IPC-CM-770D-1996.pdf - 第33页

IPC-CM-770

Januaty

1996

be used. A typical method of mounting a component of this

type is shown in Figure 6-9.

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-00185

Figure 6-9 Horizontal Mounting of Radial Leaded

Component

6.2.5 Metal Power Packages

Metal power packages

shown in Figure 6-10 should be mounted in accordance to

the following:

Lead holes may be plated through but should not be inter-

facial or interlayer if the component body is mounted in

contact with the board or circuitry thereon.

Component body should be spaced a minimum of 0.25

mm above the board surface if lead holes are interfacial

or interlayer (to permit solder flow to and onto the com-

ponent land).

Leads may be tempered or exceed 1.25 mm in diameter

provided they are not clinched against the printed board

land.

A washer should be inserted between each screw head

and the land (to preclude damage to the land).

A washer should be inserted between each screw head

and the board material (to preclude board damage).

Nut should be lock type or should be retained by locking

devices.

The heat sink or device mounting flange may be provided

with threads to match mounting screw for solder lug.

Resilient material and/or insulating material used as

required functionally or as standoffs.

Torque specification.

IPC-1-00049

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Figure 6-1

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Metal Power-package Transistor Mounted on

Resilient Standoffs

6.3 Surface Mounting

Radial-leaded components are

primarily suited for use in through-hole mounting technol-

ogy. However, when it is necessary to surface mount such

components, the guidelines of Section 22.1 should be fol-

lowed.

6.4 Mixed Technology

The assembly of surface mounted

devices in combination with radial-leaded through-hole

mounted devices requires two assembly processes if sur-

face mounted devices are positioned on the same printed

boards through which through-hole devices are placed. A

combination of reflow and wave solder assembly is

employed. See Section 25 for more information on mixed

assembly technology.

6.5 Manual Assembly

Procedures to be employed follow

practices to reduce contamination and damage to the com-

ponents. Identification marking is desirable on printed wir-

ing to assure proper placement of components during

assembly. Manual assembly practices as covered in Part

Five should be followed.

6.6 Automated Assembly

General information on auto-

mated techniques are covered in Part Five. Attention

should be given to control insertion forces on preformed

leads

so

as to not compromise clearance of components

from printed board surface to assure sufficient clearance for

cleaning after assembly.

6.7 Handling and Storage

The handling and storage of

radial-leaded components should be in accordance with the

guidelines of Section 26.

6.8 Soldering

General soldering techniques applying to

all types of components are described in Section 27.

6.9 Cleaning

General techniques for cleaning applying

to all types of components are described in Section 28.

6.1

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Conformal Coating

General techniques for confor-

mal coating for all types of assemblies are described in

Section 29.

7.0 CHIP (TWO-TERMINAL) COMPONENTS

Almost every kind of two (and many three and four) termi-

nal device is available in chip component form. The most

common are chip resistors, chip capacitors and diodes but

linear magnetic devices (chokes and transformers), crystals

and even more complex devices are made in chip form.

7.1 Part Description

Chip component sizes range from

chip diodes on the order of 0.6 mm x 0.6 mm to large

capacitors that are over 9.0 mm x 2.5. Most other chip

components can be solder assembled to printed board

structures with assembly technologies similar to those

described for through-hole mounted components.

2-8

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

Pratectlve Thlck

Fllm

Glass

film

Reslstance Element TerminatIan Termlnatlon

Land

Edge

\

Alumna

Hlgh Purity Solderable

Substrate

Coatlng Nlckel Barrler

IPC-1-00074

Figure 7-1 Typical Rectangular Chip Resistor

Capacitor

Ribbon

Leaded Capacitor

Termination

IPC-1-00189

Figure 7-2 Common Configurations of Rectangular

Capacitors

The most common package shape for resistors, capacitors,

and many other types of component is the rectangular chip.

It is rectangular in profile and in cross section and it has

metallized terminations on each end. Figure 7-1 shows a

typical rectangular chip resistor and Figure 7-2 shows sev-

eral common configurations of rectangular capacitor. Elec-

tronic Industries Association standard RS-198 covers

ceramic dielectric chip capacitors and RS-228 covers fixed

electrolytic tantalum chip capacitors.

Some components, especially diodes and high power resis-

tors, are available in tubular, or cylindrical metal electrode

face form (commonly called MELF). Other devices have

the terminations arrayed on the bottom of the device rather

than on the ends or may have pads located on the top sur-

face of the component for wire bonding to lands on the

mounting substrate. Special care is required to assemble

most of these custom components.

The end terminations typically have a solder plate or sol-

der dip finish but some have gold plate or silver-palladium

finish. For solder assembly tin or tin alloy finish is pre-

ferred with a copper or nickel barrier to protect the internal

contact from silver leaching. The end terminations may

exhibit any of the configurations shown in Figure 7-3. The

“five-faced’’ termination is preferred. Single-faced termi-

nations exhibit significantly poorer assembly yields than

either the three- or five-faced configurations.

5

face

3 face

Cylindrical

Chips

1

face

bottom Dad

Rectangular

Chips

IPC-1-00190

Figure 7-3 Cylindricallrectangular Terminations

7.1.1 Chip Inductors

Chip inductors are available in

either ceramic or ferrite variations. Typical wire used is

polyurethanelnylon insulated copper magnet wire, ranging

in size from 38 AWG to 48 AWG. The terminations are

usually molymanganese or nickel, coated with 95% tin,

5%

silver solder. Typical outlines are shown in Figure 7-4 and

7-5.

7.1.2 Switches

Switches in surface mount configura-

tions are now becoming available. DIP, slide, pushbutton,

rotary DIP and toggle switches withstand the temperatures

of batch or mass soldering as well as immersion cleaning.

SMD switches usually have

“I”

lead,

“J”

lead or

“L”

lead.

To ensure solder connection reliability, the lead configura-

tion must comply with thermal expansions while providing

a mechanically sound solder joint during switch actuation.

2-9

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

Figure 7-4

A

Chip Inductor

IPC-I-

I

Figure 7-5 Typical Surface Mount Inductor

7.1.3 Other Devices

Other passive devices are being

adapted to surface mounting applications. One noteworthy

example is a surface mount sealed single-turn cermet trim-

mer in a vertical and horizontal style. This particular prod-

uct is available in a 1K to 200K ohms resistance range.

Figure 7-6 shows an outline of this trimmer.

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Figure 7-6 Surface Mount Cermet Trimmer

7.2 Through-Hole Mounting

Chip components are nor-

mally leadless or have ribbon leads that are not appropriate

for through-hole assembly.

7.3 Surface Mounting

7.3.1 Component Preparation

When surface mounting,

good solderability of chip component terminations is

essential for high assembly yield. The solderability of the

end terminations should be tested on all lots of new com-

ponents upon receipt and after any prolonged (more than

one month) storage.

The preferred termination may be reflowed or dipped only

if excess solder is removed from the end terminations. Bare

silver-palladium terminations should be avoided because

such terminations tend to lose solderability when exposed

to a solder bath or any other large volume of solder.

Component terminations should have a diffusion barrier

layer (typically nickel or copper) under the solder to pre-

vent the leaching of silver from the underlying silver-

palladium termination.

No

silver should be detectable on

the surface of the component.

Conductors may be connected to a land at any portion of

the land perimeter but vias should not be located on or in

contact with the land. However, when reflow soldering is

used, conductors should be covered with soldermask to

minimize scavenging of solder away from the component

termination. This is especially important when conductors

connect to plated-through-holes near the component termi-

nation land.

7.3.2 Land Patterns

Land patterns for mounting chip

components should be designed for high assembly yield in

order to maintain product performance and high relia- bil-

ity of the finished product. Various land shapes are used to

solder attach chip components, the most common being

rectangular, circular or diamond shaped lands. Some vari-

ants such as ovals, semi- circles or rectangles with rounded

comers are also used. Figure 7-7 shows some typical land

configura- tions. (See PC-SM-782 for more details.)

Recommended

Alternative

IPC-1-00191

Figure 7-7 Chip Component and Lands

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services