IPC-D-859.pdf - 第60页
Where: N = R ρ R = Resistance (ohms) N = Number of squares ρ = Sheet Resistivity (ohms/square) The results should be a value between 0.3 and 10. Enter this value on the worksheet. 5.3.4.3.3 Length, Width, and Power After…
combination with Figures 5–35 and 5–36 defines recom-
mended values. These dimensions apply equally to rectan-
gular and meander configurations.
5.3.4.2 Resistor Loops Closed resistor loops are not
allowed where trimming must be performed. The proper
method for establishing closed loops is shown in Figure
5–37.
Resistor loops are allowed where the trim system has guard
measurement capability, but this is complex and requires
probing. Shorting bonds may be used to close the loop.
5.3.4.3 Resistor Layout Rules
5.3.4.3.1 Thick-Film Resistor Paste Selection
In some
cases, the selection of pastes may be obvious, especially if
there are only a few resistors or if the values are neatly
clustered. In other instances there may be a large number
of resistors that are widely spread and it may be difficult to
find three or fewer pastes that will cover all resistor values.
A good design avoids the use of more than three resistor
pastes per substrate. The numbers in the left hand column
of Figure 5–38 are standard paste values. The brackets
extending from those pastes show the range of resistor val-
ues that may be screened with each paste. Note that they
overlap.
Figure 5–39 contains generalized design/layout guidelines
for screen printing and locating resistors on a thick-film
substrate.
5.3.4.3.2 Number of Squares After making the selection
of the pastes, the next step in resistor design is to determine
the number of squares that are required to form each resis-
tor value from its assigned paste. The number of squares is
simply the value of the resistor (in ohms) divided by the
value of the paste (in ohms/square):
IPC-859-5-28
Figure 5–28 Wire bonding
Protective
Dielectric
Capacitor
Chip
IC
Chip
Transistor
Chip
Intermediate
Bonding Point
for Chip-to-
Chip Bond
Protective
Dielectric
0.32 x 0.32
[0.0125 x 0.0125]
Bonding Land
Semiconductor
Wire Bonding
Passive Component
Wire Bonding
Exit Land
Wire Bonding Land
Edge of
Substrate
Diode
▼
▼
▼
▼
▼
▼
▼
▼
December 1989 IPC-D-859
53
Where: N =
R
ρ
R = Resistance (ohms)
N = Number of squares
ρ = Sheet Resistivity (ohms/square)
The results should be a value between 0.3 and 10. Enter
this value on the worksheet.
5.3.4.3.3 Length, Width, and Power After selecting the
pastes and calculating the number of squares, then the
length and width of the resistor can be determined using
Figure 5–40. Several factors must come together here. The
length and width must each be on grid or must split grids
IPC-859-5-29
Figure 5–29 Clearances
IPC-859-5-30
Figure 5–30 Autobonding substrate reference locations
▼
▼
D
D
▼
▼
▼
▼
• Reference flag may be rotated 180°
• D = 0.06mm [0.0025"]
• Substrate
▼
IPC-D-859 December 1989
54
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
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