IPC9850_Surface Mount Equipment Characterization.pdf - 第16页

IPC-9850 Official Proposal May 2001 16 Land sizes are defined by IPC-SM-782. Termination sizes used here are the nominal values (average of the minimum and maximum values) (see Table 3-1). The Cpk is calculated relative …

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IPC-9850
Official Proposal
May 2001
15
evaluated with mathematical equations that quantify total error, i.e. the combined impacts of the X, Y, and θ
deviations. The ‘Termination-to-Land Calculations.xls’ spreadsheet provides an instruction guide and a sample lead-
to-land spreadsheet that supports these equations.
For leaded components, total error is referred to as maximum lead tip error (MLTE) . The word maximum is a part
of the metric name because the metric refers to the tip error experienced by the lead most impacted by the particular
combination of the X, Y, and θ deviations as shown in Figure 3-3. By definition, S
x
refers to the component span
(lead tip to lead tip) in one direction (X) while S
y
refers to the component’s span in an orthogonal direction (Y). For
area array components, total error is referred to as maximum ball error (MBE). Total error is then used to calculate
termination-to-land. Termination-to-land evaluation for the 1608 component is not performed, since the end-cap to
land proportions do not necessarily provide meaningful results. In this standard, the widely embraced IPC/EIA J-
STD-001 and IPC-A-610 standards are utilized.
Figure 3-3
Total error (for leaded and area array components) is then used to calculate termination-to-land, based on the
nominal component dimensions and the minimum land dimensions (IPC-SM-782). For leaded devices, termination-
to-land is based on the width of the lead. For area array components, termination-to-land is based on the percentage
of the land’s area that is in contact with the ball or column based on a two-dimensional mathematical model (not the
percentage of the ball that is on land since the land typically is designed to have a smaller surface than the ball). The
mean and standard deviation of these values for a group of placements are computed. The mean and standard
deviation are used to attain the Cpk’s values.
For this standard, Cpk for termination-to-land coverage is the parameter that quantifies the placement machine’s
capability of placing the component terminations on the associated lands. That is lead-to-land (LTL), ball-to-land
(BTL), depending upon the component type, in terms of the amount of termination that ends up on the land as a
result of the machine’s placement.
IPC-9850
Official Proposal
May 2001
16
Land sizes are defined by IPC-SM-782. Termination sizes used here are the nominal values (average of the
minimum and maximum values) (see Table 3-1). The Cpk is calculated relative to the 50% and 75% coverage called
for in the assembly standards.
Table 3-1 NEEDS TITLE
Component
Type
Component
Length x
Width
Termination
Width
Land
Length x
Width
Spec Limits for
Total Error
(Class 1,2)
Spec Limits for
Total Error
(Class 3)
SOIC-16 8.89 x 6.0 0.42 NA x 0.60 0.300
1
0.195
1
QFP-100 16.0 x 16.0 0.20 NA x 0.30 0.150
2
0.100
2
QFP-208 32.0 x 32.0 0.20 NA x 0.30 0.150
2
0.100
2
BGA-228 15.0 x 15.0 0.50 (Dia.) 0.45 (Dia.) 0.207
3
0.114
3
Notes
1. Requiring Total Error (for leaded this is MLTE) to be ≤ 0.195 for this SOIC is equivalent to requiring Lead-to-
Land to be ≥ 75%. The Total Error limit of 0.3 corresponds to LTL ≥ 50%.
2. Requiring Total Error (for leaded this is MLTE) to be ≤ 0.100 for this QFP is equivalent to requiring Lead-to-
Land to be ≥ 75%. The Total Error limit of 0.15 corresponds to LTL ≥ 50%.
3. Requiring Total Error (for area array this is MBE) to be ≤ 0.114 is equivalent to requiring Lead-to-Land to be ≥
75%. The Total Error limit of 0.207 corresponds to LTL ≥ 50%.
Calculation Method Calculation methods are provided for three component types: two leaded, one area array.
Termination-to-land calculations are based on IPC-SM-782 designed termination and designed land dimensions,
rather than actual ones. At time of publication of this standard, there is no standard for ball to land ratios for area
array components. A 0.5 mm diameter ball with a 0.45 mm land for the BGA228 was selected for calculation of this
metric.
Lead-to-land (LTL): is based on the percentage of the lead’s width that is placed on land. This measurement
quantifies the error for a component’s lead that experiences the most offset (of all the leads) due to the joint x, y, and
θ errors.
Calculation for a component with leads on four sides is shown here:
LTL=
⋅−
−
−
%100100
2
LeadWidth
LeadWidthLandWidth
MLTE
, where MLTE is defined below:
For a rectangular or square component with leads on four sides:
Maximum Lead Tip Error (MLTE) =
⋅+
⋅+
)sin(
2
)sin(
2
max
dev
S
xdev
dev
S
ydev
y
x
θ
θ
For a rectangular or square component with leads on its two longest sides:
If the component is placed so that the long sides are along the board’s X-axis,
Maximum Lead Tip Error (MLTE) = )evsin(
2
),max(
d
SS
xdev
yx
θ
⋅+
IPC-9850
Official Proposal
May 2001
17
If the component is placed so that the long sides are along the board’s Y-axis,
Maximum Lead Tip Error (MLTE) = )evsin(
2
),max(
d
SS
ydev
yx
θ
⋅+
Ball-to-Land (BTL) refers to the percentage of the BGA ball or column of an area array components that is placed
on its (round) land. BTL quantifies the error for the “ball” that experiences the most offset (of all the balls) due to
the joint x, y, and θ errors.
BTL= 100
R
A
1
2
1
•
−
π
, where
( ) ( ) ( )
+−−−
∆−
+∆−−
∆−
−
−−−−
2
1
2
2
22
2
1
2
2
1
1
2
1
2
2
1
1
2
1
R
c
sin
2
R
cR
2
c
1sin
2
R
2
R
rc
sin
2
R
rcR
2
rc
1Sin
2
R
2 =A
where:
∆X = |Xdev| + |
2
x
S
- (
2
x
S
· cos (θdev) -
2
y
S
· sin (θdev)) |
∆Y = |Ydev| + |
2
y
S
- (
2
y
S
· cos (θdev) +
2
x
S
· sin (θdev)) |
22
YX= ∆+∆∆r
c =
r
RrR
2
B
22
L
∆
−∆+
R
L
= radius of the land
R
B
= radius of the ball
R
1
= min(Radius of Ball, Radius of Pad)
R
2
= max(Radius of Ball, Radius of Pad)
Xdev = Offset of component along the X-axis from the target placement
Ydev = Offset of component along the Y-axis from the target placement
θdev = Rotational offset of component from the target placement
∆X = Maximum offset along the X-axis of the ball from its target placement
∆Y = Maximum offset along the Y-axis of the ball from the target placement
∆r = Radial offset of ball center to pad center. Synonymous with MBE in this document.
Computational note: the values of
Sin
1−
(x) need to be expressed in radians (rather than degrees).
Example of Computing Cpk limits for Termination-to-Land Coverage: After the x,y, and theta errors have been
determined by the CMM (for a four board run using one particular component type), and the individual components’
Termination-to-Land computations have been made as well (using the equations above), the next step is to calculate
the mean and standard deviation of this group of Termination-to-Land values. The Cpk equation should then be
applied, using first the 50% specification limit and then the 75% limit. These 2 Cpk values then go in the
Performance Reporting Form. Suppose the mean of the 144 Termination-to-Land values in a QFP100 run is 85%
and the standard deviation of these values is 5%. Then Cpk versus 50% requirement is 33.2
5
3
5085
=
•
−
.