IPC-CM-770D-1996 - 第101页

Januaw 1996 IPC-CM-770 Second level Top clamp plate heat sink or second level Clamping screw Edge connector Thermal grease I Buried via Spacar post IPC-I- Figure 19-4 Flexible Printed Board with Metal Support Plane core,…

IPC-CM-770

Januaty

1996

Power

plane

Metal

Supporting Plane

IPC-I-

Figure 19-2 Sequentially-processed Structure with Supporting Plane

Power

distribution

Leadless

ceramic

Wire

encapsulation

Metal support plane

Insulating dielectric Insulated copper wire

IPC-I-

Figure 19-3 Discrete-wire Structure with Low-expansion Metal Support Plane

polyimide flexible printed wiring, as shown in Figure

19-4.

These assemblies can be constructed in multilayer form

while retaining the low-modulus feature that reduces

residual strain at the solder joints. Furthermore, lasers can

drill very fine holes in the thin printed wiring laminate.

These holes can be plated-through or filled with solid cop-

per, as required.

To retain inherent flexibility while dissipating head from

the solder joint, cutouts in the flexible circuit accommodate

pillars from the metal heatsink support plane. Although this

appears to be heavy and cumbersome, if the heatsink base-

plates are made from thin sheets of aluminum, the result-

ing density of the combined circuidheatsink assembly

might actually be less than other constructions.

19.4 Constraining Core Printed Board Structures

with supporting-plane printed board structures, one or more

supporting metallic or non-metallic planes can serve as a

stiffener, heatsink, and/or CTE constraint in constraining

core printed board structures.

19.4.1 Porcelainized-Metal (Metal Core) Structures

integral core of low-expansion metal (e.g., copper-clad

Invar), can reduce the CTE of porcelainized-metal printed

board structures

that it closely matches the CTE of the

ceramic chip carrier. Also, the printed board structure size

is virtually unlimited. However, the low melting point of

the porcelain requires low-firing-temperature conductor,

dielectric and resistor inks.

A number of composite printed board structures use lead-

less components. An integral material with a lower CTE

than that of the printed boards controls the CTE of these

structures.

19.4.2 Printed Board With Constraining (not electrically

functioning) Core

Printed boards bonded back-to-back to

a constraining core can be used for high-density, low-

warpage printed board structures. The core acts as a heat-

sink, but in this case is not electrically functional. For opti-

mum density with this approach, use a multilayer

construction with a centrally-located predrilled, low-CTE

4-32

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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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