AN1231 Motorola / Freescale Semiconductor, AN1231 Datasheet - Page 7

• Manufacturer Part Number: AN1231
• Description: Plastic Ball Grid Array (PBGA)
• Manufacturer: Motorola / Freescale Semiconductor

devices is in profiling. The profiling thermocouple(s) (TC)
must be placed underneath the package and preferably with-
in an actual solder joint. This requires using thinner TC wire
(30 or 36 gauge recommended) than may typically be used
as to allow the device to solder to the board almost normally
with minimal tilt or non-contacting balls. A suggested proce-
dure for making a profile board is to solder the TC bead to a
ball under the device. The center ball is recommended since
it is likely to exhibit the minimum peak temperature and is
therefore the worst-case position for a cold solder joint. It is
advantageous to also TC the outermost ball on the leading
edge of the device to obtain worst case maximum tempera-
tures. The device is then hand placed onto a prefluxed board
footprint, secured with the minimal amount of polyimide (i.e.,
Kapton ) tape possible and reflowed. The tape is then re-
moved and the board can be used for repeated profiles, as-
suming the TC did not break free and that the device
reflowed somewhat normally onto the board. An alternate
way of more securely fastening a TC is to remove a PBGA
ball with a solder sucker or wick and to attach the bead to the
site using high-temperature solder or thermally conductive
epoxy. Drilling a hole through either the top of the device or
the bottom of the board for subsequent thermocouple place-
ment can also be done successfully. Also, inserting a TC with
thermal grease on the bead under an already mounted pack-
age can sometimes yield sufficient results.
COPLANARITY
coplanarity is currently 0.15 mm (5.91 mils), regardless of
package size or pin count. This coplanarity is defined in the
standard as the maximum distance from the highest ball to a
seating plane formed by the three balls that the package
would rest on if placed on a perfectly flat surface. Any lack of
PBGA coplanarity is a result of two elements, the warpage of
the overmolded substrate and differential substrate pad to-
solder ball tip heights. The substrate warpage is typically the
major contributor to any lack of coplanarity, while the solder
MOTOROLA FAST SRAM
Another difference between PBGA and typical leaded
The JEDEC standard for maximum allowable non-
Figure 5. PBGA IR Reflow Profile Obtained by Placing TCs Underneath Devices
250
225
200
175
150
125
100
75
50
25
0
0
1
2
3
BENEATH LEADING EDGE CORNER DEVICE
BENEATH DEVICE AT BOARD CENTER
TIME (MINUTES)
4
ball heights are relatively uniform. At room temperature, the
typical PBGA has a slight upward curvature, such that 225
pin PBGAs with a 27 mm body size have been measured to
have a worst case coplanarity of around four mils.
dard, scanning all the PBGA bumps and determining the rel-
ative positions of their tips in space. Software that takes into
account the center of mass of the part then determines which
three or more balls the device would rest on and the distance
from the remaining ball tips to a plane formed by these three
seating balls. An automated system, the Model 830B, to do
this has been developed by View Engineering and is avail-
able now (priced in the mid-$100K range). The system also
has the capability to determine the coplanarity to a best fit
plane, ball volumes, the absence of balls and the deviation of
ball tips from the expected x-y grid. A more expensive and
flexible system that also performs printed solder paste height
inspection is also available from Synthetic Vision Systems,
who are affiliated with View. Among others, RVSI is also a
potential equipment supplier.
nology is the fact that, as with any array package, the interior
joints are not visible to be readily inspected. Perimeter joints,
can be readily inspected. High volume users have presented
data showing that the 169 and 225 pin PBGA has one to two
orders of magnitude fewer solder-related defects than the
208 PQFP.
development and for failure analysis. Due to the atomic den-
sity of the lead in the solder joints, standard resolution real-
time x-ray systems may only be useful in determining shorts
and missing or double balls, which are readily observable
(see Figure 6). More subtle joint assembly defects like voids,
total wetting of the motherboard pad (i.e., full or partial
opens) and solder splattering/balling require more sophisti-
cated systems to detect. Very costly x-ray laminography sys-
tems (i.e., Four Pi Systems) can detect such features,
Motorola determines the coplanarity per the JEDEC stan-
One of the perceived drawbacks to using PBGA tech-
X–ray inspection can be used during assembly process
5
SOLDER JOINT INSPECTION
6
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AN1231
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