Here are some opinions on the use of direct mounting of BGA packages on via
holes.

The use of via hole mounting of BGA device does have advantages and
disadvantages which I have experienced during the trials for the SMART Group BGA
report. This basic cook book report has made well over two thousand pounds for
charity.  We also featured the technique on our training video on BGA. The new
"Introduction to X-Ray Inspection of BGA and Fine Pitch" also mentions the
technique but does not show any examples being x-rayed. The videos are available
from the SMTA main USA office. Contact Joanne

Disadvantages of BGA via mounting

When wave soldering secondary reflow of the topside BGA joints can be
experienced. It is necessary to mask these positions with the soldering fixture
or with peelable mask. But you must be very careful of peak temperature. Many
people have seen secondary reflow of fine pitch parts when high solder bath
temperatures 250C are used. Most wave soldering is now conducted between
235/240C. High mass boards can hold the temperature in the board after wave
contact. Sealed nitrogen wave soldering systems also maintain high internal
temperatures.

Double sided BGA  assembly can be a problem due to the solder paste reflowing,
filling the via and causing a bump on the alternative side. This will make
second side printing of paste difficult due to the stencil being lifted locally
off the pad surface.
You can not use filled or plugged vias with resist as then you will get random
voiding in the joints due to blow back of the volatile in the paste. During
reflow the paste slumps into the hole prior to reflow not allowing the solvents
to escape.

This is the same type of fault seen on wave soldering when components like
sockets are hard down on the PCB surface, it looks like outgassing of the joint
but it is not. It is not a direct benefit for component alignment of the balls
in the via holes when using paste. If the parts are depressed to this degree you
will excessively displace the paste around the pads. This will then not allow
complete via filling to take place, it may also cause shorting.  

After reflow you will tend to find that the stand off height from the board is
less than with standard techniques. This is due to the loss of some of the
solder volume to the via. Any drying operation after cleaning would be more
difficult. From an inspection point of view using vias and the associated
smaller surface pads will reduce the x-ray inspection efficiency. The joint area
will all be smaller than the ball not allowing wetting indicators to be used. If
any track is left exposed the solder will not wet out that far as an indicator.

During rework it will take longer for complete reflow to take place unless
correct pre heating is used. Due to the direct connection of the solder pads to
barrels of copper and solder slugs through the board the dissipation of the heat
will be higher.

Advantages of via mounting BGA
The process works fine and all the via can be filled with solder if you use the
rights size hole and pad, paste and low temperature balls. You do not have to
use high temperature balls.
It does make track routing easier due the reduced via pad size, but does take up
a lot of through board space.

It the technique is used and large vias are used they can be used for
positioning thermocouples under the BGA by passing them up through the via. If
via filling is considered a problem or blind vias are considered then a pre
reflow of paste could be used. This would fill the via or blind via prior to
final paste printing, BGA placement and reflow. This technique of building does
subject the board to one unnecessary reflow cycle to be avoided with copper
boards. If solder levelled boards are used then the PCB manufacturer could fill
the vias during levelling.

Rework, test and inspection can benefit from the technique as well as heat
dissipation. In the case of rework direct access can be made to the solder
connection allowing easy reflow for part removal. A solder fountain used for
conventional component removal may be used for BGA. 

It is a disappointment that after conducting the process trials we did not
subject the via mounted samples to environmental test like the standard parts.
That's life you cant think of everything.

(It was interesting to see some of Greg Bartletts BGA product recently in
Chelmsford,  Essex  England not Chelmsford  MA. It looked like a nice product.)

Bob Willis
SMART Group
Tel 01245 351502
Fax 01245 496123
Email [log in to unmask]