Ball Grid Array, BGA
- an overview or reference tutorial providing the basics about what is a
Ball Grid Array or BGA and BGA packages used for SMT technology circuit boards,
and BGA rework and BGA repair.
A Ball Grid Array or BGA package is a form of surface mount
technology, or SMT package that is being used increasingly for integrated
circuits. The BGA offers many advantages and as a result it is being used
increasingly in the manufacture of electronic circuits.
The Ball Grid Array, BGA package was developed out of the
need to have a more robust and convenient package for integrated circuits with
large numbers of pins. With the levels of integration rising, some integrated
circuits had in excess of 100 pins. The conventional quad flat pack style
packages had very thin and close spaced pins, and these were very easy to
damage, even in a controlled environment. Additionally they required very close
control of the soldering process otherwise the level of solder bridges and poor
joints rose. From a design viewpoint, the pin density was such that taking the
tracks away from the IC also proved to be problematic as there could be
congestion in some areas. The BGA package was developed to overcome these
problems, and improve reliability from the soldered joints.
What is a BGA package?
The Ball Grid Array, BGA, uses a different approach to the
connections to that used for more conventional surface mount connections. Other
packages such as the quad flat pack, QFP, used the sides of the package for the
connections. This meant that there was limited space for the pins which had to
be spaced very closely and made much smaller to provide the required level of
connectivity. The Ball Grid Array, BGA, uses the underside of the package, where
there is a considerable area for the connections.
The pins are placed in a grid pattern (hence the name Ball
Grid Array) on the surface of the chip carrier. Also rather than having pins to
provide the connectivity, pads with balls of solder are used as the method of
connection. On the printed circuit board, PCB, onto which the BGA device is to
be fitted there is a matching set of copper pads to provide the required
connectivity.
Apart from the improvement in connectivity, BGAs have other
advantages. They offer a lower thermal resistance between the silicon chip
itself than quad flat pack devices. This allows heat generated by the integrated
circuit inside the package to be conducted out of the device onto the PCB faster
and more effectively. In this way it is possible for BGA devices to generate
more heat without the need for special cooling measures.
In addition to this the fact that the conductors are on the
underside of the chip carrier means that the leads within the chip are shorter.
Accordingly unwanted lead inductance levels are lower, and in this way, Ball
Grid Array devices are able to offer a higher level of performance than their
QFP counterparts.
BGA assembly
When BGAs were first introduced, BGA assembly was one of the
key concerns. With the pads not accessible in the normal manner would BGA
assembly reach the standards that could be achieved by more traditional SMT
packages. In fact, although soldering may have appeared to be a problem for a
Ball Grid Array, BGA, device, it was found that standard reflow methods were
very suitable for these devices and joint reliability was very good. Since then
BGA assembly methods have improved, and it is generally found that BGA soldering
is particularly reliable.
In the soldering process, the overall assembly is then
heated. The solder balls have a very carefully controlled amount of solder, and
when heated in the soldering process, the solder melts. Surface tension causes
the molten solder to hold the package in the correct alignment with the circuit
board, while the solder cools and solidifies. The composition of the solder
alloy and the soldering temperature are carefully chosen so that the solder does
not completely melt, but stays semi-liquid, allowing each ball to stay separate
from its neighbours.
As many products now utilise BGA packages as standard, BGA
assembly methods are now well established and can be accommodated by most
manufacturers with ease. Accordingly there should be no concerns about using BGA
devices in a design.
Ball Grid Array, BGA, inspection
One of the problems with BGA devices is that it is not
possible to view the soldered connections using optical methods. As a result
there was some suspicion about the technology when it was first introduced and
many manufacturers undertook tests to ensure that they were able to solder the
devices satisfactorily. The main problem with soldering Ball Grid Array devices,
is that sufficient heat must be applied to ensure that all the balls in the grid
melt sufficiently for every joint to be satisfactorily made.
The joints cannot be tested fully by checking the electrical
performance. It is possible that the joint may not be adequately made and that
over time it will fail. The only satisfactory means of inspection is to use
X-ray inspection as this means of inspection is able to look through the device
at the soldered joint beneath.
It is found that once the heat profile for the solder machine
is set up correctly, the BGA devices solder very well and few problems are
encountered, thereby making BGA assembly possible for most applications.
Ball Grid Array, BGA rework
As might be anticipated, it is not easy to rework boards
containing BGAs unless the correct equipment is available. If a BGA is suspected
as being faulty, then it is possible to remove the device. This is achieved by
locally heating the device to melt the solder underneath it.
In the BGA rework process, the heating is often achieved
removed in a specialised rework station. This comprises a jig fitted with
infrared heater, a thermocouple to monitor the temperature and a vacuum device
for lifting the package. Great care is needed to ensure that only the BGA is
heated and removed. Other devices nearby need to be affected as little as
possible otherwise they may be damaged.
BGA repair
Once removed, the BGA can be replaced with a new one.
Occasionally it may be possible to refurbish or repair a BGA that has been
removed. This BGA repair may be an attractive proposition if the chip is
expensive and it is known to be a working device once removed. Undertake a BGA
repair it needs to have the solder balls replaced. This BGA repair can be
undertaken using some of the small ready-made solder balls that are manufactured
and sold for this purpose.
Summary
Ball Grid Array, BGA, devices are now widely used. Although
they do have some drawbacks, their advantages of being more mechanically robust,
providing high levels of connectivity, improved heat transfer, and shorter lead
connection lengths outweigh these problems. Accordingly they are being used
increasingly in many areas of electronics design and BGA assembly methods are
now well established.
|
