IPC-D-859.pdf - 第62页
For example, suppose one must design a 39 Kohm resistor using 10 Kohm/square paste. It must dissipate 500 mW . ( 1 ) A = P 0.0775 = 0.500 0.0775 = 6.45 mm 2 A = P 50 = 0.500 50 = 0.01 m 2 ( 2 ) N = R ρ = 39,000 10,000 = …
to permit digitizing without dimensioning. Each has mini-
mum permitted values. The total resistor area must be large
enough to dissipate the specified power. The designer must
also be aware of and take into consideration variations that
may occur between materials from different suppliers.
The design aid in Figure 5–40 is a simple tool for handling
all of these factors. The left hand column lists resistor
length in half-grid increments. The top row lists widths in
half-grid increments. All other entries in the table are num-
bers of squares:
N =
L
W
Diagonal bands are shown in Figure 5–40. Each of these
bands corresponds to a power dissipation range. The out-
side borders show the power dissipation ranges.
To use these data, simply find a square entry that is close
to the calculated value and which is also within a power
band thatmeets specifications. If there is not a square entry
that is exactly equal to the calculated valued, then select
one that is slightly lower. This will yield a slightly lower
value resistor that can be trimmed to the proper value.
For example, suppose one needs 0.7 squares (a 7-ohm
resistor from 10 ohm/square paste) that must dissipate
160mW. Look in the 150 to 200 mW band. A 0.714 square
resistor might beused. It is 1.27 x 1.78 mm [0.050 x 0.070
inch]. Or a 0.666 square resistor could be used.
The latter would be a better choice for two reasons. First,
0.714 is slightly over the required 0.7 squares. It might be
close enough but it may well yield a resistor that is too
high in value even before trimming. Remember that a resis-
tor can be trimmed only to a higher value. Second, the
0.714 square resistor is right on the 150 mW boundary. It
might be marginal for a 160 mW power dissipation.
In fact, if there is room on the substrate, it would be a good
design practice to use the 0.687 square resistor (1.40 x 2.03
mm [0.055 x 0.080 inch]. This would provide at least a 40
mW safety margin.
5.3.4.3.4 Resistor Design Equations Occasionally there
will be a case where the use of the chart in Figure 5–40 is
not sufficiently precise for the required values. In that case
one will have to calculate the values using equations.
Start by calculating the number of squares and minimum
area required. From these, one will have to calculate the
minimum width and then the minimum length.
The equations are as follows:
(1) A =
P
0.0775 [50]
(2) N =
R
ρ
(3) W =
√
A
N
(4) W = NW
where: A = Minimum area square millimeters
[square inches]
P = Maximum power dissipation (watts)
N = Number of squares
R = Resistor value (ohms)
ρ = Resistor paste value (ohms/square)
W = Width mm [inches]
L = Length mm [inches]
IPC-859-5-31
Figure 5–31 Minimum distance between wedge bond and
component as a function of component height.
IPC-859-5-32
Figure 5–32 Preferred thick-film resistor configurations
W
R
R
R
R
W
L
Rectangular
Top Hat
(For Registration Tap)
0.51 [0.020]
0.32 [0.0125]
1.02 [0.040]
0.19 [0.0075]
0.32
[0.0125]
Overlap
Overlap
0.32
[0.0125]
0.51
[0.020]
L
H
L
December 1989 IPC-D-859
55
For example, suppose one must design a 39 Kohm resistor
using 10 Kohm/square paste. It must dissipate 500 mW.
(1) A =
P
0.0775
=
0.500
0.0775
= 6.45 mm
2
A =
P
50
=
0.500
50
= 0.01 m
2
(2) N =
R
ρ
=
39,000
10,000
= 3.9
(3) W =
√
A
N
=
√
6.45
3.9
= 1.29 mm
=
√
[0.01]
3.9
=[0.051 inch]
(4) L = NW = 3.9 x 1.29 = 5.05mm
= 3.9 x [0.051]=[0.199 inch]
The resistor example would have to be dimensioned for
digitizing, would be screened at the exact value (hopefully
not higher) and would run at maximum dissipation. This is
not good design practice. If the design permits, one could
arbitrarily boost the area from 6.45 mm
2
[0.01 inch
2
]to
9.68 mm
2
[0.015 inch
2
] so that the resistor would run
cooler. Then one could recalculate the width and round it
off to the nearest grid or split grid. Finally, it is necessary
to recalculate the length and round it down to the next
lower grid or split grid.
IPC-859-5-33
Figure 5–33 Three methods showing the use of resistors with multilayer thick-film designs (see Figure 5–34 for window
allowance).
IPC-D-859 December 1989
56
5.3.4.3.5 Screening Resistors may be screened directly
onto the substrate or on any equi-planar level. They must
never be designed into a dielectric ‘‘hole’’ or terminated by
different layers of conductors. Resistors should not be
screened on screened dielectric directly over underlying
conductors.
5.3.4.3.6 Piggy Back/Daughter Substrates If space or
paste limitations make it necessary, resistors may be
screened onto a small piggy-back or daughter board sub-
strate. The small substrate is then bonded to the main sub-
strate. Connections between the two substrates are made
with wire, ribbon, or solder bonds.
5.3.4.3.7 Probe Lands Probe lands must be available for
measurements during trimming. Probe ‘‘shadow’’ must
always be a consideration. Normally the run to the resistor
terminal is adequate but if this run is covered by dielectric
than a separate land at least 0.38 x 0.38 mm [0.015 x 0.015
inch] must be available.
5.3.4.3.8 Overglaze All resistors screened with 10, 30 or
100 ohm/square paste must be protected with overglaze as
must all conductors leading to them.
5.3.5 Vias Design techniques and precautions to be used
in designing multilayer vias include:
Table 5–4 Dimensional Constraints for Thick-Film Resistors
Resistor Sizes and Spacing
Minimum
Dimension
mm [inch]
Nominal
Dimension
mm [inch]
Maximum
Dimension
mm [inch]
Figure
Item
Resistor height (H), top hat configuration 0.76 [0.030] None 12.7 [0.500] 5–32
Resistor width (W), top hat configuration 1.52 [0.060] 2.03 [0.080] None 5–32
Resistor-to-resistor spacing 0.38 [0.015] 0.51 [0.020] None 5–35
1
Resistor-to-conductor overlap (length) 0.25 [0.010] 0.32 [0.0125] None 5–35
3
Resistor-to-conductor overlap (width) 0.13 [0.005] 0.19 [0.0075] N (1) 5–35
4
Resistor length (ink value ≤100 K ohms/sq) 0.76 [0.030] 1.02 [0.040] N (1) 5–35
4
Resistor length (ink value >100 K ohms/sq) 1.02 [0.040] 1.52 [0.060] N (1) 5–35
3
Resistor width (ink value ≤100 K ohms/sq) 0.76 [0.030] 1.02 [0.040] N (1) 5–35
6
Resistor width (ink value ≤100 K ohms/sq) 1.02 [0.040] 1.52 [0.060] N (1) 5–35
7
Conductor land length (beyond resistor) 0.13 [0.005] 0.25 [0.010] None 5–35
8
Air abrasive trimming allowance (from resistor
to adjacent conductor, resistor, or land)
1.27 [0.050] 0.51 [0.020] None 5–36
9
Air abrasive trimming allowance for top-hat designs 0.76 [0.030] 0.89 [0.035] None 5–36
10
Resistor overglaze overlap (2) 0.13 [0.005] 0.19 [0.0075] None 5–36
11
(1) N=Aspect ratio=L/W 0.2≤Nfor rectangular designs of resistors whose tolerance is <20 percent. N=30 maximum for top-hat
resistor designs after trimming
(2) Overglaze is required for resistors made from inks of a sheet resistivity of less than 32 ohms per square.
IPC-859-5-34
Figure 5–34 Window allowance for thick-film resistors.
Window for
Resistor
▼
R
▼
▼
▼
▼
▼
▼
0.75
[0.030]
0.75
[0.030]
▼
▼
0.75
[0.030]
0.75
[0.030]
0.75
[0.030]
IPC-859-5-35
Figure 5–35 Nominal thick-film rectangular resistor
dimensions (see Table 5–5)
December 1989 IPC-D-859
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