Impact of Quad Flat No Lead Package (QFN) on
Automated X-ray Inspection (AXI)
Andy Pascual
Agilent Technologies
Singapore
andy_pascual at agilent dot com
Chwee Liong, Tee
Intel Corp
Kulim, Malaysia
Copyright © 2007 IEEE.
Reprinted from 2007 ITC International Test Conference, Paper 14.3
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Impact of Quad Flat No Lead Package (QFN) on Automated X-ray Inspection
(AXI)
Chwee Liong, Tee
Test Development Engineering (Malaysia), Intel Kulim, Kulim, Malaysia
Andy Pascual
Measurement Systems Division, Agilent Technologies Singapore (Sales), Singapore
Abstract
QFN is increasingly being used on wireless cards,
handhelds etc. However, QFN unique solder joints pose
great challenges for AXI. This paper discusses lack of
industry specification for QFN inspection, how AXI
methodology was improved to detect QFN solder joint
defect, design for inspection and future work.
1.
Introduction
Today, in the electronics sector the drive towards
smaller, lighter and more added features are becoming a
norm. As such, QFN (Quad Flat No Lead) packages are
increasingly being used on cell phones, handhelds and
other portable equipments [1]. Figure 1 shows the
volume of QFN will overtake other conventional
packages like leaded QFP and array packages in 2010.
electrical performance characteristics, ease of use and
small footprint [2].
X-ray has been widely used in printed circuit board
assembly (PCBA) processes for decades now because of
its ability to inspect “hidden” solder joints which are
either under a component package or shielded by larger
components, such as an RF shield. As the complexity
and density of boards has increased and test access points
needed for in-circuit test reduced, the Automated X-ray
Inspection (AXI) machine has emerged as one of the
primary test solutions for PCBA manufacturers. AXI not
only can detect gross structural solder defects, such as
opens, bridging, and tombstones, but it can also flag out
solder joint reliability issues such as insufficient or
excess solder and solder voiding. AXI also acts as an
important “gate” to reduce the number of test escapes
reaching end customers and provides information that
helps improve front of line processes.
However, as AXI is a vision inspection it is susceptible
to high false calls if test program is not fully optimized.
High false calls may lead to escapes. A false call is
defined as a defect called by the inspection machine but
verified as not a true defect by a repair operator while
escape is defined as falsely accepted by the AXI but later
found as true defect by downstream test such as electrical
test.
Figure 1 Tremendous growth in QFN volume in the future
(Source: PRISMARK Nov 2006)
One QFN supplier mentioned that they have shipped a
billion QFN packages. According to the QFN supplier,
the popularity of QFN is due to its superb thermal and
Since AXI monitors the health of the process, large
variation in process will also cause extremely high false
calls. As such, high false calls will only decrease the
effectiveness of AXI implementation in the
manufacturing line. This is because the human operators
are required to decide the failure images called by the
AXI are real or just false calls. If the false calls are too
high, this will tire the human operator who may not go
through all the failure images flagged out by AXI. If this
happens, there is a potential risk of escapes from AXI
station to the downstream stations like In-Circuit Test
(ICT) or Functional Test.
Figure 2 shows typical QFN that is being used in the
industry. QFN is quite similar to QFP (Quad Flat Pack)
but with no leads and large pad in the center. Similar to
BGA (Ball Grid Array) packages, QFN also forms solder
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joints underneath the package. Since the solder joints are
hidden, x-ray can be used to inspect for any
manufacturing defects as well as to feedback any process
characterization data to improve the process.
Figure 2 QFN package (Source : IPCA610D)
Thus, with the wide proliferation of QFNs today, it is
important to understand its impact on AXI. Hence, this
paper will discuss in detail the challenges of QFN on
AXI, the improvements made to increase its detection
rate, design for inspection (DFI) and future work. The
AXI mentioned in this paper uses 3D laminography
technology. The study presented here was based on the
QFNs populated on wireless boards.
2.
QFN challenges
2.1 Lack of Clear Industry Standard
For AXI to act as an effective process monitoring station,
it must be supported by a set of clear acceptance criteria
of good and bad solder joints agreed upon by AXI and
process engineers. Without this, results from AXI will
often be challenged. For example, why certain solder
joints are being failed as insufficient.
Since IPC A610D standard is widely adopted by industry
to define assembly process, this paper refers to it for
QFN acceptance criteria. Table 1 extracted from IPC
A610D standard describes the acceptability of QFN
solder joints.
Table 1 IPC A-610D Requirements on QFN
However, the standard is too generic to be a useful
requirement for AXI. For example, minimum side joint
length is specified as not a visually inspectable attribute;
solder fillet thickness as wetting is evident and solder
coverage of thermal pad as not a visually inspectable
attribute. These requirements do not take into
consideration that x-ray is able to inspect the nature of
the solder joints and wetting is evident does not describe
how much solder wetting is considered acceptable.
Figure 3 shows the variation of QFN solder joints under
X-ray. The ones highlighted shows solder is present but
whether they are sufficient to be considered reliable is
unknown. Unless there is a clear specification on the
QFN solder joints acceptance, there will always be
argument on whether these solder joints should be
accepted or considered as insufficient.
Figure 4 shows extensive voids found under the package
body which acts as a thermal pad. As void in BGA
always provoke serious discussion on the health of the
process, should we also be concern on the QFN thermal
pad voiding? Since QFN boasts superior thermal
performance compare to other packages, does the
extensive voids decrease its thermal conductivity; hence
causing thermal damage to the device?
Figure 5 is an AXI image which shows voiding exist on
the terminal solder joints. (Note: The term terminal is
used instead of lead as QFN is a no lead package). On
some terminals, there are even multiple voids exist. To
verify whether the solder voids found on the terminals
are real, the sample was cross sected. Figure 6 shows the
cross section photos of the voids tally with what was
shown by AXI.
Again, the same question is posed as to how much void
for QFN is acceptable is not mentioned in the IPC
A610D standard.
Although recent IPC report [3] concluded that there is no
evidence that solder joint voiding has any significant
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impact on solder joint reliability, can the same be said on
QFN? The test vehicle used in the study did not include
QFN. Besides that, the voids as shown in Figure 6 are
very large should raise eyebrows as its impact to the
solder joint reliability.
Therefore there is a need for the industry community to
drive for a much clearer specification
on what is considered as acceptable QFN solder joint.
Although x-ray inspection reveals huge amount of
information on the quality of QFN solder joints, lack of
industry standards hamper its usage as effective
monitoring gate.
Thus, one way to define the acceptance criteria for
insufficient and voiding is to select the worst case for
insufficient and voiding and then subject them to quality
and reliability tests such as temperature cycling. For
example, if the worst case thermal pad voiding is 30%
and fail after temperature cycling, then 30% will be the
thermal pad voiding acceptance criteria of the QFN
package. On the other hand, if the temperature cycling
test passes, then 30% thermal pad voiding is still
considered acceptable. This effort is essential to avoid
false rejection from AXI.
Figure 4 Center thermal pad voiding
Pin 9
Pin 11
Figure 5 Voiding observed on QFN terminal solder joint
Void
Pin 9
Figure 3 Potential insufficient solder joints
Void
Void
Pin 11
Figure 6 Cross section of pin 9 and 11 showing massive
voids on the terminal solder joints
2.2 QFN Solder Joint Open Detection
AXI looks for a set of consistent features of a good
solder joint type and sets acceptable criteria on them. If
any of these criteria is not met, it will be flagged out as
defect. A good gullwing joint, for example, has high
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heel fillet that usually extends above the lead thickness, a
low toe fillet and a thin center. Center is the solder
underneath the lead between heel and toe fillets. One
condition for open joint used by AXI is how high the
heel fillet compared to the center. Measured center
thickness is subtracted from measured heel fillet
thickness to quantify this magnitude. If there is no heel
fillet formation, the measurement difference is small and
sometimes zero or negative value as illustrated in Figure
7.
(a)
(b)
Figure 8
(a) X-ray image (b) QFN with outer edge
termination
(a)
(b)
Figure 7 Gullwing heel minus center, (a) good and (b) open
joints
Heel minus center and other criteria that were derived
from good solder joint features are applicable only
because of standardization of SMT packages. Commonly
used packages have established standards like gullwing,
chip, BGA, CGA, PTH, Jlead, and SMT Connectors.
They have consistent solder joint features that can easily
be learned by AXI.
These good QFN solder joints have high toe fillets and
consistent distance from the heel and center.
This is due to the terminal pads extending all the way up
the sides of the package. This QFN category is called
“Outer-Edge termination” where IPC J STD-001D
requires a toe fillet formation as shown in Table 2 on
Minimum Toe (End) Fillet Height.
Table 2 IPC J STD-001D Requirements on QFN
When QFN was initially inspected under x-ray, its
terminal solder profile looked like a reversed gullwing
solder joint whereby the toe fillet is higher than the heel
fillet.
Therefore, AXI users normally reverse the QFN CAD
pin orientation during test program setup to detect open
by using the concept of heel fillet thickness minus center
thickness. Open solder joint will show a negative value
and vice-versa. Combining this concept with fillet length
and fillet thickness, insufficient solder could be detected
reliably.
However, it was found later that this open detection was
not always effective for all types of QFN. In the
following paragraphs it will be discussed why the basic
techniques for open were not always applicable.
Depending on packaging technology, there are QFNs
that have terminal pads extending outside its body like
the one shown in Figure 8.
Paper 14.3
Toe fillet can vary in formation and size after reflow.
This is shown in Figure 9a to 9d. Depending on the
surface mount process, toe fillet can be very dense;
forming a large convex shape as shown by Figure 9a,
Figure 9b, and Figure 9d or just nice wetting profile in
Figure 9c. This feature can be used by AXI to separate
good solder joints from solder joints that do not have toe
fillet formation.
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(a)
(b)
Figure 10 Bottom-Only termination QFN
(c)
(d)
Figure 9 QFN with outer-edge termination of different toe
formation (Source: Amkor Paper on Board Assembly and
Reliability Considerations for QFN)
AXI allows users to set the allowable deviation from a
nominal value of a certain solder feature before flagging
out the solder joint as failure. As such, no toe fillet or
just slightly low toe formation can be set to trigger an
open failure or insufficient solder. Insufficient toe,
therefore, can be easily detected. However, since neither
IPC A610D nor IPC J STD-001D specifies the required
toe fillet height at certain percentage of the termination
height, the definition of defect is dependent on the
specification from the PCBA manufacturers.
Figure 10 shows another type of QFN which does not
have outer-edge terminations. This type of QFN is not
required to have toe fillets. The toe fillets for this type of
QFN may form after reflow but the size and height is not
consistent. Figure 11 shows the terminals of this QFN
type having variations from no toe fillet at all to thicker
than the heel fillet. None consistency of a good joint
using toe fillet as a basis leaves AXI to inspect only the
heel and the center features. This type of QFN is called
“Bottom-Only Termination” which poses the greatest
challenge to AXI.
Besides that, it is difficult for visual inspectors to
accurately spot defects at the terminals, even with the use
of magnifying instrument without any means to view
underneath the component.
Figure 11 Large variation of toe height for Bottom-Only
Termination type of QFN
One of the inspection approaches for Bottom-Only
termination, which does not rely on toe, is to use heel
fillet slope. This is taking the heel profile and measure
the highest slope along the heel end of the joint knowing
that an open joint will have lower measurement.
Figure 12 shows a heel profile and where its slope is
taken, while Figure 13 shows a comparison between heel
slopes of a good and bare reflow board of one type of
Bottom-Only Termination with 20 by 50 mils pad size
and 30 mils pitch. A bare reflow board was created from
unpopulated board which was paste printed and
reflowed. The bare reflow board was used to simulate
QFN open solder joints. This board was used as baseline
inspection criteria for AXI. Theoretically, AXI should
fail all the solder joints on the bare reflow board.
Furthermore, Outer-Edge termination types may get
categorized as Bottom-Only termination by their
manufacturers. Like one QFN supplier that regards the
presence of toe after reflow only as additional strength
for reliability [4].
Paper 14.3
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Open
Good
Figure 14 QFN toe slope comparison of good and open
joints
Figure 12 Location of heel and toe slope on the solder joint
The result in Figure13 shows heel slope of good joints
was higher than the open joints. However, its variation
was too large which could cause high false calls.
Although measurements for good solder joints did not
overlap open solder joint, the smallest gap between the
two was zero.
However, adding toe slope to heel slope was found to
give a good separation between good and open solder
joints. This is the sum of heel and toe slopes of each
QFN joint, and is shown in Figure 15. With about 10%
gap, this is a promising parameter.
Good
Open
Good
Open
Figure 15 QFN sum of heel and toe slopes comparison of
good and open joints
Figure 13 QFN heel slope comparison of good and open
joints
Figure 14 shows the toe slopes of the same package used
in the previous example on heel slope. Bottom-Only
Termination varying toe slopes gave overlapping values
between good and open. Clearly, this parameter cannot
be used.
As there are many sizes of this package, it was necessary
to test this approach on a smaller size terminals kind of
Bottom-Only Termination. Figure 16 shows the slope
sum comparison of a QFN with half the pitch and pad
size of the former type. There is overlapping between
good and open. It shows that if the threshold is set at
0.7, AXI will have 80% to 85% detection but 15% to
20% false calls.
This slope measurement turned to be an interim solution
for the immediate need of AXI. It was not a perfect
solution but it gave confidence of some extent in
capturing real defects.
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profile from the bare reflow board may not be exactly
similar to the real open sample, actual open sample is
desirable.
Good
Open
Figure 17 shows AXI image of a real manufacturing
process open QFN defect. The QFN was slightly tilted
causing the solder of those five land pads not to wick up
to make contact with the QFN pads. The board was
found to fail functional test later.
Figure 16 Slope sum comparison of good and open joints
for 12 x 26 mils QFN terminals
With the lack of guideline for underneath joints, it
became great challenge for AXI to accept large solder
joint variations without having high false call rate and at
the same time capable of detecting true defects.
Therefore, studies were made to get to a level of
acceptable false call rate and increase the capability to
detect solder defects.
3.
Improvements made to the current
algorithm
The search for the characteristics of a good QFN solder
joint that will differentiate it from a bad joint was not
easy. Different pitch and terminal sizes, package types
and even component suppliers must be considered.
Another requirement is that the surface mount process
must be ready to provide samples of defects.
Figure 17 QFN open solder joint
Figure 18a, is another example of open joint but this time
it was fault injected. A resistor was placed underneath to
tilt the component because the previous open solder joint
sample was not easily reproducible. A picture of a crosssectioned open joint is shown in Figure 18b with a
graphic drawing on the right to help illustrate the real
scenario.
AXI had to deal with changes made by these variations
in solder thickness profile, joint size and shading. It must
be noted that QFN package from different suppliers used
gives different shading during x-ray inspection which
may contribute to false failures if not considered earlier
in the test development process. A change in level of
background shading is caused by the different materials
used by component manufacturers.
Among the common types of solder joint defects, open
joint was the big challenge and so this will be the
discussion in the following paragraphs of this section of
the paper.
For every process change, a bare reflow board was
produced in order to get samples of open joints.
Therefore, all open joint measurements in the charts
shown here were from bare reflow board. The parameter
settings used in getting those measurements were first
validated using real defect examples. As the solder joint
Paper 14.3
(a)
(b)
Figure 18 (a) X-ray image of open solder joint by tilting QFN
using chip resistor (b) Cross-section showing open solder
joint
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Having actual open solder joint samples helps the R&D
from the AXI supplier to enhance the algorithm.
The defect detection techniques added for open are heel
sharpness, joint (fillet) width, center region thickness,
and upward curvature. These are necessary to cover the
various types and configurations of QFN.
Heel sharpness is a variation of heel slope that computes
the curvature of the heel edge to take advantage of heel
regularity as compared to toe. It helps find real defects
but not all. Its measurements of good joints are
compared to the open (bare reflowed board) in Figure 19.
(a)
(b)
Figure 21 Cross-section view along the pad of showing
(a) good and (b) open QFN solder joint
Open
The charts in Figures 22 and 23 show the separation of
good from open joint measurements of each parameter,
center thickness and fillet width.
Good
Good
Figure 19 Heel sharpness of a good joint is charted against
open joints from bare reflowed board
The center region thickness and fillet width were
considered and gave a very good separation of good from
open joints. Figure 20 illustrates why these two features
can be used. A good joint is always flat because it is
sandwiched by the package terminal and the land pad
while an open forms a gentle sloping hill across the pad.
Cross sectioned good and open joints, with their
corresponding AXI images, showing real scenarios in
Figure 21a and 21b.
Open
Figure 22 Comparison of center thickness between good
and open solder joints
Good
Good
Open
Open
Figure 20 Feature of good and open QFN fillet across the
pad
Figure 23 Comparison of fillet width between good and
open solder joints
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The upward curvature is fairly effective on smaller size
QFN terminals. Figure 24 charts its values, good and
open. This data is from 12 by 26 mils QFN terminals.
Upward curvature is measuring the concavity formed by
the joint along the pad, from heel to toe. A good solder
joint normally has a flat region while an open solder joint
region has a downward curve.
Open
Figure 25 Good solder joints with visible fillet as
highlighted
Good
Figure 24 Comparison of upward curvature between good
and open solder joints
Among these new parameters, fillet width measurement
is the most effective and easiest to set because it works
well with today’s commonly used terminal sizes.
However, its open joint measurements are too close to
good joints and it is not useful on smaller size joints
where upward curvature shows good results.
For that reason, it is recommendable to use multiple
parameters looking at different features to get the best
coverage and acceptable false call rate.
4.
Figure 26 Solder joints from bare reflow board with no
visible fillet
Figure 27 shows clear separation between good and bad
solder joints when the measurements are plotted
together. This enable the test programmer to set a robust
threshold that will able to distinguish between good and
bad solder joints.
Design for Inspection (DFI)
In order to have effective AXI implementation, it must
be able to distinguish good and bad solder joints
accurately. This means that solder joint formation of bad
joints must look much different to good solder joints.
With this in mind, the fillet formation of the solder joint
at the end of the terminal can be used as an attribute to
clearly distinguish between good and bad solder joints.
Figure 25 shows x-ray image of good solder joint
formation where there is visible fillet formation while
Figure 26 shows x-ray image of bad QFN solder joints
formation where there is no visible fillet formation.
Paper 14.3
Figure 27 Clear separation between good and bad (bare
reflow board) measurement of the solder joints
Therefore, board designers when choosing QFN could
choose QFNs with side terminal so that a fillet can be
formed at the end of the terminal. This will aid AXI in
making more accurate calls.
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5.
Agilent team: Wong, Sam TS; Rivas, Rich; Jessen,
Jeremy R, Koczera, Barbara; Liu, Gary
Summary
The paper highlights the increasing trend of QFN usage
in the electronics sector due to its various advantages in
terms of size, weight and performance. However, the
current industry standard on QFN is still inadequate. This
has hampered the effectiveness of AXI implementation
to inspect on QFN.
Also not forgetting Inetest AXI test support engineers.
8.
References
Also mentioned in the paper is how QFN solder joints
present great challenges to current AXI inspection
methodology. However, after extensive effort was done
to characterize the behavior of QFN solder joints had
helped to improve the AXI detection rate of bad solder
joints.
[1] C.James “Big Growth for Tiny Packages”,
www.purchasing.com/article/CA6361109.html
This paper also presented design for inspection for QFN
in order for effective AXI implementation in the
manufacturing line.
[3] IPC Solder Products Value Council, “Round Robin
Testing and Analysis of Lead Free Solder Pastes with
Alloys of Tin, Silver and Copper,” IPC Solder Products
Value Council Final Report, July 2005.
6.
[4] S. Ahmer and K. WonJoon, “Board Level Assembly
and Reliability Considerations for QFN Type Packages”.
Future Work Needed
As QFN is a type of low Z height package, the solder
joint formations do not have clear distinction between
open and good solder joint. It is predicted more low Z
height packages are coming as product getting thinner
and smaller. As such, this is going to push the AXI
technology and inspection methodology to the limit.
[2] www.amkor.com/news/PressReleases/
showpr.cfm?id=244
Product specifications and descriptions in this document
subject to change without notice.
© Agilent Technologies, Inc., 2008
Printed in USA, March 6, 2008
5989-8135EN
In order to protect customer investment on AXI, it is
essential for AXI suppliers to continue developing and
improving the current inspection methods so as to cater
to future packages.
It is also equally important to develop good DFI (design
for inspection) on either the packages or PCB that will
enable AXI inspection to be more effective. Normally,
this effort will require close collaboration work among
AXI suppliers, package suppliers and OEMs.
More characterization work is also needed to define the
acceptability criteria of the QFN solder joints.
Finally, industry groups like IPC need to update its
current standards to reflect the changing needs and
advances in the PCBA industry.
7.
Acknowledgements
The authors would like to acknowledge the following
people to make this project successful:
Intel team: Chin Chuan, Ng; Tzyy Haw, Tan; Jui Ang,
Tan; Kook Hsiung, Ooi; Thomson, Barbara A
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