BGA solder process and soldering BGA components
- an overview or reference tutorial providing essential information about
the BGA solder process and soldering BGA components used for PCB assembly
including prototype PCB assembly and BGA rework.
With the increasing component density of today's electronics
printed circuit boards and the very high track densities that result,
connectivity on many boards has become a problem. Even migrating to greater
numbers of layers for the PCB cannot overcome many of the problems. To assist in
resolving this problem an integrated circuit package known as the Ball Grid
Array, BGA was introduced. The BGA components provide a far better solution for
many boards, but care is required when soldering BGA components to ensure that
the BGA solder process is correct and that the reliability is at least
maintained or preferably improved.
What is a Ball Grid Array?
The Ball Grid Array or BGA, is a very different package to
those using pins, such as the quad flat pack. The pins of the BGA package are
arranged in a grid pattern and this gives rise to the name. In addition to this,
rather than having the more traditional wire pins for the connections, pads with
balls of solder are used instead. On the printed circuit board, PCB, onto which
the BGA components are to be fitted there is a matching set of copper pads to
provide the required connectivity.
BGA packages offer many advantages over their quad flat pack
rivals and as a result they are being used increasingly for the manufacture of
electronics circuits:
- Improved PCB design as a result of lower track density:
Track densities around many packages such as the quad flat pack become very
high because of the very close proximity of the pins. A BGA spreads the
contacts out over the full area of the package greatly reducing the problem.
- The BGA package is robust: Packages such as the quad
flat pack have very fine pins, and these are easily damaged by even the most
careful handling. It is almost impossible to repair them once the pins are
bent owing to their very fine pitch. BGAs do not suffer from this as the
connections are provided by pads with the BGA solder balls on them which are
very difficult to damage.
- Lower thermal resistance: BGAs 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.
- Improved high speed performance: As the conductors are
on the underside of the chip carrier. This 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 solder process
One of the initial fears over the use of BGA components was
their solderability and whether soldering BGA components could be made as
reliable as soldering devises using more traditional forms of connection. As the
pads are under the device and not visible it is necessary to ensure the correct
process is used and it is fully optimised. Inspection and rework were also
concerns.
Fortunately BGA solder techniques have proved to be very
reliable, and once the process is set up correctly BGA solder reliability is
normally higher than that for quad flat packs. This means that any BGA assembly
tends to be more reliable. Its use is therefore now widespread in both mass
production PCB assembly and also prototype PCB assembly where circuits are being
developed.
For the BGA solder process, reflow techniques are used. The
reason for this is that the whole assembly needs to be brought up to a
temperature whereby the solder will melt underneath the BGA components
themselves. This can only be achieved using reflow techniques.
For BGA soldering, the solder balls on the package 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.
BGA solder joint inspection
BGA inspection is one area of the manufacturing process that
has raised a considerable amount of interest since the introduction of the first
BGA components. BGA inspection cannot be achieved in the normal way using
straightforward optical techniques because, quite obviously, the solder joints
are underneath the BGA components and they are not visible. This creates
problems for BGA inspection. It also created a considerable degree of unease
about the technology when it was first introduced and many manufacturers
undertook tests to ensure that they were able to solder the BGA components
satisfactorily satisfactorily. The main problem with soldering BGA components is
that sufficient heat must be applied to ensure that all the balls in the grid
melt sufficiently for every BGA solder joint to be satisfactorily made.
The solder joints cannot be fully tested by checking the
electrical performance. While this form of test of the BGA solder process will
reveal conductivity at that time, it does not give a full picture of how the BGA
solder process has succeeded. It is possible that the joint may not be
adequately made and that over time it will fail. For this the only satisfactory
means of test is a form of BGA inspection using X-rays This form of BGA
inspection is able to look through the device at the soldered joint beneath.
Fortunately, it is found that once the heat profile for the
solder machine is set up correctly, the BGA components solder very well and few
problems are encountered with the BGA solder process.
BGA rework
As might be anticipated, it is not easy to rework BGA
assemblies unless the correct equipment is available. If a BGA component is
suspected as being faulty, then it is possible to remove the device. This is
achieved by locally heating the BGA component 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.
Summary
BGA technology and the BGA solder process have shown
themselves have proved themselves to be very successful since they were first
introduced and they are now an integral part of the PCB assembly process used in
most companies for mass production and for prototype PCB assembly. Despite some
early scepticism, they are now widely used and levels of reliability for BGA
assemblies is very high. Along side this various process issues have been
improved and elements such as BGA inspection are now easier to achieve so that
the manufacturing process can be optimised and the BGA solder element of the
process can be as reliable as possible.
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