On 03/07/12 11.15, Dr. H. Nikolaus Schaller wrote:
...
Now comes the crucial question: how can this be improved?
The specialists of the production company have spent whole weekends
to do more experiments but did not yet find the right trick. It is not that
they do not want to solve the problem (they see it as a challenge) they
simply have not yet found the key.
...
Unless one of the readers on these list knows some real specialist
for such PoP soldering of BGAs who is willing and capable to
consult (even if it costs money). So if you know someone, please
let me know.
...
Hi Nikolaus
I have no experience with BGA soldering, but here are some (untested)
suggestions. I have no idea what is possible. Maybe it might add to a
solution?:
* Use an heat infrared camera to see during soldering (and passive
cooling) how the BGA-chip temperature is distributed during the process
- if possible - even of the PCB underside.
* If BGA-chip bulges it maybe that it is too cool at the periphery?
Can the periphery be heated more? Or be heat-isolated more during
passive cooling - or cool the BGA-center more than the periphery? Maybe
the PCB just opposite the BGA should have the opposite treatment during
the cooling process?
* Make hollow BGA PCB-islands where to much or to little solder can flow
through? The melted solder adhersion should keep the solder where it
belongs?
* Maybe this paper has the answer?:
BGA rework
A comparitive study of selective solder paste deposition for area array
packages.
Ray Cirimele BEST, Inc. Rolling Meadows, Illinois rcirimele@solder:
http://www.solder.net/stencilquik/papers/SMT_PANPAC_Paper_Rev032704.pdf
Quote: ...
There has been much controversy over whether BGA solder joint
reliability is better when processed with solder paste versus paste
flux. Some studies1,2 indicate that the solder joint shape may have a
greater impact on the solder joint reliability than the volume.
...
Figure 1. An open connection caused from poor coplanarity and the use of
flux instead of paste.
...
Many things can affect the coplanarity. If the BGA has solder balls that
are not coplanar it can result in a open connection (a ball that has
not contacted and wetted the land while surrounding balls have made
contact). Variations in surface finish height, warp of the board or
area, and warp or bow of the device itself can create coplanarity
problems. The use of solder paste can help to overcome variations in
solder ball height. The use of solder paste may also improve wetting as
a result of a greater area of intimate contact between the ball and the
paste and the land and the paste. Solder paste may provide better tack
than flux alone. On newer technology PCBs, the utilization of drilled
(usually laser) micro-vias in the center of the BGA land requires the
use of solder paste to reduce voiding and to prevent loss of the solder
ball volume as a result of filling the via.
...
Although there are many ways to replace the Area Array devices using
flux, solder paste, or solder bumps, only the four most common methods
of solder paste deposition will be discussed.
...
Stand Off
There are some BGA devices with eutectic solder balls that tend to
exhibit a great deal of ball collapse after reflow. This can be caused
from design (land size) or BGA weight. Many times this excessive
collapse can result in solder shorts (usually in the corners).
Generally, greater stand-off height in conjunction with proper solder
volumes and shape can provide better solder joint reliability. The semi-
permanent stencil will prevent excessive collapse of the solder balls
and can be used to provide a minimum stand-off height.
Solder Shorts
Solder shorts can occur on BGA devices for a number of reasons. Most of
these reasons have to do with the solder paste deposition or excessive
collapse of the solder balls causing an increase in the ball diameter
and a reduction in the ball spacing. One clear advantage that the semi-
permanent stencil has over the other stencils is that it acts as a
physical barrier to prevent solder shorts from occurring. The
non-wettable polyimide film holds the solder paste in the stencil
apertures, and once the solder becomes molten it coalesces and cannot
migrate over the stencil.
...
Solder Volume
Based on previous works done on the relationship between BGA solder
joint quantity and solder joint strength and reliability, greater solder
quantities generally result in greater joint strength. Some studies
indicate that solder joint shape may have as much to do with joint
strength as the solder volume. Greater stand-off heights with a fillet
shaped like an hour glass may provide better reliability than a
traditional rounded collapsed ball shape1,2.
...
CONCLUSION
Although stencils have been used in the electronics industry for many
years, the use has been plagued by the impact of the removal of the
stencil from the substrate. Through the use
