WHITE PAPER
Wynces Silvoza
Package Development
Sr. Manager
Cypress Semiconductor Corp.
Soldering Techniques for Gull Wing Packages
Using Nickel-Palladium-Gold Finishes
Introduction
Abstract
This white paper discusses soldering
techniques for gull wing packages
with nickel-palladium-gold (NiPdAu)
finishes to achieve optimal solder
joint wetting on the leads and the
PCB pad.
A design of experiment using a
TSOP 28 Cypress package with a
NiPdAu finish was mounted on a
customer PCB board to assess the
influences of several variables
during surface mount. The Pb-free
solder paste used was tin-silvercopper (SnAgCu).
Key variables were pad stencil
aperture, placement force and reflow
profile while solder joint quality and
lead push were the output
responses.
Cypress IC packages are offered in different lead finishes to suit customer
requirements and applications. One of the popular lead finishes offered by Cypress
today is nickel-palladium-gold (NiPdAu). This finish has been accepted by the
electronics industry since Texas Instruments introduced it to the market in 2001 to
meet the ever-growing demand for lead-free components and boards.
NiPdAu is an offspring of nickel-palladium, another type of surface finish. With this
finish, however, studies have shown that the top palladium surface oxidizes easily and
presents wettability issues during soldering. Adding a gold flash top layer helps the
palladium to survive further oxidation, promoting optimal wetting.
Today, the semiconductor industry has shipped millions of NiPdAu lead finish
packages. The demand for it is increasing exponentially because of its environmental
friendliness. Lead with its high toxicity has been reduced dramatically through
legislation such as the European Council Directive on Waste of Electrical and
Electronic Equipment (WEEE), which proposes restrictions on the use of Pb, among
other materials, in electronic products.
Like any other finish, NiPdAu brings challenges in soldering. This paper addresses
those challenges in an attempt to help make soldering easy.
Data results from the completed
experiment showed the key SMT
factors that optimize wetting for
NiPdAu lead finishes, which this
paper discusses and recommends.
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Using Nickel-Palladium-Gold Finishes
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Nickel-Palladium-Gold Flat Lead Gull Wing Package 101
The Cypress Pd-based lead finish structure is the NiPdAu finish. It typically possesses a plating thickness where nickel
(Ni) ranges from 20–80 microinches, palladium (Pd) is at 0.8 microinches minimum, and a gold (Au) flash ranges from
0.12–0.6 microinches, as shown in Figure 1.
Au: 0.12–0.6 µin
Pd: 0.8 µin min
Ni: 20–80 µin
Flash
Cu:
Base Metal of Lead
Plated
Plated
Figure 1. NiPDAu Finish
Table 1 shows the Cypress packages with a NiPdAu lead finish that have passed qualification testing per JEDEC
reliability standards. Table 2 through Table 7 summarize the package reliability tests.
Cypress QTP
Number
Qual Description
Qual Level
MSL + Reflow
015108
SOIC 20/24L Ni/Pd/Au Finish
MSL1, 235 °C
010612
SOIC 20/24L Ni/Pd/Au Finish
MSL1, 220 °C
013805
SSOP 48/56L Ni/Pd/Au Finish
MSL1, 235 °C
013802
TSSOP 16/20L Ni/Pd/Au Finish
MSL1, 235 °C
015107
TSSOP 16/20L Ni/Pd/Au Finish
MSL1, 260 °C
Table 1. Summary of Package Qualifications
Test Vehicle
Before MSL1 Pre-con
After MSL1 Pre-con
Result
Die
Top
Lead
finger
Paddle
Bottom
External
Visual
Die
Top
Lead
finger
Paddle
Bottom
External
Visual
TV1
0/15
0/15
0/15
0/15
0/15
0/15
0/15
0/15
Passed
TV2
0/15
0/15
0/15
0/15
0/15
0/15
0/15
0/15
Passed
TV3
0/15
0/15
0/15
0/15
0/15
0/15
0/15
0/15
Passed
Table 2. Acoustic Monitor Stress (C-SAM) Test
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Test Vehicle
Electrical Test Results
Remarks
TV1
0/150
Passed
TV2
0/50
Passed
TV3
0/50
Passed
Table 3. Moisture Preconditioning (MSL1) Test
Test Vehicle
Electrical Test Results
Remarks
TV1
0/50
Passed
Table 4. Pressure Cooker Test (PCT)
Test Vehicle
Electrical Test Results
(After 300 Cycles)
Remarks
TV1
0/50
Passed
TV2
0/50
Passed
TV3
0/50
Passed
Table 5. Temperature Cycle Test (TC)
Test Vehicle
Electrical Test Results
Remarks
TV1
0/50
Passed
Table 6. Highly Accelerated Stress Test (HAST)
Steam Age
Duration
Steam Age
Temperature
Lead Visual Inspection
(Reject/Sample Size)
0 hr
93 oC
0/2
1 hr
4 hrs
8 hrs
o
0/2
o
0/2
o
0/6
93 C
93 C
93 C
Table 7. Solderability Data (Ceramic Plate Test) with Steam Age
Facing the Challenges
Several defects are formed during soldering, with the most undesirable being non-wetting. This type of defect can spring
from various causes ranging from materials and environment to handling, equipment, and tools alike. This paper tackles
the wettability issues related to typical gull wing lead packages widely seen on most electronic boards for various
applications.
Non-wetting is a phenomenon where adjoining metal surfaces fail to fuse or adhere to each other to form a metallic bond.
Such failures can cause direct opens to complex field failures due to reliability issues. IPC standards are very clear in
defining what is acceptable and rejected, but customers still follow their own preferences based on the application of the
package, be it commercial or automotive. The following are some non-wetting anomalies that most board mount sites
encounter, as shown in Figure 2.
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Figure 2. Common Non-Wetting Anomalies

Lead toe non-wetting: Insufficient to zero solder fillet formation on the toe of the lead

Lead bottom non-wetting: Insufficient to zero bottom fillet on the bottom surface of the lead

Lead sidewall non-wetting: Zero solder fillet on the side wall of the lead

Low heel non-wetting: Insufficient to zero solder heel fillet on the lead

Solder paste and flux pooling on bottom of package
These defects are common during the soldering of flat lead gull wing packages, and the root cause may lie in the
component and board mount level. This paper presents soldering solutions for these defects—from simple reflow
profiles to complex paste printing and PCB pad geometry.
Regarding the previously mentioned solderability issues, IPC specifically classifies lead bottom and heel non-wetting as
valid rejects, which are critical for a strong solder joint. Sidewall wetting and toe wetting are not classified as valid rejects,
as they do not put much weight on solder joint strength.
Following are the IPC standards for solder wetting on gull wing packages, applicable to high-volume PCB manufacturing.

Class 1 – General Electronic Products

Class 2 – Dedicated Service Electronics Products

Class 3 – High-Performance Electronics Products
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Figure 3. Solder Joint Anomalies
Surface Mount Technology Solutions Lead Sidewall Non-Wetting
Reflow Process
The convectional reflow furnace is perhaps the simplest off-the-shelf solution to most solderability issues, particularly
lead sidewall wetting on a NiPdAu lead finish. In a lead-free solder paste application, typically the reflow plays a major
role in properly activating the solder paste to protect the leads from further oxidation during the time at reflow and
achieve a good wetting.
In contrast to a pure matte Sn-based finish, a NiPdAu finish does not dissolve easily into the lead-free solder paste, so
sidewall wetting is very difficult to achieve. The slow ramp reflow profile is a standard lead-free reflow profile that is
generally used for board mount. This type of generic profile can easily make a pure Sn-based lead finish wet properly to
a lead-free solder paste. However, for a NiPdAu finish, this may not be the case, as the nickel plating does not dissolve
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into the lead-free solder paste. In this case, a much hotter profile with fast ramp characteristics is highly recommended.
This fast ramp profile allows the lead-free solder paste basically to wick upwards and wet the sidewall of the NiPdAu
lead finish. See “Evaluation of NiPdAu Lead Finish Flat Lead Package TSOP28.”
Slow Ramp Reflow Profile (Generic Lead-Free Reflow Profile)

This profile is characterized by a slow ramp with an extended dwell at a reflow temperature above 200 °C.

When applied on most lead-free solder paste, this profile causes solder graping, so the flux gets easily exhausted
and the solder spheres oxidize and fail to wet properly.
Figure 4. Slow Ramp Reflow Profile
Fast Ramp Reflow Profile (Generic Lead-Free Reflow Profile)

This profile is slightly hotter than the slow ramp profile.

Flux is not exhausted easily, as there is a fast ramp immediately before 200 °C.

There is still a good soak, but at a much lower temperature not extending to 200 °C.

When applied on most lead-free solder paste, this profile eliminates solder graping, so the flux is not exhausted
easily and protects the solder spheres from oxidation, enabling them to wet properly.
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Figure 5. Fast Ramp Reflow Profile
Lead Side Wall Wetting Comparison for Slow Ramp and Fast Ramp Reflow Profile

Solder paste wicked upwards more for fast ramp profile

Good sidewall wetting observed on fast ramp profile

Insufficient sidewall wetting on slow ramp profile
Insufficient Sidewall Wetting
Sufficient Sidewall Wetting
Figure 6. Lead Side Wall Wetting
Solder Paste Application Process
Solder paste application is critical in achieving a reliable solder joint. Today, automated optical inspection systems are
put in place to tightly monitor the amount of solder paste deposited on the PCB pad post-paste printing, even before a
component is mounted on the solder paste. Too little solder paste can trigger an insufficient solder joint and weak joint
strength, while too much can yield solder joint shorts. Therefore, it is important to achieve the optimum amount of solder
paste deposit applicable to the lead finish of the package.
As mentioned previously, a pure Sn-based lead-free finish easily dissolves into the lead-free SAC solder paste, so a
slightly reduced solder paste volume would not starve the solder joint as the pure Sn plating homogenously joins with
the lead-free solder paste. With the NiPdAu lead finish, the nickel does not dissolve to the lead-free solder paste at
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reflow temperature, so a lesser solder paste volume would significantly reduce the solder joint formation. Thus, the
amount of solder paste volume required for this type of lead finish needs to be optimized by defining the correlation
between the stencil aperture, thickness, pad geometry, and lead dimension of the package. All these aspects need to
support one another to achieve the ideal solder joint.
With the advent of the fine-pitch leaded package, the spacing between leads has become very small, and in effect, the
PCB pad geometry has also been reduced. This presents more challenges for the NiPdAu finish, as the width of the
PCB pad is either less than or equal to the lead width to meet fine-pitch requirements. In this scenario, the solder paste
printed on the narrower PCB pad remains beneath the bottom of the lead, making it more difficult to wick upwards to
form a sidewall wetting. Unlike the pure Sn-based finish, the pure Sn plating easily melts and dissolves together with the
lead-free solder paste at the bottom of the lead, resulting in proper wicking to form a sidewall wetting.
There are two ways to achieve a good sidewall wetting for the NiPdAu finish leads, and both are related to fanning out
solder paste on the sides of the leads. One way is by doing an overprint without creating shorts to allow the solder paste
to wet the side of the wall. Another way is to design a slightly larger PCB pad with respect to the lead width dimension,
but this option is more costly than the stencil change.
Overprinting Technique

Increasing the stencil aperture and widening it allows a wider paste deposit area.

A wider solder paste deposit area means paste can easily extend to the sidewall.

This PCB pad design makes solder paste wicking easy to form sidewall wetting.
Figure 7. Effects of Overprinting
Wider Pad Geometry

Wider pad design means wider solder paste deposit.

Wider solder paste deposit means paste can easily extend to the sidewall.

This PCB pad design makes solder paste wicking easy to form sidewall wetting.
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Figure 8. Effects of Wider Pad Geometry
Pick and Place Process
Pick and place plays a major role in achieving a good solder joint fillet, especially sidewall wetting. The amount of force
applied to the solder paste dictates the amount squeezed out and the distance the paste will travel. Typically, the higher
the pressure, the further the squeeze-out goes. For NiPdAu, where the nickel does not dissolve into the SAC solder
paste, the position of the solder paste deposit is very important. If the squeeze-out does not reach the edge of the lead
width, then the solder paste will have a hard time wicking upwards to form a sidewall wetting and fillet.
Ideally, it is recommended that the solder paste have a squeeze-out breaching the lead width, but not to the extent that it
shorts to adjacent leads or minimal solder paste is left at the bottom of the lead. This approach to resolving sidewall
wetting is a good containment when the option to change the stencil or PCB pad is not available. Board mount
manufacturers should define the optimum placement force to achieve good solder paste squeeze-out and in effect good
sidewall wetting.
Figure 9. Effects of Placement Force
Accurate placement of the component on the PCB plays a significant role in achieving sidewall wetting. If the lead is
offset on one side of the PCB pad, the opposite side of the lead achieves a better sidewall wetting, as there is an
incidental overprint on one side, with little paste squeeze-out on the affected side. So, in principle, the more centered the
lead is to the PCB pad, the higher the probability of a uniform sidewall wetting between the left and right side of the lead.
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Figure 10. Effects of Placement Accuracy
PCB Pad Type
Two major types of PCB pad types are available for board mounts depending on the application: non-solder mask
defined (NSMD) and solder mask defined (SMD). NSMD presents challenges for NiPdAu, as the solder paste is
restricted by the canal of the NSMD from wicking to the side of the lead, as illustrated in Figure 11. In this case, the need
to overprint becomes mandatory using techniques from stencil change to pick and place, as discussed previously.
On the other hand, SMD, with the absence of the canal that separates the land from the solder mask, simply allows free
movement of the solder paste during squeeze-out or overprinting, making it easier for the solder paste to wick upwards
to the sidewall of the lead.
Figure 11. Effects of PCB Pad Type
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Lead Toe Non-Wetting
Trim and Form Process
The lead frame package undergoes a trim and form process to cut and form the leads. Due to the mechanical nature of
this process, some degree of exposed copper is allowed, especially on the lead toe where a direct cross section of the
lead is performed, completely exposing the base copper metal of the lead. Having copper exposed makes the lead tone
susceptible to copper oxidation, which results in poor solderability due to non-wetting. Similarly, some portions of the
NiPdAu plating are smeared on the lower portion of the toe, allowing solder to wet depending on the smearing coverage.
For pure Sn lead finishes, similar phenomena occur, but the pure Sn plating dissolves into the SAC solder paste and
manages to cover the entire lead toe as shown in Figure 13.
Figure 12. Lead Toe Non-Wetting
Figure 13. Lead Toe Non-Wetting
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Evaluation of NiPdAu Lead Finish Flat Lead Package TSOP 28
Previously in this paper, various aspects of the board mount process were discussed in relation to their effects on lead
sidewall wetting and lead toe wetting. This section summarizes an evaluation performed to support the aforementioned
solutions.
This evaluation uses a TSOP 28 Flat Lead Package with NiPdAu finish and was done on an actual board mount facility
to accurately simulate the surface mount conditions. Following are the details of the evaluation.
Key Factors Identified Affecting Wetting and Solderability

Stencil aperture opening

Reflow profile

Pick and place force
Design of Stencil
The test stencil was designed to accommodate five different stencil aperture openings for an individual panel on the
PCB so that constant print parameters applied for various stencil designs to easily identify which aperture would bring
the best results.
Figure 14. Customer PCB
Stencil aperture openings were designed based on a package lead to PCB pad to stencil aperture opening dimension
analysis and factored in effects of a narrow aperture width and a shorter aperture length.
PCB Pad Dimensions

PCB pad size: 75 mil x 12 mil

Pad pitch: 21.5 mils
Package Lead Dimensions

Lead width maximum: 10.6 mils

Lead end distance: 535 mils

Pad end distance: 583 mils

Package size: 468 mils
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Lead to PCB Pad to Stencil Superimposition
Figure 15. Lead to Pad to PCB Superimposition
New Stencil Aperture Dimensions Per Panel Versus Old Stencil Aperture for 10.6-Mil Lead Width

Original stencil aperture size: 73 mil x 10 mil (existing stencil)

Test stencil aperture size1 (S1): 60.0 mil x 15 mil

Test stencil aperture size2 (S2): 57.5 mil x 15 mil

Test stencil aperture size3 (S3): 56.0 mil x 14 mil

Test stencil aperture size4 (S4): 54.0 mil x 13 mil

Test stencil aperture size5 (S5): 53.0 mil x 12 mil
Design of Reflow Profile
Two types of reflow profile were used in this experiment, the current slow ramp profile and the fast ramp profile, as
shown in Figure 16.
Slow Ramp Profile
Figure 16. Slow and Fast Ramp Profiles
Reflow profile temperature settings per zones set at low, medium, and high:

Low: Typical reflow profile used at board mount

Medium: Faster ramp with higher peak temperature settings of 255 °C

High: Fastest ramp with higher peak temperature settings of 255 °C

Actual settings for various reflow zones for DOE:
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Design of Pick and Place Force
When modified, placement force can dictate how much squeeze-out of solder paste will happen during package
placement. This paste squeeze-out will mechanically force sidewall creeping of the paste.
Consequently, during reflow, the paste coverage on the sidewall of the lead will more easily wet and cover the sidewall
during reflow.
The following settings were selected based on process capabilities, as shown in Figure 17.

Low: 150 grams, standard SMT setting

Medium: 300 grams to double the effect

High: 500 grams to test the maximum load
Figure 17. Placement Force Settings
Evaluation Matrix
Table 8 is the summarized evaluation matrix to check the interaction of the three key factors mentioned.
Table 8. Evaluation Matrix
Figure 18 shows the stencil aperture orientation per panel in the PCB.
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Figure 18. Stencil Aperture Orientation per Panel
Experiment Output Responses
Visual Mechanical Inspection and Cross Section (SEM)

Lead toe non-wetting: Insufficient to zero solder fillet formation on the toe of the lead

Lead bottom non-wetting: Insufficient to zero bottom fillet on the bottom surface of the lead

Lead sidewall non-wetting: Zero solder fillet on the sidewall of the lead

Low heel non-wetting: Insufficient to zero solder heel fillet on the lead

Solder paste and flux pooling on bottom of package
Figure 19. Solder Joint Anomalies
Lead Push Test (on Best Leg)
The lead push test is a mechanical test in which the package is separated from the board mechanically through a force
gauge jig while the PCB is mounted on a bench vise by a custom jig. The methodology is primarily manual and may
incur noise depending on the speed of shear, which is done manually. In this evaluation, the shear values were noted
with the break modes as the key indicating factor for wetting. Figure 20 shows how the lead push test works.
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Figure 20. Lead Push Test
The lead push test is performed to document the shear mode for qualitative analysis. No shear values are used to gauge
the results of the push test. Key break modes are as follows:

Pad rips off from PCB: Suggests very good wetting

Lead rips off from bulk solder: Suggests good wetting with good trace of solder on lead

Lead rips off clean from solder: Suggests poor wetting without any trace of wetting
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Data and Results Summary
Visual Mechanical Inspection (Optical)

Leg 1 remains the most inferior of the settings.

Leg 3 and Leg 4 improved when the stencil aperture was widened and reduced in length.

Leg 4 showed consistent satisfactory wetting when reflow was set at median.

Leg 5 showed the most consistent very good wetting.

Leg 6 to 7 had close results with leg 5 when the pick and place parameter was increased.
Figure 21. Soldering DOE Results
Visual Mechanical Inspection (Cross Section and SEM of Best Leg)

Good toe fillet height

Good lead sidewall wetting (offset placement becomes critical when aperture width decreases)

Good heel fillet

Good bottom solder fillet

Zero flux pooling
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15-mil
Aperture Width
15-mil
Aperture Width
14-mil
Aperture Width
13-mil
Aperture Width
12-mil
Aperture Width
Toe,
Bottom
and Heel
Fillet
Sidewall/
Bottom
Fillet
Lead Push Test
Figure 22. Cross Section and SEM Results

Lead push strength increased significantly (although should be used for trend only).

Significant improvement in break mode when fast ramp reflow and increased stencil aperture width were used.
Table 9. Lead Push Test Summary
Summary of Results

Visual mechanical inspection showed good wetting on all surfaces of the lead.

Lead sidewall was achievable when stencil aperture was modified to force an overprint of paste on the sidewall, as
it allowed solder paste wicking on the lead sidewall.

Reflow profile improved wetting on all surfaces of the lead.

There was zero flux pooling when the length of the aperture was shortened away from the bottom of the package.

Lead push test yielded good consistent break mode where majority was PCB pad lift.

Pick and place parameter showed slight improvement on wetting, but contribution was low.
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Best Practice Recommendations for Soldering NiPdAu
Based on the evaluation and root cause analysis, a slow ramp profile yields inferior wettability for NiPdAu finish. The
most compelling parameter is the reflow profile and stencil aperture design, which yielded drastic improvements in lead
sidewall wetting and toe wetting.
This paper recommends the following settings to achieve maximum wettability for Cypress products with a NiPdAu finish.
Reflow Process
It is recommended to use a fast ramp profile to avoid flux exhaustion, which contributes to solder sphere oxidation.
Board mount personnel must work with the solder paste supplier to learn the correct flux activation temperature and set
the fast ramp somewhere at the end of that activation temperature. Too much exposure or soak time at a higher
temperature than the flux activation would cause flux exhaustion. Following is a sample reflow profile setting for a solder
paste whose flux activation plays on the 150–180 °C range. These values may be used only as a reference as reflow
conditions may vary depending on equipment.
Figure 23. Fast Ramp Reflow Profile
Profile
Z1
Z2
Z3
Z4
Z5
Z6
Z7
Min
120
160
170
170
170
255
255
80 cm/min
Conveyor Speed
Max
120
160
170
170
170
255
255
90 cm/min
Proper profiling and maintenance of profilers needs to be observed at all times. For cases where N2 purge is available,
use it as it drives out the unwanted oxygen within the convection reflow and reduces the risk of oxidation on the solder
spheres of the solder paste and leads. For best results, a design of experiment using these parameters as a baseline is
recommended.
Stencil Aperture
Solder paste reaching the sidewall of the lead is key in promoting sidewall wetting for a NiPdAu finish as seen in the
experiment. It is highly recommended that the solder paste be overprinted and that it cover the width of the lead without
sacrificing solder bridging. As a rule of thumb, a minimum 13 percent increase in stencil aperture width is required
against the lead width dimension to create an overprint without solder bridging.
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Figure 24. Stencil Aperture Design
PCB Pad Geometry
Like the stencil aperture pad, geometry plays a role in achieving good lead sidewall wetting. The key still is that the pad
should be at a minimum 13 percent wider than the lead width, and the stencil will follow a minimum of 1:1 ratio with the
pad width. The maximum value for increasing the pad width is dependent on the pitch of the leads to avoid solder
bridging. It is recommended that a design of experiment with varying pad geometry be initiated to optimize the paste
print settings with lead sidewall wetting as a response for the NiPdAu finish.
Best Practice
Common Practice
Lead Width
Lead Width
Pad Width at minimum
is 13% wider than lead
Pad Width is 1:1 ratio
at maximum setting
Figure 25. PCB Pad Width Design
The PCB pad length is also critical during the design stage. It should not exceed the inner perimeter of the molded
package area, as solder paste may creep at the bottom of the package due to the pad extrusion. It is recommended that
the PCB pad length be reduced in such a way that it limits the coverage of the solder paste from getting to the bottom of
the plastic package. As a general rule of thumb, the inner edge of the pad should not exceed the perimeter of the
package.
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Figure 26. PCB Pad Length Design
If the PCB pad length extends to the package perimeter, printed solder paste touches the bottom side of the package
during placement. The capillary effect pulls the flux on the solder paste to pool on the bottom side of the package. This
phenomenon starves the solder paste area on the lead, allowing the solder beads on the paste to oxidize during reflow.
Oxidized beads cause cold solder joints, spot non-wetting, and solder beading, as shown on the optical photo and SEM
photo. See Figure 27.
Figure 27. Effects of Very Long Pad Length Extending to Bottom of Package
PCB pad width and length are a critical geometric aspect of the pad design, but the type of pad is also important when
trying to achieve good solder wetting and fillet on the lead sidewall and other lead surfaces. SMD design is
recommended as a best practice, as it will not restrict the solder paste from squeezing out to wick to the lead sidewall,
as shown in Figure 28.
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Figure 28. Effects of SMD over NSMD on Solder Joint
Pick and Place Parameter
The placement parameter can be used as a quick workaround when a new stencil or pad design is not available.
Combined with a fast ramp reflow profile, it can achieve maximum wetting, as seen in the evaluation. As a general rule
of thumb, the maximum placement that would yield the most optimum paste squeeze-out is the objective when using this
parameter. A design of experiment is highly recommended to achieve maximum squeeze-out and promote good lead
sidewall wetting.
Figure 29. Effects of Placement Force
Conclusion
A NiPdAu lead finish based on the process optimizations is easy to solder provided the best practices recommended are
used as a baseline. With simple general guidelines and best practices, users can easily solder this type of finish
consistently and at the same time enjoy the long-term advantages of a NiPdAu finish, which include zero whisker and
environmental friendliness.
In the various practices mentioned in this paper, the solder paste overprint on the lead sidewall significantly contributes
to achieving good lead sidewall wetting, toe wetting, and heel wetting. This aspect combined with the second most
important aspect, which is a fast ramp reflow, generally provides the best soldering results for NiPdAu.
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This paper also notes that there are two approaches in achieving side overprint of paste. One is by modifying the stencil
aperture width, making it larger than the lead width, and another is by modifying the PCB pad geometry. Both ways have
their pros and cons in terms of lead time to application but will generally yield similar satisfying soldering results.
This white paper serves as a guideline that surface mount personnel can use as a baseline when trying to optimize their
SMT line and make it more robust when soldering NiPdAu finishes. Other aspects that involve package optimization and
solder paste selection, such as halogenated type, are also options that are viable depending on the end-user application.
In the end. it is the SMT engineer’s responsibility to diligently create a design of experiment to maximize the effects of
the predefined guidelines of this paper and suit the SMT conditions. The user needs to evaluate and validate the
information provided in this paper to achieve soldering success.
References
1.
PC/EIA J-STD-002A, Joint Industry Standard, “Solderability Tests for Component Leads, Terminations, Lugs,
Terminals and Wires,” October 1998
2.
IPC-A-610E, “Acceptability of Electronics Assemblies,” April 2010
3.
“Qualification Report of Nickel/Palladium/Gold-Finish for Integrated Circuits,” Mike Burke, Bo Chang
4.
“Factors that Influence Side Wetting Performance,” Donald C. Abbott, Bernhard Lange, Douglas W. Romm, and
John Tellkamp
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Soldering Techniques for Gull Wing Packages
Using Nickel-Palladium-Gold Finishes
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October 2014