IPC-CM-770D-1996 - 第82页

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

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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

January

1996

IPC-CM-770

A. Heights

Although low profile sockets are available,

sockets (with their device) present a higher assembly pro-

file. Wave solder holding plates are one point affected.

B. Automatic Assembly

As sockets are increasingly

unique, there may be a tendency to hand load one (or sev-

eral) which are an anomaly to the board. Designers should

be led to reassign socket placement to common boards if

possible.

C. Land Patterns

As it is wise to use a device insertion/

extraction tool, clearance real estate about sockets is

required. The increasing use of chips also necessitates use

of a great deal of device I/O patterns which also has a ten-

dency to open board space. These open land areas offer

“hand hold” and/or probe points which make the board

more susceptible to surface damage.

16.5 Manual Assembly

Sockets are normally handled

manually using special tools designed for ease of alignment

and insertion. On certain sockets a locating pin is used to

correctly orient it to the land pattern, such as pressure fit

type sockets. Manual assembly as always offers a greater

susceptibility to damage. Major points to be addressed

when using these procedures with sockets are:

A. Board Loading

Always load from component side

with lands and holes in view. Immediately clinch diagonal

terminals on solder side.

B. Handling

Socket contact springs are susceptible to fin-

ger oils, handling damage, particulate entrapments. Use of

lint free gloves when assembling is appropriate. Handling

the sockets with tools or by the insulator is also necessary.

C. Storage

Leads are extremely susceptible to displace-

ment. Remove sockets from the shipping container one at

a time and immediately load in the board. Never

accumulate/store sockets on the work bench unprotected.

D. Clipping

If

leads must be clipped, they should be

shortened after they are in the board (never in an

unmounted condition).

16.6 Automated Assembly

Although the bulk of sock-

ets, DIPS, may be automatically loaded with stick loaders,

the balance do not lend themselves to automation. High

board frequency and quantity sockets may be loaded by

pick and place type equipment; however, operation cost

must be closely assessed for value.

16.7 Handling and Storage

The handling and storage of

component sockets should be in accordance with the guide-

lines of Section

26.

16.8 Soldering

Due to the terminakontact configura-

tion, sockets have a tendency to sink more heat than other

components. As a consequence, they will require a slightly

longer dwell time and will draw more once started. Due to

inability to adequately clean terminal area, the user/

manufacturer may restrict the use of activized fluxes. In

wave soldering through-hole mounted sockets, the socket

must be mounted on the component side of the printed cir-

cuit board or carrier. Care must be taken to ensure that flux

or solder does not wick up into the receptacle area of the

socket.

Soldering techniques are outlined in Section

27.

16.9 Cleaning

Special care and attention must be given

to avoid entrapment of particulates in the socket contact

cavity which is difficult to flush. Particulates may not only

leach corrosive products, but may also hinder contact

spring actions and defeat the socket.

Sockets rarely, if ever, require aggressive cleaners or

techniques.

Non-normal cleaners and reagents should be cleared for

use with the supplier

so

that contact plating or insulators

are not affected by the process.

Cleaning agents used after soldering should not have any

harmful effects upon the socket housing material. Check

with the socket manufacturer to verify the compatibility of

any cleaning agent with the socket housing material. Any

residue left on the contact surfaces must be removed.

Ideally, sockets should be provided with standoffs and

should not have blind holes, to permit cleaning agent to

pass between the socket and printed board. Cleaning is

more thoroughly covered in Section

28.

16.1

O

Coating

Care must be exercised to prevent coating

from getting on the contact surfaces. Wicking can present

a problem, and it may be necessary to seal around the

socket with tapes or fixtures to prevent it.

17.0 INTERCONNECT COMPONENTS

This section covers those devices used for providing per-

manent connections from one point to another and includes

terminals, pins, jumpers, bus wire, bus bars, stranded and

solid wire, and test points. For further information, see

IPC-PD-335.

17.1 Part Type Description

17.1.1 Terminals

Terminals is a name applied to a broad

range of devices used in or on a printed board. These

devices are usually chosen for their optimum electrical val-

ues or physical characteristics via systems testing; for

quick repair or replacement of compatible components or

devices; for their ability to be automatically inserted; to

connect printed boards to each other; or to connect the

printed wiring to other parts of the system external to the

4-15

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services