IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第22页
ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ The IPC-7351 Library Documentation spreadsheets included with this standard provide an analysis of tolerance assumptions and resultant solder joints based on the fin- ished land pattern dimensions. T oleranc…
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The equations for determining if the clearance ‘‘N’’ or the
attachment overlap ‘‘M’’ are sufficient is as follows:
M =
[
W + X
2
]
−
√
C
2
+ F
2
+ P
2
N = E-
[
W + X
2
]
+
√
C
2
+ F
2
+ P
2
3.1.5.1 Tolerance and Solder Joint Analysis The fol-
lowing tolerance concepts are used to determine the land
patterns for electronic components. These concepts are
detailed in Table 3-1 and reflect the tolerances on the com-
ponent, the tolerances on the land pattern (on the intercon-
necting substrate), and the accuracy of the equipment used
for placing components.
Solder joint minimums are shown for toe, heel and side fil-
lets. These conditions are minimums, since the equations in
3.1 address the component, board, and placement accuracy
tolerances (sum of the squares). The minimum solder joint
or land protrusion is increased by the amount that the tol-
erance variation does not use up. The courtyard excess is
added to the maximum dimension that the land pattern or
component occupies. The courtyard excess number is
added to each side of the dimension in question. It is
intended that this addition provides sufficient room for
electrical and physical clearance between components
and/or land patterns. Since the total of all the number cal-
culations may not result in a reasonable numerical equiva-
lent, a suggested round-off (up or down) feature has been
added to the tables to identify a rounding up value for the
final number to be used in the design.
IPC-7351-3-04
Figure 3-4 Pitch for Multiple Leaded Components
0.63 Pitch
0.3 - 0.2
▼
▼
Pitch
▼
▼
▼
▼
Leads (W)
Lands (X)
Pitch
▼
▼
E
M
G
M =
W+X
-
C
2
+
F
2
+
P
2
2
Note: Positional tolerance takes angularity into account
N
▼
▼
▼
▼
N = E -
[
W+X
]
-
C
2
+
F
2
+
P
2
2
▼▼
▼
▼
Table 3-1 Tolerance Analysis
Elements for Chip Devices
Tolerance
Element Detailed Description
Component
Tolerance
The difference between the MMC and the
LMC of each component dimension, length,
width and distance between terminations or
leads. This number is the ‘‘C’’ tolerance in
the equations.
Board
Tolerance
The difference between the MMC and the
LMC of each land pattern dimension. This
number is the ‘‘F’’ tolerance in the equations.
Positional
Accuracy
Positional accuracy is defined as diameter
of true position (DTP). This is the variation
of the part centroid related to the land pattern
theoretical center (includes feature location
tolerance from Table 3-17).
Toe Fillet The land protrusion beyond the lead or
termination extremities (see Tables 3-2
through 3-14).
Heel Fillet The land protrusion beyond the internal lead
or termination dimensions (see Tables 3-2
through 3-14).
Side Fillet
Width
The land protrusion to either side of the lead
or termination (see Tables 3-2 through 3-14).
February 2005 IPC-7351
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The IPC-7351 Library Documentation spreadsheets
included with this standard provide an analysis of tolerance
assumptions and resultant solder joints based on the fin-
ished land pattern dimensions. Tolerances for component
dimensions, the land pattern dimensions (fabrication toler-
ances on the interconnecting substrate), and the component
placement equipment accuracy are all taken into consider-
ation. These tolerances are addressed in a statistical mode,
and assume even distribution of the tolerances for compo-
nent, fabrication and placement accuracy.
Individual tolerances for fabrication (‘‘F’’) and component
placement equipment accuracy (‘‘P’’) are assumed to be as
given in spreadsheet documentation. These numbers may
be modified based on user equipment capability or fabrica-
tion criteria. Component tolerance ranges (C
L
,C
S
, and C
W
)
are derived by subtracting minimum from maximum
dimensions given. The user may also modify these num-
bers, based on experience with their suppliers.
The dimensions for minimum solder fillets at the toe, heel,
or side (J
T
,J
H
,J
S
) have been determined based on industry
empirical knowledge and reliability testing. Solder joint
strength is greatly determined by solder volume. An
observable solder fillet is necessary for evidence of proper
wetting. Thus, the values in the library documentation
spreadsheets usually provide for a positive solder fillet.
Nevertheless, if the user of any of the three land pattern
geometry variations desires a more robust process condi-
tion for placement and soldering equipment, individual ele-
ments of the analysis may be changed to new and desired
dimensional conditions. This includes component, board or
placement accuracy spread, as well as minimum solder
joint or land protrusion expectation. In addition, this stan-
dard recognizes the need to have different goals for the
solder fillet or land protrusion conditions.
Tables 3-2 through 3-14 indicate the principles used for the
three goals established by this standard. The tables reflect
maximum (most), median (nominal) and minimum (least)
material conditions for the land protrusions used to develop
land patterns for surface mounting various lead or termina-
tions of components. Unless otherwise indicated, the IPC-
7351 identifies all three goals as Density Levels A, B, or C.
3.1.5.2 Component Dimensions Illustrations of compo-
nent dimensions in each library documentation spreadsheet
are accompanied by a table of figures for each of the dif-
ferent part numbers, as taken from multinational compo-
nent standards organizations. The standards organizations
provide many more dimensions to define the requirements
for manufacturing the specific components in a family
class; only those dimensions that are necessary for land
pattern development are repeated within the library docu-
mentation spreadsheets. At times, the component tolerances
or component gauge requirements do not necessarily reflect
the exact tolerance on a manufacturer’s data sheet. Compo-
nent dimensions are provided according to the concepts of
maximum and least materials condition (MMC and LMC).
Both conditions are presented in the tables within the
library documentation spreadsheets. The component manu-
facturers may not always dimension their components in
accordance with the limits shown in the tables. However,
these limits may be used as criteria for go/no-go accep-
tance of the component. The LMC dimensions of the fig-
ure are those that have been used in the equations
described in 3.1 for determining the recommended land
pattern.
Table 3-2 Flat Ribbon L and Gull-Wing
Leads (greater than 0.625 mm pitch) (unit: mm)
Lead Part
Minimum
(Least)
Density
Level C
Median
(Nominal)
Density
Level B
Maximum
(Most)
Density
Level A
Toe (J
T
) 0.15 0.35 0.55
Round-off factor
Round off to the nearest two place
even decimal, i.e., 1.00, 1.20, 1.40
Heel (J
H
)
1
0.25 0.35 0.45
Round-off factor
Round off to the nearest two place
even decimal, i.e., 1.00, 1.20, 1.40
Side (J
S
) 0.01 0.03 0.05
Round-off factor
Round off to the nearest two place
decimal, i.e., 1.00, 1.05, 1.10, 1.15
Courtyard excess 0.1 0.25 0.5
1. For gullwing components where dimension Smin is less than or equal to
dimension Amax, use the following heel fillet goals:
Density Level C - 0.05 mm
Density Level B - 0.15 mm
Density Level A - 0.25 mm
Note: This does not apply to gullwing components where the lead terminals
have a tolerance T1 that is greater than 0.5 mm.
Table 3-3 Flat Ribbon L and Gull-Wing Leads
(less than or equal to 0.625 mm pitch) (unit: mm)
Lead Part
Minimum
(Least)
Density
Level C
Median
(Nominal)
Density
Level B
Maximum
(Most)
Density
Level A
Toe (J
T
) 0.15 0.35 0.55
Round-off factor
Round off to the nearest two place
even decimal, i.e., 1.00, 1.20, 1.40
Heel (J
H
)
1
0.25 0.35 0.45
Round-off factor
Round off to the nearest one place
decimal, i.e., 1.0, 1.1, 1.2, 1.3
Side (J
S
) -0.04 -0.02 0.01
Round-off factor
Round off to the nearest two place
decimal, i.e., 1.00, 1.05, 1.10, 1.15
Courtyard excess 0.1 0.25 0.5
1. For gullwing components where dimension Smin is less than or equal to
dimension Amax, use the following heel fillet goals:
Density Level C - 0.05 mm
Density Level B - 0.15 mm
Density Level A - 0.25 mm
Note: This does not apply to gullwing components where the lead terminals
have a tolerance T1 that is greater than 0.5 mm.
IPC-7351 February 2005
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Table 3-4 J Leads (unit: mm)
Lead Part
Minimum
(Least)
Density
Level C
Median
(Nominal)
Density
Level B
Maximum
(Most)
Density
Level A
Heel (to find Z
dim) (J
H
)
0.15 0.35 0.55
Round-off factor
Round off to the nearest two place
even decimal, i.e., 1.00, 1.20, 1.40
Toe (to find G dim)
(J
T
)
-0.30 -0.20 -0.10
Round-off factor
Round off to the nearest two place
even decimal, i.e., 1.00, 1.20, 1.40
Side (J
S
) 0.01 0.03 0.05
Round-off factor
Round off to the nearest two place
decimal, i.e., 1.00, 1.05, 1.10, 1.15
Courtyard excess 0.1 0.25 0.5
Table 3-5 Rectangular or Square-End Components
(Capacitors and Resistors) Equal to or Larger
than 1608 (0603) (unit: mm)
Lead Part
Minimum
(Least)
Density
Level C
Median
(Nominal)
Density
Level B
Maximum
(Most)
Density
Level A
Toe (J
T
) 0.15 0.35 0.55
Round-off factor
Round off to the nearest two place
even decimal, i.e., 1.00, 1.20, 1.40
Heel (J
H
) -0.05 -0.05 -0.05
Round-off factor
Round off to the nearest two place
even decimal, i.e., 1.00, 1.20, 1.40
Side (J
s
) -0.05 0.00 0.05
Round-off factor
Round off to the nearest one place
decimal, i.e., 1.0, 1.1, 1.2, 1.3
Courtyard excess 0.1 0.25 0.5
Table 3-6 Rectangular or Square-End
Components (Capacitors and Resistors)
Smaller than 1608 (0603) (unit: mm)
Lead Part
Minimum
(Least)
Density
Level C
Median
(Nominal)
Density
Level B
Maximum
(Most)
Density
Level A
Toe (J
T
) 0.00 0.10 0.20
Round-off factor
Round off to the nearest two place
decimal, i.e., 1.00, 1.05, 1.10, 1.15
Heel (J
H
) -0.05 -0.05 -0.05
Round-off factor
Round off to the nearest two place
decimal, i.e., 1.00, 1.05, 1.10, 1.15
Side (J
S
) 0.00 0.00 0.05
Round-off factor
Round off to the nearest two place
decimal, i.e., 1.00, 1.05, 1.10, 1.15
Courtyard excess 0.1 0.15 0.20
Table 3-7 Cylindrical End Cap
Terminations (MELF) (unit: mm)x
Lead Part
Minimum
(Least)
Density
Level C
Median
(Nominal)
Density
Level B
Maximum
(Most)
Density
Level A
Toe (J
T
) 0.2 0.4 0.6
Round-off factor
Round off to the nearest two place
even decimal, i.e., 1.00, 1.20, 1.40
Heel (J
H
) 0.02 0.1 0.2
Round-off factor
Round off to the nearest two place
even decimal, i.e., 1.00, 1.20, 1.40
Side (J
S
) 0.01 0.05 0.1
Round-off factor
Round off to the nearest one place
decimal, i.e., 1.0, 1.1, 1.2, 1.3
Courtyard excess 0.1 0.25 0.5
Table 3-8 Bottom Only Terminations (unit: mm)
Lead Part
Minimum
(Least)
Density
Level C
Median
(Nominal)
Density
Level B
Maximum
(Most)
Density
Level A
Toe (J
T
) 0.15 0.35 0.55
Round-off factor
Round off to the nearest two place
even decimal, i.e., 1.00, 1.20, 1.40
Heel (J
H
) 0.25 0.35 0.45
Round-off factor
Round off to the nearest one place
decimal, i.e., 1.0, 1.1, 1.2, 1.3
Side (J
S
) 0.01 0.03 0.08
Round-off factor
Round off to the nearest two place
decimal, i.e., 1.00, 1.05, 1.10, 1.15
Courtyard excess 0.1 0.25 0.5
Table 3-9 Leadless Chip Carrier with
Castellated Terminations (unit: mm)
Lead part
Minimum
(Least)
Density
Level C
Median
(Nominal)
Density
Level B
Maximum
(Most)
Density
Level A
Heel (J
H
) (to find
Z dim)
0.45 0.55 0.65
Round-off factor
Round off to the nearest two place
even decimal, i.e., 1.00, 1.20, 1.40
Toe (J
T
) (to find G
dim)
0.05 0.15 0.25
Round-off factor
Round off to the nearest two place
even decimal, i.e., 1.00, 1.20, 1.40
Side (J
S
) -0.15 -0.05 0.05
Round-off factor
Round off to the nearest two place
decimal, i.e., 1.00, 1.05, 1.10, 1.15
Courtyard excess 0.1 0.25 0.5
February 2005 IPC-7351
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