IPC-CM-770D-1996.pdf - 第68页

IPC-CM-770 Januaty 1996 3-32 COPYRIGHT Association Connecting Electronics Industries Licensed by Information Handling Services COPYRIGHT Association Connecting Electronics Industries Licensed by Information Handling Serv…

January

1996

IPC-CM-770

environment, since bare chips are being handled. General

cleaning guidelines are discussed in Section 28. Attention

should be placed on component and solvent compatibility.

14.1

Coatings

General conformal coating characteris-

tics applying to all types of components and assemblies are

described in Section 29.

Coatings are used in COB technology to protect the bonded

chips on the board from handling damage as well as for

environmental protection. Typical coatings used for this

purpose are room- temperature vulcanizing @TV) disper-

sion coatings, silicon dielectric gel, and epoxy type coat-

ings, applied locally at the bonded chip site, as opposed to

entire board coating.

14.10.1

RTV

Dispersion Coating

Typical RTV disper-

sion coatings are one-component, room-temperature vulca-

nizing (RTV) silicone-rubber coatings that are supplied as

a xylene dispersion.

(No

mixing is required.) The curing

process used a crosslinking mechanism that generates

methanol during cure. Once applied and exposed, the mate-

rial vulcanizes by reaction with moisture from the air to

form a soft, resilient elastomeric coating that will withstand

long term exposure to temperature as high as 250°C

(482°F).

14.10.2 Silicone Dielectric Gel

Special controlled tech-

nology can yield a silicone gel that affords the nonflowable

permanence of a solid, but also gives the freedom from

large mechanical and thermal stresses of a fluid. Chemi-

cally, a typical silicone gel is very similar to silicone flu-

ids, but with just enough crosslinking to prevent separation

of the individual polymer chains and give nonflow thermal-

set properties. The fully-cured dielectric gel is a soft, jelly-

like material that exhibits tenacious pressure-sensitive

adhesion to virtually any substrate.

14.10.3 Epoxy Coatings

Epoxy coatings are also avail-

able for COB self-crowning or “glob-top’’ applications.

Typical materials are two-component, liquid epoxy/

anhydride systems that have been formulated for their

superior thermal shock performance, substrate adhesion,

moisture resistance, and glass-transition temperatures in the

range of from

165”

to 180°C.

3-3

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

3-32

Licensed by Information Handling Services

January

1996

IPC-CM-770

Part Four

Electromechanical and Interconnect Components

15.0 CONNECTORS

This section provides the details on selection, description

and technical considerations for mounting electrical con-

nectors. Through mount, surface mount and mixed mount-

ing situations are addressed as well as automation.

Interconnection needs must be considered during the initial

phases of printed board design and process design. This

section reviews basic connector selection criteria by con-

sidering requirements for performance, packaging and pro-

cessing.

15.1 Performance Considerations

The required perfor-

mance of an electrical connector can be sub-divided into

mechanical, electrical and environmental categories.

15.1.1 Mechanical

These factors are important because

they affect electrical and environmental performance.

Mechanical aspects of a connector include normal force,

insertion/extraction forces, wipe length, and shock and

vibration.

“Normal force” is the force a contact beam exerts on a

mating half in a direction normal to the surface of the mat-

ing half. In the case of multi-beam systems, a normal force

is not additive. For example, if a contact has four beams

each exerting

100

gms. normal force, the normal force for

that contact is designated as

100

gms.-not

400

gms. Nor-

mal force must be sufficient to maintain contact pressure to

maintain electrical requirements through environmental

exposures.

Normal force affects insertiodextraction forces. Therefore,

a proper balance must be considered. For example, as the

number of I/Os increases on a connector, the total mating

force could increase to the point where special equipment

would be required to mate. An alternative would be to

select a connection containing terminals that have lower

insertion/extraction forces. This would reduce the total

mating force for the same number of I/Os.

Wipe length is an important connector requirement because

connectors are often bowed. Therefore, as connector length

increases, the possibility of not mating at the ends of the

connector increases. Adequate wipe length eliminates this

problem.

The shock and vibration performance of a connector sys-

tem is significantly dependent on the supporting structure

or framework of the chassis or cabinet where utilized. Con-

nectors are available with a variety of hold-downs for

applications where high vibration loading is anticipated. In

applications where extreme density is required, the normal

force of the contact may be the only means of retaining the

component or mating half of a connector. Hence, rudimen-

tary analysis in these situations may dictate the selection of

an appropriate connector. MIL-STD-202 is the predomi-

nant document in describing vibration tests for connectors.

15.1.2 ElectricallElectronic Considerations

Contact

resistance, capacitance and inductance (impedance) are sig-

nificant variables in the selection of connectors in high

speed applications. Power rating coupled with millivolt

drop and temperature rise carry equal significance in power

oriented applications. Special note is given to published

power ratings since test conditions significantly affect

actual performance. Dominant variables include the pres-

ence of a housing, housing population density, housing

materials, contact materials, ambient temperatures and air

flow. Recent rulings by the Federal Communications Com-

mission (FCC), pertinent to EMCRFI control, embodied in

Docket 20780 are accepted as a controlling application

document. Shielded and/or filtered connectors provide one

solution for the equipment manufacturer.

15.1.3 Environmental Requirements

Connectors must

retain their performance requirements in their designed

environment. Therefore, connector durability and electrical

performance should be evaluated through environmental

exposures. Data are typically published regarding perfor-

mance in standard corrosion tests (such as nitric acid), in

on-site exposure tests, and in simulated accelerated envi-

ronmental requirements.

15.2 Selection Considerations

Design philosophy in

equipment design often puts severe limitations on connec-

tor selection. Key issues include, system performance,

goals, board design, space allocations, process and safety

considerations.

15.2.1 Polarization

Whenever it is possible to install a

connector on the printed board improperly or install a con-

nector on the wrong board, a polarization feature is some-

times cut into the board to assure proper installation.

Polarization features may be provided in the connectors to

preclude improper installation. (See Figure

15-1.)

15.2.2 Keying

Keying a connector enables the same

basic connector to be used with different signal patterns

without the possibility of mix-up. This feature is an advan-

tage of two-part connector systems and is illustrated in

Figure 15-2.

Licensed by Information Handling Services