IPC-CM-770D-1996 - 第102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…

Januaw

1996

IPC-CM-770

Second

level Top clamp plate

heat

sink

second

level

Clamping screw

Edge

connector

Thermal

grease

Buried via

Spacar

post

IPC-I-

Figure 19-4 Flexible Printed Board with Metal Support Plane

core, (Figure

19-5).

The holes in the core are filled with a Molybdenum can be used as the core in these printed board

compatible resin prior to lamination and the printed board structures for special applications that require inherent

structure completed with conventional fabrication stiffness in extreme environments, but molybdenum and

techniques.

copper-clad Invar are difficult materials to fabricate using

Dielectric

Circuit lines

Supportingare

IPC-I-

Figure 19-5 Printed Board with Supporting Plane (Not Electrically-functional Constraining Core)

4-33

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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

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

and

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.

Graphite

",",Ed

\Mounting hole

Thermal vias

IPC-I-

Figure 19-7 Balanced Structure with Constraining Core

not at Neutral Axis

material

-Adhesive

giz:d \Mounting hole

Thermal vias

IPC-I-

Figure 19-8 Balanced Structure with Constraining Core

on Neutral Axis

4-34

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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:

Components

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

double sided multilayer (type 2)

b) number of assemblies to be manufactured

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

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

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