IPC-CM-770D-1996 - 第83页

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

IPC-CM-770

Januaty

1996

printed boards. To provide mechanical strength to the ter- minal serves as the feed through device through the glass

minal and the resultant connection, most terminals are seal. See Figure

17-3.

designed for through-the-board mounting, either with

crimping, rolling, or flaring on the back side of the board,

or by an interference fit or “press fit” into the board.

Solder-only mounting may be used when the terminals are

only used as test points.

D.

Cup Terminals

cup terminals are not normally

attached to printed boards, but are often used as part of a

connection technique which does interface with a printed

board. Cup terminals are designed for either soldering (see

Figure

17-4)

or crimping (see Figure

17-5).

Terminals can be generally categorized as follows:

A.

Turret Terminals

This type of terminal has guide slots

to assist in retaining the attaching wires, and is available

with mounting bases designed for funnel flange or roll

flange retention, or for press-fit installation. See Figure

17.1.2 Solderless (Wire) Wrap PinslPosts

Pins are

available in single and double ended types, with and with-

out solder carrying characteristics; single pin end with a

socket on the other end, with and without self solder carry-

ing characteristics; rectangular and square varieties. Vari-

17-1.

I

guide

slot

dase

clearance

IPC-I-O0251

Figure 17-1 Typical Turret Terminals

B. Bifurcated Terminals

This type of terminal contains ous self retaining features for securing to the printed board

two posts for lead or wire attachment. Bifurcated terminals are also available, as are plating and base material. There

are normally available only with their bases designed for are those that require soldering to the printed board and

funnel flange or roll flange retention. See Figure

17-2.

those that do not. Compliant pins depend on an expanded

lnsulatlon

clearance

Polnt

o1

entry

IS

same for

upper conductor

IPC-I-O0252

Figure 17-2 Typical Bifurcated Terminals

C. Pin Terminals

These terminals are designed for use section of the pin for retention in the printed board. Other

where space is limited, and are normally used only in the types require the insertion machine to crimp the pin on

“press-fit’’ style. One variation of the pin terminal is the both sides of the printed board for retention.

A

third type is

hook terminal. This type of terminal is usually found on held in place completely by the solder. The choice depends

relays and other hermetic seal devices, where the hook ter- on consideration such as the type of tooling available, the

4-16

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services