IPC-D-859.pdf - 第43页
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 wirin…
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
36
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
37
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
T
▼
▼
▼
▼
B. Reflow ConfigurationA. End Cap Configuration
A B
Reflow
Terminal
Area
End
Cap
▼
▼
W
V
▼
▼
▼
▼
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
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