IPC-7095C-2013.pdf - 第128页
is the maximum recommendations, so reballing a removed BGA and reinstalling it would exceed this number . There- fore, a reballed BGA will always exceed this limit. Many OEMs do not allow re-balled BGAs, or re-use of any…
7.8.8 Defect Correlation/Process Improvement It is
important to use inspection information to control the
manufacturing process to maximize quality and yield. The
manufacturing process leaves a signature (acceptable or
unacceptable) on each component; this signature can be
observed through inspection. The signature can be
observed using methods and tools discussed previously.
In many cases a visual inspection of a BGA is the first clue
to any problems. An operator can look at the edge of the
BGA on all four sides. The distance between the BGA and
the circuit board should appear uniform and the solder balls
should appear consistent in shape.
To directly observe the solder connections under a BGA,
X-ray or optical inspection (endoscope) is necessary. These
methods can be used to inspect for obvious defects such as
bridges and missing balls. They are also useful for charac-
terizing the BGA reflow process. During inspection the
BGA solder balls should be examined for uniformity of
size and shape. In the absence of wetting indicators, the
solder balls should appear round and of equal size through-
out the package. A PBGA with a 0.75 mm diameter ball
prior to reflow will swell to a nominal 0.90 mm diameter
after reflow; a 36% increase. A 10-15% variation in solder
ball area from the center to the edge of the package is nor-
mal, but a larger variation can indicate a problem with the
reflow process.
X-ray inspection of the BGA from an angle is also useful
to examine the shape of the BGA solder ball in the area at
which it makes contact with the land. By changing the
angle of X-ray inspection, the land is shifted so that it does
not obscure the rest of the solder ball. This allows the
operator to inspect the shape of the solder connection as it
forms onto the land to verify that the land is in contact with
the solder ball and the solder is completely wetted to the
land.
Quantitative measurement of the solder bond X-ray image
can be performed using image analysis software. Such soft-
ware is useful, but not required for the inspection of BGAs.
The advantage of the software is in its ability to identify
and display subtle variations in the size and shape of the
solder bond image which is not easily observed by an
operator. These subtle variations are a signature of the
process used to manufacture the part and can be used to
monitor the process and to correct for deficiencies. A num-
ber of signatures can be correlated with known process
deficiencies.
7.9 Repair Processes
7.9.1 Rework/Repair Philosophy
Plastic ball grid arrays
are a forgiving component package. With their self align-
ment characteristics, a ball grid array can be placed as
much as 50% ball off land and when reflowed, the package
will properly align itself. With a controlled process and
appropriate equipment, rework should be kept to a mini-
mum.
There are many rework systems on the market, most of
which use a prism for placement, which allows the view-
ing of the land pattern on the board with the ball image
superimposed over it. Most systems also allow board pre-
heat and stored reflow profiles for many different compo-
nent sites. This section will focus on conditions that should
be met in order to successfully rework a plastic BGA.
There are four main rework functions for BGAs: removing
the component, redressing the site, placing the component,
and reflowing the component. These will be discussed in
the following paragraphs.
7.9.2 Removal of BGA When removing a BGA a deci-
sion has to be made regarding whether the component will
be reballed and used again or simply discarded. Special
considerations need to be made if the BGA will be reballed
and reused, i.e., maximum number of reflow cycles as rec-
ommended by the component vendor. Typically 3 reflows
IPC-7095c-7-56a.b
Figure 7-56 X-Ray Image Showing Warpage in a BGA
The x-ray image (a) shows a distortion of the ball bonds in the lower
right corner; characterized by large elongated bonds. The same
location as viewed through a visual microscope (b) shows warpage
and the solder ball peaking away from the package.
(a)
(b)
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is the maximum recommendations, so reballing a removed
BGA and reinstalling it would exceed this number. There-
fore, a reballed BGA will always exceed this limit. Many
OEMs do not allow re-balled BGAs, or re-use of any com-
ponent. One should check with the customer before pro-
ceeding.
If the component is to be reused and it is overmolded plas-
tic, it will have to be baked. These packages are nonher-
metic and therefore absorb water if they have been out of
a moisture controlled atmosphere longer than the time
allowed in J-STD-020. The baking process drives off mois-
ture and prevents the ‘‘popcorning effect,’’ which is water
vaporizing within the component during reflow and caus-
ing catastrophic failure. Another consideration before
removal concerns the components adjacent to the BGA. If
hot air is used, and if the profile to be used exceeds 4°C per
second, the components surrounding the BGA may need to
be shielded due to thermal shock or secondary reflow.
Polyimide tape or water soluble mask that is commonly
used in the wave solder processes can be used as shielding.
These deficiencies can be resolved with proper equipment
design. See Figure 7-57 for shielding with polyimide tape
to prevent damage to adjacent components when using hot
air for BGA repair.
With lead-free technology, it is recommended to minimize
the temperature delta between the bottom side of the board
and topside of the board during the profiling process for
BGA removal and reattach. The temperature of the bottom
side of the board heating should be increased to minimize
the topside nozzle heat when setting up the profile. This
will minimize exposure of potential delamination of the
bare board or heat transfer to adjacent components.
7.9.3 Replacement
7.9.3.1 Land Pattern Site Dressing
Once the BGA has
been removed from the board, solder will have to be
removed from the land pattern. Solder vacuums and solder
wick works well for removing solder off the lands. Caution
is recommended when using any of these tools because the
land can lift with excessive heat or pressure. Each land
must be completely flat and clean before placing the new
BGA. With the higher temperatures of lead-free alloys, it is
critical to minimize contact with the solder mask dams
between the lands and the vias (dog bone design). The
higher temperatures may increase the potential to damage
this solder mask. It can also be affected by the surface fin-
ish of the bare board. Two variables that can influence the
adhesion of the solder mask to the land are the length of
the solder mask dam (encroached vias are recommended)
and the surface finish of the bare printed boards.
7.9.3.2 Flux Application Although the solder mask
encroached on the via land is over bare copper, adhesion
can be affected when subjected to the surface finish chem-
istry.
Two different methods of flux application can be used:
paste/liquid flux or solder paste; however, using flux only
(in a liquid or paste form) is only applicable for eutectic
BGA reattachment. In addition, some applications require
the addition of solder paste in order to promote a robust
solder joint. If paste flux (also called tack flux) is to be
used ensure that the solder balls are Sn63/Pb37 (eutectic).
Many ceramic BGAs use Pb90/Sn10 solder balls which
reflow at 302°C. If the solder balls are not Sn63/Pb37, then
solder paste has to be used. If solder balls are SN63/Pb37,
Figure 7-57 BGA/Assembly Shielding Examples
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then liquid flux or the paste flux are commonly used when
reattaching a BGA, especially BGAs with less than 208
balls. Flux is applied to the land pattern or the BGA solder
spheres and the BGA is placed over it. One drawback of
this method is the coplanarity issue. If the lands are not
perfectly flat, some of the balls may not touch. Excessive
flux application may also cause bridging between solder
balls. With most plastic BGAs the ball alloy will be one
which will reflow into the solder joint.
7.9.3.3 Paste Application Paste application is the pre-
ferred method, but it does add time to the rework process
as well as tooling cost. Paste can be applied locally with
the use of mini-stencils. When ordering mini stencils for
ceramic packages, the same aperture/thickness should be
used as used for initial attach. This will ensure the reliabil-
ity of the ceramic packages for reworked components.
These stencils can be purchased from many different sup-
pliers and are tailored to fit specific land patterns. A fixture
or tape can be used to hold the stencil in place during
application on the land pattern.
When using these methods, considerations have to be made
for solder paste handling and stencil cleaning. Solder paste
can also be applied to the BGA using a syringe or paste
dispensing frame with other proper tooling. The amount of
solder paste applied should be carefully controlled. When
printing solder paste for CSPs, there may not be enough
room on the bare board for mini-stencils. In this situation
it is a common process to screen the solder paste onto the
package (bottom side of the balls) and then place the mod-
ule onto the board for replacement.
7.9.3.4 Rework Issues Interpackage spacing is decreas-
ing constantly. Even if companies have some Design for
Manufacture (DfM) guidelines for interpackage spacing,
those on the front line of manufacturing know very well
that DfM guidelines are not always followed. So, using
mini-stencils to print solder paste is becoming more diffi-
cult. Also, because a mini-stencil is needed for each size
and type of part, it not only slows down the process but
also quickly adds to the cost of repair.
Using a mini-stencil is not the only issue with ever-
decreasing interpackage spacing. Using different hot air
nozzles for each size and type of part being removed also
adds to the cost and complexity of rework. Additionally,
the potential for melting solder joints of neighboring com-
ponents is a serious concern. In addition to increased inter-
metallic thickness because of unnecessary reflow which
weakens the solder joints, the boards must be baked before
rework, increasing cycle time.
Throughput in rework is very important. BGAs and some
of the larger components can take at least 20 minutes per
component for removal and replacement.
Another important issue in rework is PCA board warpage.
Warpage is partly due to intense local heating for a rela-
tively long time, which is necessary to remove the compo-
nent.
For BGA repair, there are two rework processes in use
today: hot air and laser. Hot air is the most common. The
new process for removal and replacement of surface mount
components, including BGAs and CSPs, is laser-based.
Multiple rework on the same BGA site can result in barrel
cracking of the printed wiring board. Consideration must
be made for printed wiring board materials and number of
heat cycles the materials can withstand.
7.9.3.5 Hot Air Systems for BGA Repair Hot air systems
are either totally manual or semi-automated. Using a
nozzle, they blow hot air on the part to be reworked. The
part is pulled away from the board when the solder on all
joints is molten. Bottom-side heating is used to uniformly
heat the entire board to a preheat setting prior to applying
topside nozzle heat, thus reducing thermal shock.The hot
air usually is directed on the BGA package by a nozzle
designed specifically for that component. The package
body is heated by the hot air impinging on the package and
conduction within the package. Initially, the package is
preheated with the nozzle some distance away (typically
25 mm or more) from the package body. Then the nozzle
is lowered to a point just above the package body and lead
temperature increases sharply until it reaches a peak. Dur-
ing this process of blowing hot air, the solder joints of
neighboring components even 12 mm away can reflow, an
unwanted and undesirable result. With higher velocity air,
smaller components such as CSPs are prone to movement
during rework.
After the component is removed, paste application for reat-
tachment is a most difficult and time-consuming process.
However, using flux only (liquid or paste form) is only
applicable for eutectic BGAs reattachment. Some applica-
tions require solder addition to promote a robust joint.
Typically, a mini-stencil or dispenser is used to apply the
paste. Both hot air nozzles and ministencils are needed for
each type and size of part being reworked. Both these items
require sufficient interpackage spacing for rework. The
forced convection heating of the bottom side of the board
will minimize the temperature of the nozzle required to
achieve an acceptable lead-free profile. Typically tin/lead
bottom side preheating is approximately 100°C. This tem-
perature should be increased to 130°C minimum for lead-
free products.
7.9.3.6 Laser Systems for BGA Repair The laser sys-
tems use from one to four diode lasers. Some of the laser
systems are limited to reworking only peripheral compo-
nents, in which the leads are in the laser’s line of sight.
However, there are other laser systems that use multiple
January 2013 IPC-7095C
115
Copyright IPC-Association Connecting Electronics Industries
Provided by IHS under license with IPC
Not for Resale
No reproduction or networking permitted without license from IHS
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