IPC-CM-770D-1996 - 第81页

IPC-CM-770

Januaty

1996

.a35

Section

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Contact

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IPC-I-

Figure 16-11 Leadless Grid Array Socket

A

r

'ACT-

HOUS

PA

STRUCTURE

?SOLDER

J

\

ING

OIN

T

P/I

STRUCTURE

COMPLIANT SECTION

L

LAND

B

IPC-I-

Figure 16-1 2 Section Through Socket Solder Contact

The low profile plastic leaded chip carrier socket serves as

a transition device to enable users to take advantage of the

benefits of four sided IC packages without making changes

in their current assembly procedures.

As previously mentioned, the chip carriers can have sur-

face mount leads on 1.27 mm spacing. The chip carrier

socket has flat spring input leads on 1.27 mm spacing to

accommodate the package, and straight solder tail output

leads on 2.54 mm spacing to meet the requirements for

standard through-hole mounting.

In pin grid array packages, the through-hole mounting is

the dominant choice. Sockets for pin grid arrays are

designed to match pin configurations.

There are three types of pin grid array sockets currently

available: the standard version which allows the component

to be pushed in; a low insertion force version which

employs a low force contact design; and a zero insertion

force, which makes use of a mechanical assist to maintain

contact pressure.

Through-hole mounting techniques are outlined in Section

21.

16.2.1 Land Patterns

The most appropriate board layout

for the component is usually offered by the socket supplier.

Automatic assembly will normally demand additional

clearance for greater positioning/placement allowance.

Sockets are somewhat unique to other board components in

that they serve as a connector for their components. As

such, they take much more stresdstrain on the fillet through

the terminal). It is strongly recommended therefore, that:

Surface land diameters be as large as allowable.

The RTH diameter should be designed to the lower side

to allow maximum capillary action, minimum hole solder.

Surface detail should allow at least two diagonally oppo-

site terminals to be clinched, or minimally to have reten-

tive mechanical actions.

See IPC-SM-782 for surface mount land patterns.

16.2.2 Lead Configuration After Assembly

Socket con-

tacts are normally stationary (nonformable), preset and

typically protrude orthogonally to the board surface. For

stamped contacts, where there is not a mechanical grip,

clinching is recommended. Clinches should be formed to

4-12

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

the inside. A post assembly terminal review should high-

light lead length conformance and separation of lead ends

from other leads or surface conductors.

16.2.2.1 Polarization

The device and socket assembly/

application specifications, as well as the board markings

are the principal controlling documents for orienting sock-

ets on the board. Sockets are visually and dimensionally

symmetrical and may only offer visual aids (dot, slanted

comer, tab, etc.) on the part to orient it. Therefore, the con-

trolling assembly document should note that visual aid as

well as pin numbers.

16.2.2.2 Lead Forming and Alignment

Lead forming is

not usually performed on sockets, but if it is necessary, the

fixture or tool must ensure that the integrity of the contact

areas remain intact, and that the lead is not deformed

beyond

10%

of the lead diameter.

Packaging and handling will dictate the lead’s proper align-

ment being available at the assembly station. Automatic

machines require an angle less than 90 true for proper jaw

grip of the part.

16.2.2.3 Seating

Sockets being relatively light in rela-

tion to their land patterns and the number of leads penetrat-

ing the board are prone to be improperly seated; either not

fully bottomed out to the board or having a lead or two

crushed down (rolled under the body). It is advised that

after assembly, a visual inspection be authorized prior to

clinching and soldering.

16.3 Surface Mounting

Instead of straight solder tails

which are inserted into through-holes, the surface mount

socket has flat

“L”

or

“J”

type leads.

Figure 16-11 shows a section through a contact that can be

used with the type of socket in Figure 16-9 to provide for

surface mounting. This contact has a long compliant lower

leg and mates with the land pattern shown in Figure 16-14.

The two registration holes mate with corres- ponding pro-

jections on the socket.

Figure 16-13 shows an alternate contact configuration. At

the cost of reduced compliance, this contact has a shorter

electrical path, allows an easier visual inspection of the

soldered joint, and required a different land pattern.

For sizes larger than the 68-position

(1.27

mm) centerline,

the interaction of the contact force and cover force may

distort the connector housing. This may in turn strain the

solder joint.

This situation is avoided by use of the four comer screws

to engage a backup plate. These transfer the forces from

the cover to the backup plate and establish the closed sys-

tem which minimizes the forces on the housing, the printed

HOLD

DOWN

SCREWS

(4)

COVER

OR

HOLD

DOWN

CHIP CARRIER

THRUSTER CLIP

BACKUP PLATE

IPC-I

-247

Figure 16-13 Screw Down Cover

board structure and the solder joints.

Another configuration is shown in Figures 16-13 and

16-14. Four comer screws transfer forces from cover to

backup plate. The backup plate in conjunction with the

dielectric spacer are two of the factors that characterize the

impedance of this socket.

A thruster clip is used to bias the leadless type A,

B

or

D

chip carrier into a zero reference position for good registra-

tion. The hold-down has compliance to accommodate the

thickness tolerance of the chip carrier

(CC).

It snaps into

place and retains the

CC.

An opening is providedto accom-

modate a heat sink.

IPC-I-

Figure 16-14 High Speed Circuit Socket

The type of socket shown in Figure 16-9 can also be

obtained with a mechanical contact pressure interface with

4-13

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

a printed board structure. The advantages of this type of

socket are the ease of replacement of the socket in the

16.4 Mixed Technology

Few considerations apply to

field. The disadvantage is that a second mechanical inter-

mixed assembly. The assembly sequence is normally dic-

face is now in the circuit. Figure 16-15 shows how the

tated by the soldering procedures to be used. Sockets are

socket is mounted on the structure. Two alignmendlocating

not intended for direct immersion in solder, therefore are

pins in the socket engage two mating holes on the printed

restricted to the component side of the printed board or

board

The

pins

Serve

as

registration

as

well

as

carrier when the assembly is to be wave soldered. For sol-

devices.

A

Screw clamps

the

socket

to

the

strut-

der flow processes the socket insulator material must be

ture by means of a backup plate and a socket plate. The

with

the

reflow

process.

clamping action of the Screw and Plate causes the lower Sockets in today's packaging schemes are being used for

half of the Contact to deflect and generate the Contact force. increasingly specialized applications and therefore less fre-

This arrangement is suitable for 68-Position

(1.27

mm> quently. As a consequence, sockets appear more often in

centerlines and smaller. In the larger sizes, the contact mixed mounting situations. Several physical obvious points

force becomes too great for a single screw clamp. Four arise unique to sockets:

comer clamping screws are used in the larger sizes.

STAND-OFF

SOCKET PLATE

BACKUP PLATE ALIGNMENT/LOCATING PIN

IPC-I

-00249

Figure 16-15 Section Through Pressure Mounted Socket

The center screw is used to hold the socket in place until

the four comer screws are engaged. The center screw

applies a preload to the bottom beams of the contact. After

the chip carrier is inserted into the socket, the cover is

secured by the four comer screws. These comer screws are

torqued until the full contact load is developed. The load is

transferred to the backup plate; this reduces the load on the

printed board structure and minimizes creep under load.

16.3.1 Component Preparation

Lead forming is not

performed on surface mount sockets.

16.3.2 Land Patterns

Surface mount sockets for the

most part have the terminals folded under the insulator,

making it difficult to visually align the densely populated

terminal to the conductor pattern. As a consequence, cer-

tain suppliers have provided alignment holes in the chip

mounting cavity to align the socket to either special non-

functional lands or holes. An adhesive is usually utilized

under the body to hold the socket in position prior to sol-

dering. Socket land patterns are normally identical to the

device land pattern in order to retain consistency but allow

a socket option. A typical land pattern pattern is shown in

Figure 16- 16.

AREA TO BE

FREE OF

1

295 SQUARE

O

210

SQUARE

(4

PLACES)

METALLIZATION

REF

1

,(4

PLACES)

!Ll.OOO

---

(INCHES

SHOWN)

Esc IPC-I-O0250

Figure 16-16 Example

of

68

110

Land Pattern on Printed

Board Structure

4-14

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services