IPC-D-859.pdf - 第42页

as mismatch of Coef f icients of Thermal Expansion, solderability/weldability of the material, and protection of its contacts from corrosion during storage. When ﬂexible cable is used, it may be soldered directly to land…

4.4 Component Lead Sockets

4.4.1 Integrated Circuit Sockets

4.4.1.1 DIP Sockets

Sockets, such as that shown in Fig-

ure 4–16 are designed for low-cost, high-density, mounting

of DIP components. They are generally designed to resist

solder wicking into the socket or cavity, and have provi-

sions that provide the needed clearance of the socket above

the substrate.

4.4.1.2 Chip Carrier Socket Sockets such as that shown

in Figure 4–17 can be used with most of the chip carriers

described in 4.3.1. They can be either surface mounted or

through-hole mounted, depending on the termination con-

ﬁguration of the socket contacts.

When they are surface mounted, they can also be obtained

with a mechanical contact pressure interface to the printed

board. The advantages of this type of socket are the ease of

replacement of the socket, and the absence of damage due

to soldering and unsoldering. A disadvantage is that a sec-

ond mechanical interface is now in the circuit path.

4.5 Connectors and Interconnects One of the critical

design requirements for multilayer hybrid circuits is to pro-

vide a method of interconnection for signal and power

input/outputs.

This is normally done by surface contact to lands on the

outer layer(s), typically with a conductive header or by the

use of ﬂexible circuitry.

When the header is used, it may be soldered, welded or

brazed to the surface lands, with its contacts projecting

beyond the periphery of the substrate. In selecting material

for the header, consideration should be given to such items

IPC-859-4-6

Figure 4–6 Plastic-leaded chip carrier lead misalignment

IPC-859-4-7

Figure 4–7 Round or coined lead

December 1989 IPC-D-859

as mismatch of Coefficients of Thermal Expansion,

solderability/weldability of the material, and protection of

its contacts from corrosion during storage.

When ﬂexible cable is used, it may be soldered directly to

land patterns on the substrate. Proper mechanical support,

using tie-down bars, or adhesives, shall be used to prevent

stresses on the solder joints.

IPC-859-4-8

Figure 4–8 JEDEC 1.27 mm [0.050 inch] chip carrier packages

IPC-859-4-9

Figure 4–9 Common features of JEDEC 1.27 mm [0.050 inch] center chip carriers.

IPC-D-859 December 1989

In either case, if the assembly is to encounter a great deal

of vibration or shock, additional mechanical support should

be provided.

4.6 Special Wiring

4.6.1 Jumper Wires

It may be necessary to add point-

to-point wiring to a hybrid. Such wiring shall beconsidered

as not being part of the hybrid circuit, but as part of the

assembly process. Therefore, their use shall be documented

on the hybrid circuit master drawing.

4.6.1.1 Types Point-to-point (jumper) wires are usually

of the following types:

A. Bare bus wire that consists of a single strand of wire

that is of sufficient cross-section to be compatible with

the electrical requirements of the circuit without the use

of sleeving or other insulation.

B. Sleeved bus wire that consists of a single-strand of bare

bus wire (see A above) that is covered by insulation

tubing

C. Insulated bus wire that consists of a single strand wire

that has its own insulation when purchased, either a

plastic or varnish coating.

D. Insulated stranded wire that consists of multiple strands

of wire that are purchased with a plastic coating

4.6.1.2 Application The use of jumper wires should

adhere to the following rules:

A. Bare bus wires should not be longer than one inch.

B. Bare bus wires should not cross over hybrid conduc-

tors.

C. Bend radii for jumper wires should conform to that of

normal component bend requirements (see 4.1.11).

D. The shortest path of jumper routing should be used

unless hybrid design considerations dictate otherwise.

E. Sleeving should be of sufficient length to ensure that its

slippage at either end of the jumper wire will not result

in a gap between the insulation and solder joint or wire

bend of more than 3.2 mm [0.125 inch]. Also, the

sleeving should not be damaged by the jumper wire or

hybrid circuit soldering operations.

4.6.2 Bus Bars Bus bars are usually preformed compo-

nents in the hybrid circuit assembly that provide most, if

not all, of the power and ground distribution over the cir-

cuit surface. They are used primarily to minimize power

and ground distribution circuitry or to provide power and

ground distribution not provided by the hybrid circuit.

The number of conductor levels in the bus bar, the type and

number of their terminals, the size and ﬁnish of their con-

ductors, and the dielectric strength of the insulation

Table 4–1 Chip Carrier Application Considerations

Consideration Leadless CC’s Leaded CC’s

Thermal expansion to match P/I structure Critical Less critical

Removal and replacement Comparatively easy with special tools.

Less risk of damaging P/I

Solder joint inspection Difficult Less difficult

Flux removal after soldering Difficult Less difficult

Socket compatible Yes (except Type C) Yes (except Type B)

Lead length (CC-land pattern) Minimal Moderate (inductance greater)

Conductive cooling Good, with direct head conduction path

(Lower proﬁle height)

Preparation for soldering Solder coating of terminals required None except for solder coating as required

for solderability

Self centering Usually Rarely

Flexure of P/I structure Critical Less critical

IPC-859-4-10

Figure 4–10 24-I/O, 1.02 mm [0.040 inch] pitch, fan-out

pattern on a custom grid

2.0

[0.080]

December 1989 IPC-D-859