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Texas Instruments Factors That Influence Side-Wetting Performance on IC Terminals Application notes
Application Report
SCEA038 – November 2007
Factors That Influence Side-Wetting Performance
on IC Terminals
Donald C. Abbott, Bernhard Lange, Douglas W. Romm, and John Tellkamp ................................................
ABSTRACT
A designed experiment evaluated the influence of several variables on appearance and
strength of Pb-free solder joints. Components, with leads finished with
nickel-palladium-gold (NiPdAu), were used from Texas Instruments (TI) and two other
integrated circuit suppliers. Pb-free solder paste used was tin-silver-copper (SnAgCu)
alloy. Variables were printed wiring board (PWB) pad size/stencil aperture (the pad
finish was consistent; electrolysis Ni/immersion Au), reflow atmosphere, reflow
temperature, Pd thickness in the NiPdAu finish, and thermal aging. Height of solder
wetting to component lead sides was measured for both ceramic plate and PWB
soldering. A third response was solder joint strength; a "lead pull" test determined the
maximum force needed to pull the component lead from the PWB.
This paper presents a statistical analysis of the designed experiment. Reflow
atmosphere and pad size/stencil aperture have the greatest contribution to the height of
lead side wetting. Reflow temperature, palladium thickness, and preconditioning had
very little impact on side-wetting height. For lead pull, variance in the data was
relatively small and the factors tested had little impact.
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1
2
3
4
5
6
Contents
Introduction .......................................................................................... 3
Experiment .......................................................................................... 4
Results: TI Versus Competitor 1 ................................................................ 10
Results: TI Versus Competitor 2 ................................................................ 14
Summary/Conclusions ............................................................................ 18
Acknowledgements ............................................................................... 18
List of Figures
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
TI SOIC Package End View ....................................................................... 4
TI SOIC Package Top View ....................................................................... 4
PWB Pad Dimensions ............................................................................. 5
Side Wetting Classification Examples ........................................................... 6
Component Placed Onto Customer Pad ........................................................ 7
Component Placed Onto TID Pad ................................................................ 7
Component Placed Onto IPC ..................................................................... 7
Soldered Device on PWB ......................................................................... 8
Package Body Cut Using Diamond Blade ....................................................... 8
Package Body Removed .......................................................................... 8
Mold Compound Removed ........................................................................ 8
One Set of Leads Bent Up For Pull Test ........................................................ 9
Pull Test Performed Vertical to PWB ............................................................ 9
Comparison of Side-Wetting Performance in CPT Method Versus PWB Soldering ..... 12
Comparison of Side-Wetting Performance in CPT Method Versus PWB Soldering ..... 16
List of Tables
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
2
DOE Input Variables ............................................................................... 4
PWB Pad and Stencil Aperture Size ............................................................. 5
Different Palladium Thicknesses ................................................................. 5
Layout of Designed Experiment .................................................................. 6
ANOVA Results for CPT Side Wetting, TI Versus Competitor 1............................ 10
Average Effects Table for CPT Side Wetting, TI Versus Competitor 1 .................... 10
ANOVA Results for PWB Lead Side Wetting, TI Versus Competitor 1 .................... 11
Average Effects Table for PWB Lead Side Wetting, TI Versus Competitor 1............. 11
ANOVA Results for Lead Pull After PWB Soldering, TI Versus Competitor 1 ............ 13
Average Effects Table for Lead Pull After PWB Soldering, TI Versus Competitor 1 ..... 13
ANOVA Results for CPT Side Wetting, TI Versus Competitor 2............................ 14
Average Effects Table for CPT Side Wetting, TI Versus Competitor 2 .................... 14
ANOVA Results for PWB Lead Side Wetting, TI Versus Competitor 2 .................... 15
Average Effects Table for PWB Lead Side Wetting, TI Versus Competitor 2............. 15
ANOVA Results for Lead Pull After PWB Soldering, TI Versus Competitor 2 ............ 16
Average Effects Table for Lead Pull After PWB Soldering, TI Versus Competitor 2 ..... 17
Factors That Influence Side-Wetting Performance on IC Terminals
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Introduction
1
Introduction
The ceramic plate test (CPT) or surface mount process simulation test (1) has been used in the industry
since the early 1990s to evaluate solderability of component terminals. The CPT simulates the
environment that surface mount devices encounter during solder reflow. In this method, solder paste is
screened onto a ceramic substrate, the test devices are placed on the printed solder paste, and the
ceramic substrate is processed through a reflow cycle and allowed to cool. After reflow, the units are
easily removed from the ceramic for inspection. The beauty of this test is that the IC devices are subjected
to the same solder paste and reflow environment seen in printed wiring board (PWB) processing, and use
of a ceramic substrate allows for inspection of the soldered lead surface (underside of lead foot).
However, use of a CPT in place of PWB soldering introduces a variable that may influence solder joint
shape: the ceramic is, by design, a nonsolderable surface, while a PWB pad is a solderable surface. This
difference in wetting properties of the substrates may influence lead side wetting. Another substrate factor
that could influence lead side wetting is the pad size/stencil aperture size. Process factors that could
influence lead side wetting are reflow atmosphere, reflow peak temperature, and solder paste. Factors
related to the components that might have an influence are palladium thickness and preconditioning. In
this study, PWB finish (ENIG) and Pb-free paste were held constant.
As the industry moves into Pb-free processing with reflow environments and materials different from
tin-lead (SnPb) soldering, it is imperative to understand the impact of these variables on solderability,
particularly solder joint shape, when testing using CPT and PWB methods.
(1)
See Test S/S1 in J-STD-002 or Method 2 in JESD22-B102D.
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Experiment
2
Experiment
A designed experiment (DOE) evaluated the effect of several variables (Table 1) on component lead side
wetting and lead pull performance.
Table 1. DOE Input Variables
I.D.
Variable
No. of Levels
L1
L2
Pad
Pad Size/Stencil Aperture
3
CUST
IPC
RA
Reflow Atmosphere
2
Air
N2
RT
Reflow Temperature
2
230C
240C
PDT
Palladium Thickness
2
0.01 µm (0.4 µ")
≥0.02 µm (0.8 µ")
AG
Precondition
2
None
16 hr, 155°C
L3
TID
The IC package used for these evaluations was an 8-pin SOIC (see Figure 1 and Figure 2).
Figure 1. TI SOIC Package End View
4
Factors That Influence Side-Wetting Performance on IC Terminals
Figure 2. TI SOIC Package Top View
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Experiment
Three levels were evaluated for the PWB pad size/stencil aperture opening. CUST is a customer design,
TID is a TI design, and IPC is from the IPC guidelines. All were included on each board. The pad
dimension correlated 1:1 with the stencil aperture. Dimensions and areas of the three pad levels evaluated
are shown in Table 2 and shown graphically in Figure 3.
Table 2. PWB Pad and Stencil Aperture Size
Pad/Aperture Opening
Length (mm)
Width (mm)
Area (mm2)
CUST
1.2
0.6
0.72
IPC
1.9
0.55
1.045
TID
1.52
0.76
1.155
Customer
IPC
TID
0.6
0.55
0.76
1.9
1.2
1.52
Figure 3. PWB Pad Dimensions
RA was either air or nitrogen (N2) purge (~50 ppm remaining 02). A commercial Pb-free SnAgCu solder
paste was used with a RT of 230°C and 240°C. To evaluate the impact of different palladium thicknesses,
Pb-free NiPdAu-finished components from three different suppliers were used, as shown in Table 3.
Table 3. Different Palladium Thicknesses
Component
Pd Thickness – µm (µ")
TI
0.01 (0.4)
Competitor 1
0.05 (1.97)
Competitor 2
0.04 (1.57)
Preconditioning (thermal aging) was another variable. The two levels were no preconditioning and
16 hours/155°C dry heat.
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Experiment
The designed experiment layout is shown in Table 4.
Table 4. Layout of Designed Experiment (1)
Run
Pad
RA
RT
PDT
AG
1
CUST
Air
230
0.01
16 hr
2
CUST
Air
230
>0.02
0
3
CUST
Air
240
0.01
0
4
CUST
Air
240
>0.02
16 hr
5
CUST
N2
230
0.01
0
6
CUST
N2
230
>0.02
16 hr
7
CUST
N2
240
0.01
16 hr
8
CUST
N2
240
>0.02
0
9
IPC
Air
230
0.01
16 hr
10
IPC
Air
230
>0.02
0
11
IPC
Air
240
0.01
0
12
IPC
Air
240
>0.02
16 hr
13
IPC
N2
230
0.01
0
14
IPC
N2
230
>0.02
16 hr
15
IPC
N2
240
0.01
16 hr
16
IPC
N2
240
>0.02
0
17
TID
Air
230
0.01
16 hr
18
TID
Air
230
>0.02
0
19
TID
Air
240
0.01
0
20
TID
Air
240
>0.02
16 hr
21
TID
N2
230
0.01
0
22
TID
N2
230
>0.02
16 hr
23
TID
N2
240
0.01
16 hr
24
TID
N2
240
>0.02
0
(1)
Pad = pad dimension, RA = reflow atmosphere, RT = reflow temperature, PDT = Pd thickness,
AG = preconditioning
Responses were lead side-wetting height in the CPT and PWB mount, and lead pull measurements after
PWB mount. The degree of lead side wetting was judged on a scale of 0 to 1, with 0 being no solder
wetting the side of the lead and 1 showing solder to the top edge of the lead, i.e., 100% of the lead side
was covered with solder (Figure 4). Statistical analysis of the output was performed using a common
statistical analysis software package, and output data is summarized in an Analysis of Variance (ANOVA)
and Effects table.
100% = 1.0
50% = 0.5
10% = 0.1
Figure 4. Side Wetting Classification Examples
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Experiment
Figure 5 through Figure 7 show examples of unit placement on the PWBs. Note the 1:1 design of the
stencil and PWB pad. The devices were not pushed into the solder paste print.
Figure 5. Component Placed Onto Customer
Pad
Figure 6. Component Placed Onto TID Pad
Figure 7. Component Placed Onto IPC
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Experiment
Figure 8 through Figure 13 show the lead pull test method using a 20-pin SOIC package. The method is
the same when testing an 8-pin SOIC package as in this experiment. First the component is soldered onto
a test PWB. The package body is cut using a diamond blade and removed. The component leads are bent
up for the pull test. The individual leads are pulled vertical to the PWB.
8
Figure 8. Soldered Device on PWB
Figure 9. Package Body Cut Using Diamond
Blade
Figure 10. Package Body Removed
Figure 11. Mold Compound Removed
Factors That Influence Side-Wetting Performance on IC Terminals
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Experiment
Figure 12. One Set of Leads
Bent Up For Pull Test
Figure 13. Pull Test Performed
Vertical to PWB
The unit of measure for lead pull test data in this experiment is kilograms (kg) pull force.
The results are presented in Section 3 as TI components versus Competitor 1 components and TI
components versus Competitor 2 components, looking at CPT, PWB mount, and lead pull results, in that
order.
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Results: TI Versus Competitor 1
3
Results: TI Versus Competitor 1
3.1
Lead Side-Wetting Height in CPT Test Method
Analysis of Variance (ANOVA) results for CPT lead side-wetting height of TI versus Competitor 1 are
shown in Table 5.
Table 5. ANOVA Results for CPT Side Wetting, TI Versus Competitor 1
Rank
Source
Df
SS
F Ratio
Prob > F
% Contribution
1
RA
1
7.287526
206.286
<0.0001
44.92
2
Pad (IPC and CUST-TID)
1
4.245326
120.171
<0.0001
26.17
3
PDT
1
1.438151
40.709
<0.0001
8.86
4
Pad (IPC and CUST-TID)*RA
1
1.325013
37.507
<0.0001
8.17
Pad (IPC and CUST-TID)*PDT
1
0.7190755
20.355
<0.0001
4.43
Pad (IPC-CUST)
1
0.2691016
7.617
0.0061
1.66
Pad (IPC-CUST)*RT
1
0.2197266
6.22
0.0131
1.35
AGE
1
0.206276
5.839
0.0162
1.27
Pad(IPC-CUST)*RA
1
0.175352
4.964
0.0265
1.08
RA*PDT
1
0.170859
4.847
<0.0001
1.05
RA*AGE
1
0.162526
4.601
0.0326
1
RT
1
0.005859
0.166
0.6841
0.04
Total
16.224792
100
RA and Pad have the strongest contribution to side-wetting height in the CPT method. Other factors (PDT,
RT, and AG) and interactions all have lesser or no contribution.
The average effects table for individual factors is shown in Table 6. An effects table shows the mean value
for each factor level setting in all runs. For instance, under the column heading RA, the average value for
AIR was 0.648 and the average value for N2 was 0.923. This tells us that N2 provided higher lead side
wetting than AIR.
Effects plots are shown for each factor immediately under Table 6. An effects plot is a graphical
representation of the average effects data. An effects plot provides an easy to understand visual
representation of the average effects data. Basically if the effects plot is flat (horizontal line), there is little
to no effect. If the effects plot has a slope (slanted line), there is some effect from the factor. The greater
the slope of the line, the greater the effect.
Table 6. Average Effects Table for CPT Side Wetting, TI Versus Competitor 1
RA
Pad
PDT
AG
RT
AIR
0.648
IPC
0.679
0.01
0.847
0
0.809
230
0.79
N2
0.923
CUST
0.744
0.05
0.724
16
0.763
240
0.782
TID
0.934
1
0.9
1
0.9
1
0.9
1
0.9
1
0.9
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.1
0
0.1
0
Air
N2
RA
10
0.1
0
0.1
0
IPC
CUST
Pad
TID
0.01
0.05
PDT
Factors That Influence Side-Wetting Performance on IC Terminals
0.1
0
0
16
AG
230
240
RT
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Results: TI Versus Competitor 1
The effects table and plots clearly show that reflow atmosphere and pad/aperture size have a strong
effect. N2 provides higher side-wetting performance. For factor of Pad, the wider the pad and aperture
opening the higher the side wetting. Thinner Pd showed higher side wetting. The effects of aging and
reflow temperature are minor.
3.2
Lead Side-Wetting Height in PWB Soldering
ANOVA results for PWB lead side-wetting height of TI versus Competitor 1 are shown in Table 7.
Table 7. ANOVA Results for PWB Lead Side Wetting, TI Versus Competitor 1
Rank
Source
Df
SS
F Ratio
Prob > F
% Contribution
1
Pad (IPC and CUST-TID)
1
1.8116
131.448
<0.0001
53.85
2
RA
1
0.735
53.332
<0.0001
21.85
3
RT
1
0.2301
16.696
<0.0001
6.84
4
Pad (IPC-CUST)*AG
1
0.18598
13.495
0.0003
5.53
Pad (IPC and CUST-TID)*RA
1
0.17824
12.933
0.0004
5.3
Pad (IPC-CUST)*RT
1
0.10973
7.962
0.005
3.26
RA*RT
1
0.065104
4.724
0.0304
1.94
AGE
1
0.04167
3.023
0.0829
1.24
Pad (IPC-CUST)
1
0.0066
0.479
0.4893
0.2
PDT
1
0
0
3.364024
100
Total
Pad and RA have the strongest contribution to side-wetting height in PWB mount. Other factors (PDT, RT,
and AG) all have lesser or no contribution.
The average effects table for individual factors is shown in Table 8.
Table 8. Average Effects Table for PWB Lead Side Wetting, TI Versus Competitor 1
Pad
RA
RT
AG
PDT
IPC
0.836
Air
0.846
230
0.865
0
0.879
0.01
0.89
CUST
0.846
N2
0.933
240
0.914
16
0.9
0.05
0.889
TID
0.987
1
0.9
1
0.9
1
0.9
1
0.9
1
0.9
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.1
0
0.1
0
0.1
0
IPC
CUST
TID
Pad
Air
N2
RA
0.1
0
0.1
0
230
240
RT
0
16
AG
0.01
0.05
PDT
The effects table and effects plots show that Pad has a strong effect and RA has a moderate effect. Once
again for the Pad factor, the wider pad opening yields higher lead side wetting. For RA, N2 provides
higher lead side wetting, but it is a minor effect in this case, most likely because of the "wettability" of the
PWB. The effects of RT, AG, and PDT are very minor.
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Results: TI Versus Competitor 1
Summary of the average values for each combination of Pad and RA is shown graphically in Figure 14.
The table contrasts the average values in CPT testing versus PWB soldering. The data demonstrates a
difference in the side-wetting performance between CPT test method versus PWB soldering, particularly
for the narrow pad/aperture openings (IPC, CUST) in air. When N2 is used, the wetting performance is
essentially the same for either method, CPT or PWB soldering.
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
CPT
PWB
TID-N2
CUST-N2
IPC-N2
TID-AIR
CUST-AIR
IPC-AIR
Figure 14. Comparison of Side-Wetting Performance in CPT Method Versus PWB Soldering
12
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Results: TI Versus Competitor 1
3.3
Lead Pull Variation in PWB Soldering
ANOVA results for component lead pull after PWB soldering of TI versus Competitor 1 are shown in
Table 9.
Table 9. ANOVA Results for Lead Pull After PWB Soldering, TI Versus Competitor 1
Rank
Source
Df
SS
F Ratio
Prob > F
% Contribution
1
RT
1
2.5438
21.264
<0.0001
48.44
2
Pad (CUST and IPC-TID)
1
2.1901
18.307
<0.0001
41.7
3
Pad (IPC-CUST)*RT
1
0.405
3.386
0.0674
7.71
4
Pad (IPC-CUST)
1
0.1128
0.943
0.3328
2.15
Total
5.2517
100
Reflow temperature and Pad have the most contribution to any variance in lead pull after PWB mount.
However, as can be seen in the following effects plots, the actual variance is small. Other factors (RA,
PDT, and AG) have no contribution.
The average effects table for individual factors is shown in Table 10.
Table 10. Average Effects Table for Lead Pull After PWB Soldering, TI Versus Competitor 1
Pad
RA
RT
AG
PDT
IPC
2.033
Air
2.042
230
1.964
0
2.083
0.01
2.1
CUST
1.973
N2
2.116
240
2.194
16
2.074
0.05
2.057
TID
2.23
4
4
4
4
3.5
3.5
3.5
3.5
3
3
3
3
2.5
2.5
2.5
2.5
2
2
2
2
1.5
1.5
1.5
1.5
1
1
0.5
0.5
4
0
3.5
3
1
0.5
CUST
TID
Pad
2
1.5
1
1
0.5
0.5
0
0
IPC
2.5
230
AIR
240
N2
RA
0
0
0
RT
16
AG
0.01
0.05
PDT
The effects table and effects plots for lead pull show very little variance from any variable in this
experiment. Basically, all lead pull data are in the same range.
3.4
Summary/Conclusions for TI Versus Competitor 1
In both CPT and PWB soldering, RA and Pad contribute strongest to component lead side-wetting height.
The other factors had negligible or no contribution. N2 provided the highest side wetting of leads and for
Pad, the wider the pad/stencil aperture opening, the higher the lead side wetting. For lead pull after PWB
mount, RT and Pad had the greatest contribution to variance. However, the effects table shows there is
very little variation in the lead pull data across all groups.
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Results: TI Versus Competitor 2
4
Results: TI Versus Competitor 2
4.1
Lead Side-Wetting Height after CPT Testing
Analysis of Variance (ANOVA) results for CPT lead side-wetting height of TI versus Competitor 2 are
shown in Table 11.
Table 11. ANOVA Results for CPT Side Wetting, TI Versus Competitor 2
Rank
Source
Df
SS
F Ratio
Prob > F
% Contribution
1
RA
1
4.6376042
146.65
<0.0001
61.53
2
Pad (IPC and CUST-TID)
1
1.622513
51.307
<0.0001
21.53
3
Pad (IPC and CUST-TID)*RA
1
0.5365755
16.968
<0.0001
7.12
Pad (IPC-CUST)*RT
1
0.2691016
8.51
0.0037
3.57
Pad (IPC-CUST)
1
0.2562891
8.104
0.0047
3.4
PDT
1
0.1426042
4.509
0.034
1.89
Pad(IPC-CUST)*PDT
1
0.0722266
2.284
0.1316
0.96
Total
7.5369142
100
RA and Pad have the strongest contribution to lead side-wetting height in the CPT. Other factors (PDT,
RT, and AG) all have lesser or no contribution. The average effects table for the individual factors is
shown in Table 12.
Table 12. Average Effects Table for CPT Side Wetting, TI Versus Competitor 2
RA
Pad
PDT
AG
RT
AIR
0.718
IPC
0.75
0.01
0.847
0
0.829
230
0.829
N2
0.938
CUST
0.813
0.05
0.808
16
0.826
240
0.826
TID
0.92
1
0.9
1
0.9
1
0.9
1
0.9
1
0.9
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.1
0
0.1
0
0.1
0
Air
N2
RA
IPC
CUST
Pad
TID
0.1
0
0.1
0
0.01
0.04
PDT
0
16
AG
230
240
RT
The effects table and effect plots show that RA and Pad have a strong effect. N2 provides higher lead side
wetting. For the factor of Pad, the wider the pad opening the higher the lead side wetting. Thinner Pd
showed slightly higher side wetting. AG and RT had no effect.
14
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Results: TI Versus Competitor 2
4.2
PWB Lead Side-Wetting Height
ANOVA results for PWB lead side-wetting height of TI versus Competitor 2 are shown in Table 13.
Table 13. ANOVA Results for PWB Lead Side Wetting, TI Versus Competitor 2
Rank
Source
Df
SS
F Ratio
Prob > F
% Contribution
1
Pad (IPC and CUST-TID)
1
1.3167187
83.324
<0.0001
48.16
2
RA
1
0.8251042
52.214
<0.0001
30.18
3
RT
1
0.2604167
16.48
<0.0001
9.52
4
Pad (IPC and CUST-TID)*RA
1
0.2200521
13.925
0.0002
8.05
PDT
1
0.0816667
5.168
0.0236
2.99
RT*PDT
1
0.0301042
1.905
0.1683
1.1
Total
2.7340626
100
Again, Pad and RA have the strongest contribution to lead side-wetting height in board mount. Other
factors (PDT, RT, and AG) show less or no contribution.
The average effects table for the individual factors is shown in Table 14.
Table 14. Average Effects Table for PWB Lead Side Wetting, TI Versus Competitor 2
Pad
RA
RT
AG
PDT
IPC
0.859
Air
0.858
230
0.879
0
0.898
0.01
0.89
CUST
0.867
N2
0.951
240
0.931
16
0.912
0.05
0.919
TID
0.988
1
0.9
1
0.9
1
0.9
1
0.9
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.5
0.4
0.3
0.2
0.1
0
0.1
0
IPC
CUST
TID
Pad
N2
RA
0.1
0
0.1
0
0.1
0
Air
1
0.9
0
230
16
0.01
0.04
240
RT
AG
PDT
The effects table and effects plots show that Pad and RA have minor effect. Once again for the Pad
factor, the wider pad opening provides higher side wetting. For RA, N2 provides higher side wetting. For
RT, effect is minor and 240°C provides higher wetting. AG and PDT have virtually no effect on
side-wetting height in PWB soldering.
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Results: TI Versus Competitor 2
Summary of the average values for each combination of Pad and RA is shown graphically in Figure 15.
The table contrasts the average values in CPT testing versus PWB soldering. The data demonstrates a
difference in the side-wetting performance between CPT test method versus PWB soldering, particularly
for the narrow pad/aperture openings (IPC, CUST) in air. When N2 is used, the wetting performance is
essentially the same for either method, CPT or PWB soldering.
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
CPT
PWB
TID-N2
CUST-N2
IPC-N2
TID-AIR
CUST-AIR
IPC-AIR
Figure 15. Comparison of Side-Wetting Performance in CPT Method Versus PWB Soldering
ANOVA results for component lead pull after PWB soldering of TI versus Competitor 2 are shown in
Table 15.
Table 15. ANOVA Results for Lead Pull After PWB Soldering, TI Versus Competitor 2
Rank
Source
Df
SS
F Ratio
Prob > F
% Contribution
1
RT
1
1.6875
12.676
0.0005
28.27
2
PDT
1
1.4352
10.781
0.0012
24.04
3
Pad (CUST and IPC-TID)*RA
1
1.0732
8.061
0.005
17.98
4
Pad (CUST and IPC-TID)
1
0.7975
5.991
0.0153
13.36
RA*PDT
1
0.6075
4.563
0.034
10.18
RA
1
0.3008
2.26
0.1345
5.04
RT*PDT
1
0.0675
0.507
0.4773
1.13
Total
5.9692
100
The variation seen in lead pull after PWB mount is spread across the main factors of RT, PDT, and RA.
The other factors (Pad and AG) show no contribution to variation.
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Results: TI Versus Competitor 2
The average effects table for the factors is shown in Table 16.
Table 16. Average Effects Table for Lead Pull After PWB Soldering, TI Versus Competitor 2
RT
PDT
Pad
RA
AG
230
1.92
0.01
2.1
IPC
1.995
Air
2.053
0
1.994
240
2.107
0.04
1.927
CUST
1.941
N2
1.974
16
2.033
TID
2.105
4
4
3.5
3.5
3
3
2.5
2.5
2
2
1.5
1.5
1
1
0.5
0.5
0
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0
230
240
RT
0.01
0.04
4
4
3.5
3.5
3
3
2.5
2.5
2
2
1.5
1.5
1
1
0.5
0.5
0
Cust
IPC
TID
Pad
PDT
0
Air
N2
RA
0
16
AG
The effect of the main factors is very small for lead pull response. RT has a slight effect, with 240°C being
best case. PDT has a slight effect, with 0.01 being best case setting. Pad also has a slight effect, with TID
being best case. RA and AG have no effect. In general, for lead pull after PWB soldering, the variation in
the data is very small, confirming that the effect of these variables is also small.
4.3
Summary/Conclusions for TI Versus Competitor 2
In both CPT and PWB soldering, RA and Pad contribute strongest to component lead side-wetting height.
The other factors had negligible or no contribution. N2 provided the highest side wetting of leads and for
Pad, the wider the pad/stencil aperture opening, the higher the lead side wetting. For lead pull after PWB
mount, RT and Pad had the greatest contribution to variance. However, the effects table shows there is
very little variation in the lead pull data across all groups.
4.4
Industry Standard Wetting Requirements
Investigation of industry standards and consult with IPC staff determined that there is no toe fillet height or
side joint height requirement in Table 8-5, section 8.2.5 of IPC-A-610D. “Climb” of the solder up the side of
the lead or toe is not required. Comments from industry experts indicate that 70-80% of strength of the
solder connection is from the heel fillet. The tables and dimensions in IPC-A-610D are identical to those in
J-STD-001D. Stated simply, the industry standard for board soldered gull wing units has no requirement
for component lead side wetting.
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Summary/Conclusions
5
Summary/Conclusions
NiPdAu finished leads from three different companies were evaluated for CPT and PWB solderability.
Reflow atmosphere and pad size gave the strongest contribution to component lead side wetting. Other
factors had little or no contribution. Nitrogen atmosphere provided the highest side wetting and for pad
size, the widest pad/stencil aperture opening showed the highest side wetting. For lead pull after PWB
mount, the variance in the data was spread across reflow temperature, palladium thickness, and reflow
atmosphere, however, the effects table shows very little variation in the lead pull data across all groups.
5.1
Conclusions
•
•
•
•
•
•
•
6
Of the factors tested, reflow atmosphere and pad/aperture size have the greatest contribution to
component lead side-wetting height.
Nitrogen gives higher lead side wetting than air.
The larger the pad/aperture width, the higher the lead side wetting.
Reflow temperature, palladium thickness, and precondition had very little impact on lead side-wetting
height performance.
The data demonstrates a difference in the side-wetting performance between CPT test method versus
PWB soldering, particularly for narrow pad/aperture openings in air. When N2 is used the wetting
performance is essentially the same for either method, CPT or PWB soldering.
For lead pull, variance in the data was relatively small. Variation in the input factor tested had little
impact on lead pull results. In other words, the mechanical strength of the solder joints is relatively
independent of changes in the inputs. side-wetting height may change but mechanical strength is the
same.
Lead pull force is little affected by side-wetting height.
Acknowledgements
The authors wish to recognize the following for their professional assistance with statistical data analysis:
• Bill Russell, Raytheon Professional Services
• Dr. Madhukar Joshi, formerly of Texas Instruments
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Factors That Influence Side-Wetting Performance on IC Terminals
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