHOWN)

IPC-1-00056

Figure 11-13 Axial Alignment

land method of flatpack mounting has the following advan-

tages over the through-the-board mounting methods:

Drilling of holes is not required.

Bending or special forming of the flat leads is minimized.

High component packaging densities are possible; greater

conductor routing on the circuit side of the printed board

structure; component mounting is possible on both sides

of the assembly. One disadvantage of the surface mount-

ing method is that the flatpack must be located accurately

and held in position during the soldering operation.

3-15

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

Lead extends beyond

edge

of

land greater

than

0.12

mm

[.005”]

NOT

RECOMMENDED

IPC-I-O0058

Figure 11 -1 4 Lead Extension

(a)

“Post”

Method

(b)

“Surface Land” Method

IPC-I-

Figure 11-15 Welded Configurations

1 1.4 Mixed Technology

There are no special procedures

required for mixed assembly of flatpack devices. The

assembly sequence is normally dictated by the soldering

procedures to be used. For instance, if the surface mounted

devices are to be hand soldered in place, all through-the-

board mounted parts would be wave soldered in place first.

The remaining devices would then be hand soldered in

place.

However, if the surface mounted devices are to be mass

reflow soldered in place, this operation would be accom-

plished prior to the wave soldering operation.

The two step sequence is selected such that the second sol-

dering operation disturbs the initial connections by the least

amount.

Flatpacks are not intended for immersion in solder. There-

fore, flatpacks mounting is normally restricted to the “com-

ponent” side of the printed board or carrier when the

assembly is to be dip or wave soldered.

11.5 Manual Assembly

When handling flatpack devices

manually, care must be taken because the fragile nature of

the leads precludes any operation where pressure could be

applied to the formed leads. The two methods currently in

use are rubber tipped tweezers, and vacuum pick-up and

handling devices. Of the two, the vacuum pick-up device is

preferable, since it allows the flatpack to be maintained

parallel to the printed board or carrier at all times.

Vacuum handling is recommended with quad packs, since

no section of the body is available for handling by twee-

zers.

11.6 Automated Assembly

Flatpack devices may be

assembled using automated techniques. “Pick and place”-

type equipment is normally used, selecting devices from

special carriers which will protect the preformed leads

from damage.

11.7 Handling and Storage

The handling and storage of

ribbon-lead components should be in accordance with the

guidelines of Section

26.

11.8 Soldering

General soldering techniques applying to

all types of components are described in Section

27.

11.9 Cleaning

General techniques for cleaning applying

to all types of components are described in Section

28.

11.1

O

Conformal Coating

General techniques for con-

formal coating for all types of assemblies are described in

Section

29.

12.0 CHIP CARRIERS

Chip carriers can be generally defined as low profile,

square packages with connections on all four sides consist-

ing of metallized terminations on “leadless” version and

leads formed around or attached to the side of the package

in the case of leaded version.

Growth in use of chip carriers has been well documented

and relates to cost savings, as well as performance advan-

tages in some specific applications (high speedhigh fre-

quency).

Some disadvantages in the component mounting process

are inherent to use of either leaded or leadless chip carri-

ers. These include the need for more precise placement

techniques and tolerances and the method of retaining the

component in place prior to the soldering operation. Other

general considerations are outlined in Table

12-1.

The use of leaded chip carriers generally has the following

advantages over the use of leadless chip carriers:

The effect of thermal expansion mismatch is less critical

with leaded chip carriers due to lead compliance.

Flexure of a substrate structure is less critical with leaded

chip carriers due to lead compliance.

Leaded chip carriers can generally be used more readily

with glass epoxy printed boards.

The leads create a standoff which aids in the removal of

soldering fluxes during cleaning.

3-16

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services