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Texas Instruments Wave Solder Exposure of SMT Packages Application notes
Application Report
SCEA041 – September 2008
Wave Solder Exposure of SMT Packages
James Huckabee, Steven Kummerl, Dominic Nguyen, Douglas W. Romm, and Larry Ting..............................
ABSTRACT
Attaching surface mount components to the bottomside of a printed circuit board (PCB)
by wave solder processing is common practice. Because surface mount components
are normally soldered using a reflow oven process the impact of wave soldering and
wave solder simulation methods must be understood.
Process characterization was performed to compare the wave solder reflow method,
the solder dip method, and standard reflow oven processing. Units processed through
the solder dip method see a very quick rise time to peak temperature which is quite
different from the other two methods. The solder dip method has excessive thermal
shock.
A designed experiment compared the contribution of moisture soak, wave time, and
PCB design. Both moisture soak and wave time have a strong effect on the variation
seen in delamination. Reliability stressing demonstrated passing results for the units.
Testing demonstrated that the solder dip method is not a good simulation method for
wave solder exposure of surface mount components. DOE testing demonstrated that
moisture soak and wave time are significant factors in the process. Reliability stressing
showed that units passed even with variations seen in package delamination.
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Contents
Introduction ..........................................................................................
Process Characterization ..........................................................................
Designed Experiment ..............................................................................
Summary/Conclusions .............................................................................
References ..........................................................................................
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3
4
7
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List of Figures
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2
3
Comparison of Time/Temperature Profiles for Three Soldering Methods................... 3
Isometric View of SOIC Gull-Wing Package .................................................... 3
Effects Plots ......................................................................................... 6
List of Tables
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3
4
5
6
7
8
Wave Solder Simulation Conditions in JESD22A111 ..........................................
Test Results .........................................................................................
Experiment Factors .................................................................................
Experimental Layout ...............................................................................
Average Effects .....................................................................................
ANOVA Results .....................................................................................
Reliability Assessment Results ...................................................................
Summary of Test Results ..........................................................................
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Wave Solder Exposure of SMT Packages
1
Introduction
1
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Introduction
It is common practice to attach surface mount components to the underside of a printed circuit board
(PCB) by processing the PCB through a wave soldering operation after gluing the components to the
PCB. This paper summarizes results of several tests performed to understand the performance of surface
mount components when exposed to the conditions outlined in JESD22A111, Evaluation Procedure for
Determining Capability to Bottom Size Board Attach by Full Body Solder Immersion of Small Surface
Mount Solid State Devices (see Reference 1).
JESD22A111 allows for processing of components by either of two different methods, wave soldering or
solder dip. The first experiment was performed to see the difference in the profile exposures for the
different methods of wave soldering, solder dip, and reflow profile.
The wave solder simulation conditions in JESD22A111 are as shown in Table 1 .
Table 1. Wave Solder Simulation Conditions in JESD22A111
TEST CONDITIONS
REFLOW METHOD
WAVE SOLDER
SOLDER DIP
Preheat temperature
25 to 140°C
145°C
Preheat time
80 seconds
15 seconds min
Solder temperature
245°C classification
245°C +5/–0°C
245°C +5/–0 C
260°C classification
260°C +5/–0°C
260°C +5/–0°C
5+/–1 seconds
5 +/–1 seconds
Single wave simulation
Solder immersion time
Dual wave simulation
First wave: 5+/–1 sec
Second wave: 5+/–1 sec
10 +/–1 seconds
The flow for testing per JESD22A111 is as follows:
1. Initial inspection: Baseline is determined using visual inspection and acoustic microscope examination.
2. Bake: Units are baked for 24 hours at 125°C to remove moisture.
3. Moisture soak: Units are submitted to appropriate soak conditions based on moisture sensitivity level of
the package.
4. Reflow: Units are subjected to either the wave solder reflow method or the solder dip procedure per
conditions in Table 1.
5. Failure Criteria: After exposure to the reflow conditions units are tested electrically, inspected using
acoustic microscope, inspected for external package cracks, and cross-sectioned for internal cracks.
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Process Characterization
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2
Process Characterization
A trial test compared performance between the wave solder reflow method, the solder dip method, and
reflow soldering. The reflow soldering profile used was based on IPC/JEDEC J-STD-020D Pb-free
assembly reflow profile (see Reference 2). In trial test we followed the recommended solder dip procedure
per 22-A111 solder dip parameters. Section 5.6.2b of 22-A111 says to hold devices within one inch above
the hot solder for 15 seconds to simulate preheat. However, the temperature profile obtained did not
match exactly with preheat temperature. We determined that when using the method described in 5.6.2b
preheat did not reach the desired 145°C for 15 seconds minimum.
A visual comparison of the time/temperature profiles for each of the three soldering methods is shown in
Figure 1.
300
Temperature (°C)
250
200
150
100
50
0
0
50
100
150
200
250
300
350
Time (s)
Figure 1. Comparison of Time/Temperature Profiles for Three Soldering Methods
Units exposed to the solder dip method (shown in blue) reach peak temperature much more quickly than
units exposed to either the wave solder method or surface mount reflow. The units processed through the
wave solder reflow (shown in red) see a slow rise to preheat temperature before rising to peak
temperature of the solder in the wave. The reflow soldering profile shows a normal rise to the preheat
range and then a gradual rise to peak temperature.
Comparison of the profiles shows similar peak temperatures for all three profiles. However, the solder dip
method has excessive thermal shock with the component going from ≈75°C to ≈260°C very quickly. This
transition does not compare to the transition seen in the wave solder profile or the reflow soldering profile.
Test units were exposed to the three different reflow conditions described above.
The test vehicle used was a 16-pin narrow body small outline integrated circuit (SOIC) package.
Figure 2. Isometric View of SOIC Gull-Wing Package
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3
Designed Experiment
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Prior to reflow exposure the units were baked (24 hours minimum at 125 +5–0°C) to dry the packages and
then soaked. After reflow exposure, the units were processed through electrical test and SAM. Results are
shown in Table 2.
Table 2. Test Results
TEST
GROUP
METHOD
SOAK
ELECTRICAL TEST SAMPLE SIZE/
NO. OF FAILS
SAM SAMPLE SIZE/
NO. OF FAILS
1
Wave
MSL 1: 168 hrs at 85°C/85 RH
10 / 0
10 / 0
2
Wave
MSL 3: 192 hrs at 30°C/60 RH
10 / 0
10 / 0
3
Solder dip
MSL 1: 168 hrs at 85°C/85 RH
4/0
4/2
4
Solder dip
MSL 3: 192 hrs at 30°C/60 RH
4/0
4/0
5
Reflow
MSL 1: 168 hrs at 85°C/85 RH
10 / 0
10 / 0
6
Reflow
MSL 3: 192 hrs at 30°C/60 RH
10 / 0
10 / 0
The results of this experiment support the claim that the solder dip method imposes more severe thermal
shock and is not equivalent to the standard reflow conditions used for MSL characterization.
The authors agree with the note in section 4 of 22-A111 that reads: "Assessment by dipping in a solder
post usually exposes devices to higher stresses than the wave solder procedure, which results in induced
failures."
Based on these results TI has decided not to use the solder dip method as a wave solder simulation.
3
Designed Experiment
A designed experiment was performed to understand the effect of processing SOIC units through wave
soldering. A 16-pin SOIC package was used for this experiment.
The three factors and the factor level setting are shown in Table 3.
Table 3. Experiment Factors
LEVEL
SOAK
WAVE TIME
PWB DESIGN
1
Soak 1: bake only
5s
Low-K
2
Soak 2: 192 hours at 30°C/60 RH
10 s
High-K
3
Soak 3: 168 hours at 85°C/85 RH
Three different soak conditions were used to see the impact of moisture absorption. The time that the
components saw in the solder wave was varied between 5 seconds and 10 seconds. Two different PWB
designs were used – JEDEC Low-K (see Reference 3) and JEDEC High-K (see Reference 4) .
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Designed Experiment
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Table 4. Experimental Layout
RUN
WAVE TIME
PWB
SOAK
1
10 s
High-K
Soak 1
2
10 s
High-K
Soak 2
3
10 s
High-K
Soak 3
4
10 s
Low-K
Soak 1
5
10 s
Low-K
Soak 2
6
10 s
Low-K
Soak 3
7
5s
High-K
Soak 1
8
5s
High-K
Soak 2
9
5s
High-K
Soak 3
10
5s
Low-K
Soak 1
11
5s
Low-K
Soak 2
12
5s
Low-K
Soak 3
Four replicates for each factor level combination (FLC) shown above were processed for a total of five
units per FLC.
Initial electrical test, visual inspection, and acoustic microscope examination were performed to establish a
baseline. The units were baked for 24 hours at 125°C. Immediately after bake, units were exposed to the
soak conditions in Table 4. Bake and soak of the units was scheduled to overlap so that all units would
exit the soak chambers at the same time. Units were processed through reflow within four hours to meet
the requirement of the standard which indicates "not sooner than 15 minutes and not longer than 4 hours
after removal from the temperature/humidity chamber, subject the sample to the reflow conditions."
For the wave soldering operation the preheat temperature, preheat time, and solder temperature were all
set according to the requirements in Table 1 of J-STD-22A111. The time in the solder was was alternated
between 5 and 10 seconds depending upon the run being processed.
After completion of the wave solder processing the units were tested electrically. Then acoustic
microscope examination was performed to determine the post reflow delamination. The average delta of
pre-reflow delamination to post-reflow delamination was calculated for each group.
The average effects table for individual factors is shown in Table 5. An effects table shows the mean value
for each factor level setting in all runs. For instance under the column heading “Soak” the average value
for Soak 1 was 1.319 and the average values for Soak 2 and Soak 3 were 6.244 and 6.174 respectively.
This tells us that Soak 1 provided the lowest delamination results, which is intuitive. The result seen for
the wave time factor is counter intuitive with the longer wave time exposure giving the lower delamination
value.
Effects plots are shown in Figure 3. An effects plot is a graphical representation of the average effects
data. An effects plot is used because it provides and easy to understand visual representation of the
average effects data. Basically if the effects plot is “flat” (horizontal line) then there is little to no effect. If
the effects plot has a slope (slanted line) then there is some effect from the factor. The greater the slope
of the line, the greater the effect.
Table 5. Average Effects
LEVEL
SOAK
MEAN VALUE
WAVE TIME
MEAN VALUE
PWB
MEAN VALUE
1
Soak 1
1.319
5s
6.26
Low-K
3.99
2
Soak 2
6.244
10 s
2.90
High-K
5.17
3
Soak 3
6.174
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5
20
18
16
14
12
10
8
6
4
2
0
Soak 1
Soak 2
Average Delamination (%)
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Average Delamination (%)
Average Delamination (%)
Designed Experiment
20
18
16
14
12
10
8
6
4
2
0
5s
Soak 3
20
18
16
14
12
10
8
6
4
2
0
Low-K
10 s
High-K
Figure 3. Effects Plots
The effects table and effects plots show that soak and wave time have a strong effect. The effect of the
PWB variation is minor.
Analysis of variance (ANOVA) was used to determine contribution of each factor to the variance seen in
the experiment (shown in Table 6).
Table 6. ANOVA Results
RANK
SOURCE
SS
MSE
F-RATIO
PROB > F
PERCENT CONTRIBUTION
(100% total)
1
X3 (Soak)
212.09446
106.0472
4.2966
0.0236
45.41%
2
X1 (Wave Time)
108.30849
108.3085
4.3882
0.0454
23.19%
X1*X3
61.0795
30.53975
1.2373
0.3055
13.08%
X1*X2
44.69765
44.69765
1.8109
0.1892
9.57%
X1*X2*X3
25.70661
12.85331
0.5208
0.5997
5.50%
X2 (PWB)
13.5375
13.5375
0.5485
0.4651
2.90%
X2*X3
1.59337
0.796685
0.0323
0.9683
0.34%
If p < 0.05, then that factor is statistically significant with 95% confidence. The ANOVA analysis showed
that the main factors of soak and wave time were significant to the delamination. The main factor of PWB
and all interactions were not significant.
After final CSAM data was collected and analyzed the units were cross-sectioned to inspect for package
cracks. No package cracks were seen on any of the cross-sectioned units.
3.1
Reliability Stressing
JEDEC Std 22-A111 indicates that to evaluate the impact of delamination on device reliability, the product
must either meet the delamination requirements listed or reliability assessment. In our evaluation we saw
a few cases where a measureable delamination change (10% absolute change between pre and post
reflows) was noted. Thus, we proceeded to perform a reliability assessment on packages built with the
same device used in the DOE.
The following flow was used to assess the reliability performance of the units:
1. Electrical test
2. Units submitted to bake + soak
a. Soak 1: Bake only
b. Soak 2: 192 hrs at 30°C/60 RH
c. Soak 3: 168 hrs at 85°C/85 RH
3. Processed through wave solder with 10 seconds exposure
4. Electrical test
5. Reliability exposure
a. One set: unbiased HAST (130°C/85 RH)
b. One set: temp cycle (–65°C/150°C)
6. Electrical test
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Summary/Conclusions
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Results for the reliability assessment are shown in Table 7.
Table 7. Reliability Assessment Results
UNBIASED HAST
(96 hrs at 130°C/85 RH)
ELECTRICAL TEST RESULTS
(NO. TESTED/NO. FAILED)
TEMPERATURE 1K CYCLES
(–65°C/150°C)
GROUP 1
GROUP 2
GROUP 3
GROUP 1
GROUP 2
GROUP 3
Prior to bake + soak
22/0
22/0
22/0
22/0
22/0
22/0
After wave solder processing
22/0
22/0
22/0
22/0
22/0
22/0
After reliability stressing
22/0
22/0
22/0
22/0
22/0
22/0
The results of all testing are summarized in Table 8.
Table 8. Summary of Test Results
CONDITION 1
CONDITION 2
CONDITION 3
BAKED DRY
MSL3–192 hrs 30°C/60 RH
MSL1–168 hrs 85°C/85 RH
Electrical test
Pass
Pass
Pass
Die delamination
Pass
Pass
Pass
External cracks
Pass
Pass
Pass
Internal cracks
Pass
Pass
Pass
Die pad delamination
Pass
Pass
Pass
Lead finger delamination
Pass
Pass
Pass
CRITERIA
4
Summary/Conclusions
Results of the process characterization showed that the solder dip method has the quickest rise time to
peak temperature because of very short preheat. Excessive thermal shock is experienced when the
components go from 140°C to 260°C very quickly. A designed experiment showed that moisture soak and
wave time have a strong effect on delamination seen during wave solder processing. Units exposed to
wave solder processing passed electrical test after reliability stressing (unbiased HAST and temperature
cycle).
4.1
Conclusions
•
•
5
The solder dip method imposes excessive thermal shock on the components. The temperature
transition in the solder dip method is not comparable to what is seen in the wave solder profile or the
reflow soldering profile. The solder dip method is not a good method for evaluating capability of surface
mount components to be exposed to bottom side board attach.
Independent of variation seen in delamination, all units passed electrical test after wave solder
processing and reliability stressing. The 16-pin SOIC units tested passed the criteria outlined in
JESD22A111.
References
1. Evaluation Procedure for Determining Capability to Bottom Size Board Attach by Full Body Solder
Immersion of Small Surface Mount Solid State Devices, JESD22A111, May 2004.
2. Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices,
IPC/JEDEC J-STD-020D, June 2007.
3. Low Effective Thermal Conductivity Test Board for Leaded Surface Mount Packages, EIA/JESD51-3,
August 1996.
4. High Effective Thermal Conductivity Test Board for Leaded Surface Mount Packages, EIA/JESD51-7,
February 1999.
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