IPC-CM-770D-1996.pdf - 第74页

January

1996

IPC-CM-770

IPC-1-00234

Figure

15-1

O

Surface Mount Connector

the high temperatures (215°C) encountered during reflow

solder processes.

The leads have been formed to make contact with the

lands on the board. The more intimate the contact is

between lead and land, the better.

The leads should be plated

so

as to assure good solder-

ability to the land. Solderability of leads can be checked

using simple tests described in MIL-STD-202, Method

208 or J-STD-002.

The connectors may require features for mechanical

attachment to the board. This is especially desirable for

longer connectors or connectors that are potentially sub-

jected to abuse during mating and unmating.

Alternative methods of surface mounting include the use of

pressure contacts to maintain electrical contact at the

board, as well as the separable interface. A point in these

applications is the need to provide board stiffening struc-

tures to neutralize the effect of "normal" forces.

An evolving technology is the use of conductive epoxies to

surface-mount a connector to board. Utilization of this

method affords the potential of lower processing tempera-

tures.

15.5.1

Component Preparation

Extra care should be

taken with surface mount connectors to assure that the

leads are not damaged prior to placement on the board.

15.5.2

Land Patterns

The size of the land on the board

is directly related to the size of the lead to be soldered plus

a tolerance needed to adapt to any inaccuracies in place-

ment. Generally, a land that is 0.25 mm larger on all sides

than the lead is recommended. The minimum land size

must be sufficient to provide a fillet on all sides of the lead.

15.5.3 LeadlLand Configurations After Assembly

Con-

nector leads are not reconfigured after surface mounting.

15.5.4

Mounted Component Configuration

The loca-

tion, or true position, of component leads becomes more

critical as the number of inputs/outputs increase. Board

hole tolerances and lead tolerances over the length of some

of the larger connectors can cause interference problems

when mounting. In such instances, mounting aids are

required and are provided by the connector manufacturer.

An important consideration with larger surface mounted

connectors is the difference in thermal expansion between

the connector plastic and the printed board. All commonly

used plastic materials have a coefficient of thermal expan-

sion (CTE) that exceeds that of epoxy glass boards by at

least

7

ppm/C"

(17

ppm/C" for ceramic substrates). The

mechanical connector design must be able to absorb this

CTE difference to avoid solder joint failures during thermal

cycling. Visual examination of leads of the mounted con-

nectors is recommended. Reposition individual leads if

required.

15.6 Mixed Technology

These assemblies generally are

wave soldered as the last attachment step. Standard

through-hole connectors can therefore generally be used for

mixed assemblies. These connectors should be applied

after the reflow solder process or must be chosen to be

compatible with the high temperatures seen during reflow

soldering.

15.7

Manual Assembly

Most connectors are inserted or

placed using manual techniques. Assembly may occur the

same time as other components or as a separate secondary

operation. Care should be exercised to not disturb previ-

ously assembled parts.

15.8 Automated Assembly

For many backplane assem-

blies, computer controlled machines are used which stake

individual pins into the backplane. Those pins that are actu-

ally needed are staked. Also, pin lengths can be selected for

staged mating of ground, power, and signal contacts.

Increasingly, robots are being used to place connectors.

Where robots are being used, it is important to specify

connectors with true position tolerances and other key

dimensions compatible with the board and robotic place-

ment system. Packaging should also be selected to make

delivery of the connector to the robot simple.

15.9 Handling and Storage

The handling and storage of

connectors should be in accordance with the guidelines of

Section 26.

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Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

15.1

O

Soldering

Care must be taken

so

that flux or sol-

der does not wick up into the contact. Some manufacturers

provide anti-wicking devices to prevent flux or solder

wicking.

If

connectors are not mechanically secured to the

board, suitable fixturing should be provided to prevent lift-

ing during soldering. This is especially important with

small lightweight connectors. General soldering guidelines

are discussed in Section

27.

15.1 1 Cleaning

Cleaning agents used after soldering

should not have any harmful effects upon the connector

housing material. Check with the connector manufacturer

to verify the compatibility of any cleaning agent with the

connector housing material. Any residue left on the contact

surfaces must be removed. When possible the open end of

the connector should be turned with the opening downward

so

that the cleaning agent can drain out of the connector

body to facilitate drying.

Ideally, connectors should be provided with standoffs and

should not have blind holes, to permit cleaning agent to

pass between the connector and the printed board.

15.1

2

Coating

Care must be exercised to prevent coat-

ings from getting on the contact surfaces. Wicking can

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

the connector to prevent it.

cessing temperature extremes.

Two classes of sockets are available; namely, low insertion

force and zero insertion force. Low insertion force

describes disconnects where the insertionlextraction forces

associated with the normal forces and component loading

actions are present. Since the magnitude of the force is

additive and related to the number of leads per device, the

maximum size of socket is limited. In situations where high

forcelpin counts are encountered, zero insertion force con-

nectors are used. These are characterized by the presence

of a cdlever arrangement which relieves the normal force

created by the fixed spring segments of connectors.

Synonymous with this is the implication that appropriate

clearances must be provided for the camllever actuation.

The use of a socket carries the inherent cautions associated

most component namely:

Assembly reliability does not deteriorate as a result of

process incompatibility.

Contaminants are not trapped to promote the degradation

of materials in the system.

Proper selection by engineering.

Added costs.

Sockets may be classified in two categories; namely, dis-

IPC-I-

L

Figure 15-11 D-subminiature Surface Mount Connector

16.0 DISCRETE COMPONENT SOCKETS

Sockets are passive interconnecting devices which serve to

create a readily separable interface between discrete com-

ponents such as DIPS, chip carriers, displays, axial compo-

nents, etc. The socket is usually soldered to the board and

embodies a plated fixed spring element which exerts nor-

mal force on an appropriate segment of an active discrete

component. Sockets have a broad range of utility including

ease of component replacement due to failure, system

updating, component substitution due to conditioning

andor testing, and protection of active devices from pro-

Crete or loose piece and array assemblies. Discrete sockets

are individually applied components which are inserted

independently at various board locations. Array style sock-

ets utilize a carrier such as a housing or tape to carry indi-

vidual contacts and ease the manufacturing task of socket

installation to the board.

16.1 Part Type Description

There are several types of

sockets used in printed board applications for the attach-

ment of discrete component leads. They generally fall into

one of the following categories.

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

January

1996

IPC-CM-770

Figure 15-1

2

Surface Mount Receptacle

Figure 15-1

3

Box-contact Surface Mount Receptacle

A.

Stake-to-the-Board Devices

that are mounted horizon-

tally or vertically. They are usually manufactured as either

dual or multimetal devices, or as singular metal devices.

B. Low Profile Pluggable Devices

assumes positive reten-

tion to the printed board prior to soldering. They usually

are designed with a cup and spring extension that allows

solder to be drawn to the top surface of the board, produc-

ing a solder fillet on the component side land without over-

flow into the socket.

C. Low Profile Grip Type Devices

are miniature connec-

tors with a singular metal device that has a portion acting

as a temporary retention device for discrete component

leads,

so

that they may be eventually soldered and, if nec-

essary,eventually removed easily without damage to the

board or to the component being removed.

16.1.1 Individual Component-Lead Sockets

16.1.1.1 Low Profile Sockets

One such device is a plug-

gable socket that assumes positive retention in the board

prior to soldering, and spring to board electrical contact

after soldering. The design of the cup and spring extension

allows solder to be drawn to the top surface of a two-sided

board, producing a solder fillet on the top land without

overflow into the socket. Soldering may be done by the

usual methods of wave soldering or hand soldering.

The socket shown is of two-part design having a heat

treated spring assembled into a drawn cup. Spring provides

multiple cycle life with high contact forces.

Many of these components come with an elastomer seal to

keep out contaminants during the soldering, washing cycles

as well as throughout its life and care must be taken during

insertion into the board

so

as not to destroy this seal. The

seal is normally left in position throughout the life of the

socket and discrete components are mounted or inserted

through it without detrimental effects. See Figures 16-1 and

16-2.

The profile of this type of device above the printed board

usually is less than

0.8

mm and can be used on centerline

spacings as close as 2.54 mm minimum center to centerline

distance. Product is available for nominal hole sizes of

1.07

4-1

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Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services