IPC-CM-770D-1996.pdf - 第88页

January

1996 IPC-CM-770

Current carrying capacity? (Select wire size. Table 17-1 switching circuits. In addition, because transistorized and

lists the current carrying capacity of small wires.)

integrated equipment requires relatively high currents with

Ease of handling? (Select wire size. Table 17-2 lists the

low voltages, it is essential that DC power distribution lines

breaking strength of typical small wires.)

have a low inductance and resistance with a high capaci-

tance.

17.1.4 Bus Bars

High-speed, solid-state circuits demand

distribution circuits be designed and placed with reference

area

Of

a

conductor.

to minimum noise pickup-a common source of pickup is

greater control of electrical noise. It is important that power Resistance can be reduced by increasing the cross sectional

Table 17-1 Current Carrying Capacity

of

Small Wires’

Rating for PVC

&

Similar

Low

Fusing Current Bare Wire, Rating for TFE

&

Similar High

Conductor Size AWG

Amperes

Temp Insulation. Amperes Temp. Insulation. Amperes

30

20.5 7 4 26

14.4 6 3 28

10.2 4 2

I

24

I

6

I

10

I

29.2

I

22

I

8

I

13

I

41.2

I

20

I

10

I

17

I

58.4

I

18 82.9 24 15

1.

Ratings are based on a single conductor at 30°C ambient temperature in still air. Other factors which affect the current carrying capacity of a conductor are

the length of the wire, the proximity to other wires, the cooling available, and the thickness of the insulation. Because of these variables, the data in this table

should be considered as an approximation only.

Table 17-2 Breaking Strength

of

Small Wires

Conductor Size

1.

Calculated in accordance with ASTM B-25, using an Ultimate Tensile

Strength value of 42,000 psi for annealed copper.

To obtain minimum, effective inductance, conductors must

be placed close together. In this manner, the magnetic

fields around each conductor, when

180

out of phase would

tend to cancel and reduce a possible source of noise.

Twisted pairs of wire have been used to achieve this effect-

however a twisted pair of wires has more inductance than

a laminated bus bar.

Since each flat conductor in a laminated bus bar can be

surrounded with ground shields to further reduce stray

electrical fields, the laminated bus is even more desirable

for power distribution. Moreover, the laminated bus bar

design permits maximum allowable distributed capacitance

between adjacent conductors, thereby reducing and sup-

pressing undesirable voltages, pulses, and noise.

The laminated bus bar is essentially a composite of flat

conductors isolated from each other with a dielectric mate-

rial and laminated into a rigid multiconductor unit. Due to

its physical structure, the laminated bus maintains very

definite electrical characteristics. When used as an integral

part of an electronic system it is a very stable circuit ele-

ment.

The number of conductors or levels, types and number of

terminals, size and finish of the conductor and the dielec-

tric strength required, depends upon the application.

Bus bars can be mounted vertically as illustrated in Figure

17-10, horizontally as in Figure 17-11, and are often

mounted under the DIP components as illustrated in Figure

17-12.

I

IPC-1-00262

Figure 17-10 Vertically Mounted Bus Bars

17.1.5 Test Points

Test points are used for safe, reliable,

temporary access to a part of the printed wiring assembly

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

or circuit. Such assess is required for main- tenance and

adjustment of circuitry. Use of a test point is preferable to

probing the printed board, since probing a printed board

can be a shock hazard, and can cause short

Test points consist of an insulator surrounding a contact for

electrical connection. The electrical contact is equivalent to

a single socket connector.

I

IPC-1-00263

I

Figure 17-11 Horizontally Mounted Bus Bars

Test points are designed for mounting on one side of the

printed board, as illustrated in Figure 17-13 and 17-14.

IPC-1-00274

Figure 17-12 Under-DIP Mounted Bus Bars

17.1.6 Insulating Tubing

Insulating tubing is available

in both heat shrinkable and non-heat shrinkable styles, and

in a variety of materials including polyvinyl chloride and

polytetrafluoroethylene.

Typical specifications for insula-

tion tubing include MIL-1-7444, ASTM D-3295, and MIL-

1-23053. Insulation tubing is used to insulate bare wires

and component leads, to insulate metal case components,

as supplementary insulation over wire bundles, and to insu-

late and protect connection terminations.

17.2 Through-Hole Mounting

17.2.1 Component Preparation

A.

Terminals

Terminals must be clean and suitable for the

subsequent operations. Terminals to be soldered in place or

which will be soldered to subsequently must be verified

solderable prior to installation. Solderability should beveri-

fied in accordance with J-STD-002.

B. Pins

Pins to be used as connectors or for subsequent

wire wrapping should be precious metal plated, clean, and

meet applicable plating thickness and porosity specifica-

tions. There are a number of plating options available to

optimize pin solderability in areas of the pin that should be

soldered to a printed board. These options include solder

coatings, gold flashes, and alternatives such as palladium

nickel with and without gold flashes.

C. Wires

Wires to be soldered should be verified solder-

able in accordance with J-STD-002. Insulated wires should

have the insulation removed using thermal, chemical, or

precision cutting-type insulation strippers. Cutting-type

insulation strippers may be automated or manual, but

should not be operator adjustable.

-The lay of the wire strands should be restored if dis-

turbed by the insulation removal process.

-

Insulation may be removed from many types of magnet

wire by the use of chemical stripping agents in accor-

dance with the stripping agent manufacturer's recom-

mendations. Particular care should be exercised when

handling chemical stripping agents, and stripped wires

should be thoroughly cleaned.

-

Polyurethane or similar heat strippable coatings may be

removed by dipping the insulated wire to the required

depth into a pot containing the appropriate solder alloy

at the temperature recommended by the wire manufac-

turer. The dipping action will have the double effect of

removing the insulation and tinning the wire. Solder

purity should be controlled in accordance with J-STD-

001.

-

Thermal stripping is preferred for AWG 22 and smaller

conductor wire sizes. Fraying should be minimized.

Insulation discoloration resulting from thermal stripping

is permissible, but charring or burning should not be

allowed. The lay of the wire strands should be restored

if disturbed by the insulation removal process.

D. Bus Bars

Bus bars must be clean and suitable for the

subsequent operations. Portions of the bus bars to be sol-

dered should be verified solderable in accordance with

J-STD-002.

E.

Test Points

Test points must be clean and suitable for

the subsequent operations. Portions of the test points to be

soldered should be verified solderable in accordance withJ-

STD-002. The section which will mate with external test

equipment should be gold plated or protected with another

plating system which will provide suitable electrical con-

tact and minimize corrosion.

F. Insulating Tubing

Insulation tubing, when specified by

design or required for insulation purposes, should be

installed prior to soldering.

17.2.2 Land Patterns

A.

Terminals

Terminals are suitable for installation in a

variety of configurations which affect land pattern. Those

installations where the terminals is intended to be soldered

to the printed board require land patterns as follows:

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COPYRIGHT Association Connecting Electronics Industries

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

January

1996

IPC-CM-770

Hardware Mount

Solder Mount

IPC-1-00265

Figure 17-13 One-terminal Printed Board Test Point

IPC-1-00266

Figure 17-1

4

Two-terminal Printed Board Test Point

Single Sided Board, Soldered on Shoulder Side.

The land

pattern should be a minimum of

0.5

mm larger in diam-

eter than the diameter of the terminal shoulder. The clear-

ance between the terminal shank and the hole should be

0.13

mm maximum. See Figure

17-15.

Single Sided Board, Soldered on Flange Side.

The land

pattern should be a minimum of

0.5

mm larger in diam-

eter than the projection of the flared flange. The difference

idiameter between the terminal shank and the hole should

be

0.13

mm maximum. See Figure

17-16.

Double Sided Board without Plated-Through-Hole

Termi-

nals are not recommended for installation where the ter-

minal would act as an interfacial connection.

Double Sided Board with Plated-Through-Hole

The land

pattern on the shoulder side of the board should be

0.5

mm larger in diameter that the solder of the terminal. The

land pattern on the flange side of the board should be

0.5

mm larger in diameter that the projection of the flared

I

IPC-1-00267

Figure 17-15 Terminal Installation, Single-Sided Board,

Soldered on Shoulder Side

flange. The difference in diameter between the hole and

the terminal shank should be between

0.25

and

0.5

mm.

To assure proper filling of the hole with solder, the termi-

nal should be installed with an elliptical swage as illus-

trated in Figure

17-17

and

17-18.

4-21

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services