Practical recommendations for production sampling and viewing various part types.
Ed.: This is the second of a two-part article; part one was published in December.
Various
procedures are suggested as a standardized approach for x-ray
inspection of specific component types on assembled PCBs in a
production environment. It is recommended that a sample of two boards
be examined from each batch for normal in-process inspection.
Inspection should also occur when changing temperature profiles or when
setting new product introduction profiles. This information can be
valuable for future reference, particularly when using new component
types.
Voiding is a common fault detected by x-ray inspection.
Voids are the presence of air bubbles or other nonmetallic material
trapped within the solder joint. Voiding in production is usually
caused by either a fault in achieving the peak reflow temperature or
the board spending insufficient time above the liquidus temperature of
the solder paste. The question of what level of voiding is acceptable
is hotly debated. SnPb and Pb-free solder joints will almost always
have some level of voiding. However, the size and quantity of voiding
seen will depend on the sample and the x-ray system’s detection
sensitivity.
X-ray inspection should be conducted after rework
of any area array devices, land grid array (LGA) or quad flatpack
no-lead (QFN) components. This will quickly confirm rework quality
nondestructively and help establish correct rework process profiles.
BGA voiding.
The presence of voids and their quantity are process quality
indicators. In particular, if the voiding level over time tends to
increase or decrease, this is often seen as a sensitive indicator of
changes in the production process. BGA void percentage is calculated on
an x-ray inspection system by totaling the lighter pixels (void pixels)
within each solder ball and presenting them as a percentage of the
total number of pixels within the entire solder ball area. This
calculation is achieved by looking at the ball from the top-down, but
recognizes that it gives values based on 2-D data for 3-D spheres. The
calculation requires setting suitable greyscale thresholds to define
the solder ball outline and void pixels.
Two primary types of
voiding can be present: process or “bulk” voids and interfacial voids.
Process voids are often relatively large and positioned in the middle
of the ball or associated with one of the solder ball interfaces, pad
side or device side. Process voids often can be reduced in size by
slightly increasing the time above liquidous for a few seconds, thereby
permitting volatiles to fully escape during reflow. Interfacial voids
are smaller than process voids and are typically associated with the
solder ball interfaces, most often to the pad or device. The position
of these voids can be confirmed by observing the solder ball at
different oblique angle views within the x-ray and observing how the
voids move relative to the surrounding materials.
Limited
greyscale capability of earlier x-ray systems and detectors was
insufficient to see the interfacial voids; only the larger process
voids could be seen. It is suggested there always has been some level
of interfacial voiding in BGA solder joints, but this could not be
detected with older systems. Newer digital x-ray systems can detect
more than their older counterparts because of advances in x-ray tube
technology (
Figure 1).
It
is suggested within the industry that any voiding at a BGA device
interface may be more detrimental to overall joint quality than if the
voiding is in the bulk of the solder ball or at the pad interface. The
known situations where interfacial voiding may cause production issues
are within the failure mode described as “champagne” voiding. It is
suggested substantial interfacial voiding at the pad interface from the
effects of the board surface finish can cause champagne voiding.
Digital x-ray inspection, with its greater greyscale sensitivity
compared to its analog counterpart, can distinguish differences between
interfacial voiding and bulk voiding and emphasize the presence of
non-reflowed or open joints (
Figure 2).
‘Wetting indicators.’
To assist BGA inspection in confirming that good reflow has taken place
or highlight the presence of open joints, it is possible to modify the
BGA termination pads during design so they include “wetting
indicators.” Wetting indicators make BGA solder joint inspection much
easier because a wetting indicator is a minor, deliberate change to the
BGA joint shape that can be easily seen in the x-ray image. For
example, this could be achieved by designing pads in elliptical forms
instead of circular shapes. Alternatively, an area of the trace from
the mounting pad could be left exposed from the design of the solder
mask. The result of either modification is to permit the solder to
deliberately wet away from the main pad in a controlled manner during
reflow, causing a characteristic solder ball shape compared to
“standard” reflow. This characteristic wetting shape will be obvious
within the x-ray image and make any non-reflowed joints much easier to
identify.
Not all pads need be defined this way. It is
sufficient to have a single example of this type of reflow indicator
near each corner of the BGA together with a few in the center of the
device. Failures are most likely to occur in these two areas. This
approach will give confidence the reflow process is efficient.
X-ray
inspection systems should make void percentage measurements on BGAs
automatically so that any trend in the voiding level over time can be
identified. Measuring the void percentage level within BGAs not only
ensures current production quality meets necessary standards, but also
is able to monitor any subtle changes that may occur in the process.
With LGAs and QFNs, care should be taken to inspect the center area of
these devices because they are being used to dissipate heat. Excessive
voiding in this area will mean a reduction in the die attach area,
which results in a reduction of the heat dissipation capacity of the
device.
QFP inspection should begin from one corner of a device and scan around all four sides (
Figure 3).
Attention must be paid to the presence of heel fillets, side fillets
and, if possible, toe fillets on gullwing leads. Toe fillets will not
always be visible during inspection due to the lack of wettable area on
the lead tip. Heel fillets should be consistent in size and are the
area that will be subject to stress during mechanical or thermal
cycling. Voiding may also be present under the lead. Heel fillets will
be visible on all gullwing leads, ideally with toe fillets present, as
well as J-lead terminations having fillets on the back and front of the
lead.
Inspecting discretes.
X-ray inspection of passives should be left until last, as they
normally will be satisfactory if all other parts are confirmed as
completely reflowed. As a result of their small mass, they are likely
to reflow before any other component and therefore less likely to
exhibit voids. However, they may exhibit voiding following double-sided
reflow.
When a chip component has successfully been soldered,
there will be evidence of a fillet on the end terminations and possibly
on the side terminations. The solder joint area under the chip
termination should also be assessed.
Small actives such as
SOT23, SOT89 and SOIC devices are also less likely to exhibit poor
reflow. These devices’ low mass makes their complete reflow relatively
straightforward. It is possible to see voiding on SOT89 components on
the center termination. As the size of transistor packages increases,
their power handling tends to increase; therefore, any substantial
voiding in the termination may affect heat dissipation from the device.
Advanced packages. Inspecting semiconductor packages
places a greater demand on x-ray inspection and provides a unique set
of challenges. Traditional analysis using 2-D x-ray imaging is often
limited with advanced package types because the multiple layers within
the device are seen at the same time. This can be confusing since
multiple dies and multiple layers of wire bonds appear to overlap each
other in a 2-D image.
Another challenge of packaging
inspection is interfacial and plating void detection within microvias.
While difficult to detect with normal 2-D x-ray imaging or
cross-sectioning, these defects can be determined with computerized
tomography (CT) inspection (
Figure 4).
Recent
advancements in CT technology have greatly improved imaging speed and
resolution of the most detailed features. Because of these
improvements, CT has become an ideal inspection methodology for complex
3-D packages since it generates a 3-D model of the entire electronic
package. The resulting 3-D model can be viewed in real-time so that
interconnections normally obscured by other joints or components within
the package can be diagnosed, ensuring complete package inspection.
As
component technology continues to drive the miniaturization of devices,
the need for x-ray inspection becomes more apparent. Traditional means
of inspection are still necessary, yet are less effective than x-ray
for detecting and resolving manufacturing defects with today’s advanced
packages and area array devices.
Dr. David Bernard is product manager x-ray systems at Dage Precision Industries (dage-group.com); d.bernard@dage-group.com.