IPC-D-859.pdf - 第44页

depends on the application. However , these parameters should be clearly deﬁned on the procurement document for these parts. For optimum board design ef f iciency , the bus bar terminals should interface with the circuit…

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

depends on the application. However, these parameters

should be clearly deﬁned on the procurement document for

these parts.

For optimum board design efficiency, the bus bar terminals

should interface with the circuit on a uniform termination

pattern.

4.6.3 Flex Cable When ﬂexible cable becomes part of a

hybrid circuit assembly the terminations shall be accom-

plished in the manner that imposes no undue stress on the

cable/assembly interconnection.

IPC-859-4-11

Figure 4–11 Modiﬁed fan-out patterns

IPC-859-4-12

Figure 4–12 Leadless discrete components

▼

B. Reflow ConfigurationA. End Cap Configuration

A B

Reflow

Terminal

Area

End

Cap

▼

IPC-859-4-13

Figure 4–13 Improper mounting of end-cap discrete

components

IPC-859-4-14

Figure 4–14 Chip resistor with deposited element

IPC-D-859 December 1989

Sometimes this interconnection uses pins, where a pin

passes through the substrate and the ﬂex cable to provided

the proper interconnection. At other times, the ﬂex cable

may be surface soldered directly to land patterns on the

hybrid circuit. Proper mechanical support, using tie-down

bars, or adhesives, shall be used to prevent stresses on the

solder joints.

5.0 Hybrid Circuit Design Requirements There are

design and documentation feature requirements that apply

to the basic layout, the artwork master, the hybrid circuit

itself, and the end-item hybrid circuit assembly. All must

be taken into consideration during the design. Therefore, it

is important to understand the relationships they have with

one another as shown in Fig. 5–1.

5.1 Design/Fabrication Sequence

5.1.1 Schematic/Logic Diagram

The initial schematic/

logic diagram designates the electrical functions and inter-

connectivity to be provided by the hybrid circuit and its

assembly. The schematic should deﬁne, when applicable,

critical circuit layout areas, shielding requirements,

grounding and power distribution requirements, the alloca-

tion of test points, and any preassigned input/out connector

locations. Schematic information may be generalized as

hard copy or computer data (manually or automated).

5.1.2 Parts List The initial Parts List should contain the

following:

A complete description of the part, quantities, manufactur-

er’s name, reference designators, and, if required, special

ordering instructions. Sometimes it is desirable to also

include the cost of the items to be used for cost analysis

purposes. The method of how the information is to be for-

matted and what must be included depends upon the stan-

dards established by each user. It is advisable to group

like-items, e.g., resistors, capacitors, integrated circuits,

etc., in some sort of ascending or numerical order. This is

of particular value to someone who must order the materi-

als.

The parts list must also have some sort of drawing number,

depending upon the design facilities system. And, in most

cases, the name of board for which it was prepared.

Included also should be the names of the persons prepar-

ing, checking, and approving this document.

Some means of identifying the revision level must also be

provided.

5.1.2.1 Non-standard Part Information When non- stan-

dard parts are used in the design, the detailed mechanical/

physical description, shall be provided as a minimum. This

information should be referenced to the schematic, so that

the appropriate layout conﬁguration can be determined.

When non-standard parts are used, usually a speciﬁcation

control drawing is required, to ensure that future parts

match the layout concepts.

5.1.3 End-product Requirements The end-product

requirements, as deﬁned in IPC-HM-860, should be known

during the design phase. In addition, the maintenance mis-

sion of the equipment is important information to insure

the correct design approach of component placement and

conductor routing.

5.1.4 Testing Requirements Normally, prior to starting

a design, a testability review meeting should be held. Test-

ability concerns, such as circuit visibility, circuit operation,

and special test requirements and speciﬁcation are dis-

cussed as a part of the test strategy. Any artwork or con-

ﬁguration concepts necessary to test the board, shall be

incorporated into the ﬁnal board layout.

During the design testability review meeting, tooling con-

cepts are established, and determinations are made as to the

most effective tool-cost versus board layout concept condi-

tions.

IPC-859-4-15

Figure 4–15 Metal electrical face (MELF) chip resistor

IPC-859-4-16

Figure 4–16 Dip socket

December 1989 IPC-D-859