IPC-CM-770D-1996 - 第103页

IPC-CM-770

Januaty

1996

conventional processes. Graphite can be used where ther-

mal conductivity per unit of weight is important.

19.4.3 Printed Boards with Electrically-functional Con-

straining Cores

More conventional multilayer printed

boards can be made as printed board structures with thin,

0.1

to

0.25

mm, copper-clad Invar as electrically-functional

ground and power planes. After the planes have been pre-

drilled they are located in a symmetrical arrangement

within the lay-up and subsequently laminated as an integral

part of the multi- layer printed board structure (Figure

19-6).

The overall CTE of the structure can be tailored by

varying the composition and thickness of the planes.

IPC-I-

Figure 19-6 Multilayer Printed Board Structure with

Copper-clad Invar Power and Ground Planes (Electrically-

functional Constraining Cores)

19.4.4 Printed Board With Constraining Core

A con-

straining fiber resin composite internal plane in a conven-

tional printed board can modify thermal expansion in the

X

and

Y

axes, improve rigidity and improve thermal conduc-

tivity, depending on the properties of the supporting plane

and its location within the printed board structure. These

constraining fibers can be graphite, Kevlar, quartz, etc. The

very high modulus of these materials requires a balanced

construction to prevent bowing or twisting (see Figures

19-7

and

19-8

for typical construction). Graphite is expen-

sive, but its cost is justified if low weight is critical. Graph-

ite is conductive; therefore via holes must be drilled over-

size and then resin filled prior to final via hole drilling.

Graphite allows excellent CTE tailoring. Kevlar and quartz

fibers require modified fabrication techniques due to their

mechanical properties.

e

Graphite

-

I

",",Ed

1

\Mounting hole

Thermal vias

IPC-I-

Figure 19-7 Balanced Structure with Constraining Core

not at Neutral Axis

L

material

-

-Adhesive

giz:d \Mounting hole

Thermal vias

IPC-I-

Figure 19-8 Balanced Structure with Constraining Core

on Neutral Axis

4-34

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

Part Five

Assembly Processing Techniques

20.0 COMPONENT MOUNTING

This Section is a general introduction to Sections 21

through 24, providing general process information.

20.1 General Considerations

When first considering

assembly options some basic decisions have to be made.

Decisions such as whether to use solder or adhesives for

component attachments, whether to surface mount, through

board mount or use a mixture of both, whether even newer

techniques such as chip on board should be considered?

Selections of placement methods and equipment and

choices of solder material form lead to questions about

material application methods and equipment.

20.2 Assembly Processes

20.2.1 Printed Board Assembly Techniques

The selec-

tion of assembly techniques depends upon four major

areas:

1)

Components

4

types

surface mounted

through hole

special considerations (heat sensitive, unsealed,

etc.)

b) number of components to be mounted

c) component securing requirements

2) Printed Board Structure

a) types of assemblies

single sided (type

1)

double sided multilayer (type 2)

b) number of assemblies to be manufactured

3)

Assembly Processes and Joining Techniques

4) Sequence of Events in Assembly

The type of component used is normally connected to the

type of assembly used. For example, single sided (type

1)

assemblies often use through-hole components only,

whereas double sided/multilayer (type 2) assemblies often

use surface mounted or intermixed components. In the

former case, the through-hole components are frequently

wave soldered. In the later case, the surface mounted com-

ponents may be reflow soldered, or those surface mounted

components on the underside of the board may be attached

with adhesives and then wave soldered.

If the printed board structure is complicated, or only a

small number of assemblies are to be made, then manual

assembly techniques are often used. However, if the printed

board structure is fairly straightforward and simple, or the

number of assemblies to be made is large, then the set-up

time and monetary investment for automated component

mounting and assembly may be worthwhile.

In either case, component mounting on double sided or

multilayer printed boards is more complicated than for

single sided boards because the former use plated-through

holes for the through-board components. These plated-

through holes require greater tolerances because plating

builds up in the holes. This may restrict component

mounting.

The joining techniques used may also influence the assem-

bly process. Although this guideline deals primarily with

component mounting, not the joining process, the two can-

not be separated in intermixed assemblies. In some sequen-

tial manufacturing operations, certain parts must be secured

or permanently attached before other components are

mounted.

The assembly process itself often influences component

placement. For example, singular (one at a time) or mul-

tiple (several at a time) component placement will affect

toolhead clearances for automatic placement equipment, set

up procedures, and other manufacturing steps.

Finally, the entire sequence of events in the assembly/

joining process will affect component placement. Previ-

ously mounted components must not interfere with other

components mounted in a second step, and secondary join-

ing techniques such as soldering must not damage compo-

nents previously placed and joined.

In some cases, problems in assembly cannot be solved sim-

ply by selecting a different assembly/joining procedure.

Perhaps the problem cannot be "solved" at all, but must be

prevented through careful design. To prevent problems and

create a board which will be manufacturable, the designer

of printed board assemblies must take into account all of

the fabrication assembly steps necessary to complete the

electronic assembly.

20.3 Process

Flow

20.3.1 Introduction

Assembly processes for production

of printed board assemblies differ according to the type of

product (e.g., through board, surface mount, mixed tech-

nology or chip on board), and within types according to

company expertise, experience and preference. Table 20-1

compares some possible process flow sequences for each of

the above types of product.

5-1

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

The manufacturing engineer usually establishes the flow

sequence for a particular line

so

that it can be used as a

planning aid, an equipment requirement guide, a manpower

requirement guide, and a list of the necessary process steps.

However, it is important that comparative process options

such as shown in Table

20-1

be available at the design

stage if the full benefits of each process sequency are to be

realized. For example, designers should be aware that the

assembly process for a double-sided or a mixed technology

PWB

will be more complex than for a single-sided

PWB.

Such a comparison also allows equipment cost and space

comparisons, and pinpoints necessary material changes.

Other decisions such as the choice of individual machine or

equipment processes may be based on component tempera-

ture sensitivity, cleanliness requirements, component mark-

ing stability, and component bond robustness. Ultimately,

all considerations will be combined in selecting an assem-

bly process.

20.4 Materials

20.4.1 Solders

Solders are generally metal alloys with

melting points in the range -150°C to -400°C. Below this

temperature range, alloys are commonly called fusible

alloys: above this they are called brazes. Tin-lead alloys are

most common, although more complex compositions have

been developed for special applications.

For electrical soldering, alloys near the eutectic composi-

tion (63% tin/37% lead) have the required combination of

properties. Although compositions either side of the eutec-

tic have higher liquidus (completely melted) temperatures

desirable for higher ambient temperature applications,

remember that the initial melting point (at which solder

softens) is 183"C, the same for all tin-lead alloys with

compositions between

20%

and

98%

tin. There is no ben-

efit of service temperature unless the tin content is less than

20%

or greater than

98%.

The range of properties of tin-lead alloys can be varied by

adding other metals such as bismuth or indium to lower the

melting point, or antimony, silver, etc. to increase hardness

and fatigue resistance. Alloys containing less than

10%

tin

are used for applications involving temperatures below

-40°C. As mentioned earlier, the choice of the soldering

flux depends primarily on the solderability of the base

material.

20.4.2 Part Type Description

Sections 4 through

19

pro-

vide guidelines for the specific component dealt with in

each section. The parts are described in detail and each

section outlines specifics affecting the part class. Descrip-

tions and classifications provided are those generally used

in the industry with reference to military and commercial

adaptations.

20.4.2.1 Lead Forming

When component leads require

forming, the leads should be formed with a bending tool.

The component leads should be formed to the final con-

figuration before assembly or installation (except for the

final crimp, where required). When being bent, welded

leads should be firmly held by a suitable tool on the side

of the weld away from the component body. Lead forming

should not damage the lead due to nicking. Energy from

the bending action detrimental to the part should not be

transmitted into the component body.

Lead forming tooling and forming tolerances have signifi-

cant impacts on maintaining functional quality of compo-

nents. Considerations must be given to lead material and

hardness when designing tooling. Component body materi-

als (glass, elastomers, metal case, plastic) react differently

to forming strains. Stresses from gripping and close bend-

ing may damage protective cases.

Nicks and cuts and section reduction to leads during grip-

ping and forming can provide failure sites and change elec-

trical characteristics. Care should be exercised in tool

design and materials. Following specific guidelines on

closeness of bend and minimum bend radii is necessary.

20.4.2.2 Component Securing

Forming of leads for

through-hole mounting components serves many purposes.

These include the desire to retain the component in the

substrate during subsequent handling prior to soldering or

provide a standoff. Such bends or loops are provided by

tooling with some forming machines and/or can be intro-

duced by hand forming. Care must be exercised such that

stresses are not introduced to the component, leads or sol-

der joint during solidification.

The shock and vibration to which printed board compo-

nents are subjected during normal handling, environmental

testing and use can damage the lead terminations and lead-

to-component body seals. For this reason, many compo-

nents should be mechanically secured to the mounting

base.

The more commonly used component securing methods

are: clips, clamps, and brackets; wire and elastic straps;

adhesives; and integral mounting provisions.

Most circuit malfunctions in a severe vibration environ-

ment are caused by cracked solder joints, cracked seals, or

broken electrical lead wires. These failures are usually due

to dynamic stresses that develop because of relative motion

between the electronic components and the board. This

relative motion is generally most severe during resonant

conditions.

Since shock and vibration vary widely with the specific

application, it is not possible to provide solutions to all

component mounting problems. The purpose is to suggest

some general guidelines which if observed will provide

reasonable assurance that the components and assemblies

will survive shock and vibration within their intended use.

5-2

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services