[Gta04-owner] Status OpenPhoneux / GTA04

Tue Jul 3 19:09:56 CEST 2012

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 at 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 of compatible materials it 
has become possible to omit the stencil removal process step and thereby 
remove many of the potential opportunities for error. The improvements 
to the process include:
* Ease of use
* No paste release problems
* No smearing during removal
* Prevention of solder shorts
* Repairs damaged solder resist
The fact that by leaving the stencil in place benefits the process of 
BGA replacement by opening the process window is an added bonus.
..."

Search:
http://www.google.dk/search?q=university+solder+bga+joint+study

-

AN 353: SMT Board Assembly Process Recommendations:
http://www.altera.com/literature/an/an353.pdf
Quote: "...
One important factor that you must consider in designing stencils is 
that lead-free pastes have higher surface tension and do not wet or 
spread on the surface of pads as easily as eutectic solder pastes. This 
higher surface tension can lead to exposed pad finish material after 
reflow soldering. You can rectify this problem by modifying the stencil 
aperture designs to increase the paste coverage on the pads.
...
To ensure that all components are heated to temperatures above the 
minimum reflow temperatures and that smaller components do not exceed 
the maximum temperature limits, you must perform reflow profiling by 
attaching calibrated thermocouples embedded in the spheres of the larger 
BGA parts as well as other critical locations on the boards (2). Because 
the components are subjected to higher reflow temperatures, select the 
appropriate moisture sensitivity level (MSL) for the components and 
component handling. You must strictly follow the storage recommendations.

Although nitrogen is not required, Altera recommends including nitrogen 
in the reflow process as its presence helps achieve better wettability 
and widen the process window. Nitrogen is especially beneficial when 
temperature differential across the board is large. Additionally, 
nitrogen improves the appearance of solder joints by inhibiting the 
effects of oxidation.
...
To ensure that all packages are successfully and reliably assembled, the 
reflow profiles studied and recommended by Altera are based on the 
JEDEC/IPC standard J-STD-020 revision D.1 (3).
...
To reduce thermal stress on boards and components, you must control the 
peak temperatures below the recommended maximums (Table 4) and minimize 
the temperature gradients across the board. High temperatures can put 
significant stress on plated through-holes and barrels, which can lead 
to cracking. High first-pass temperatures on double-sided assemblies 
increase the amount of second-side oxidation, which can cause 
solderability problems on the second pass.

Altera has worked extensively with leading EMS companies and has 
successfully demonstrated that the Pb-free parts can be soldered in air 
atmosphere (5).

However, for high-density, two-sided assemblies, you can alleviate the 
problems related to a narrow process window by selecting modern reflow 
ovens with forced convection and more heating zones with tighter process 
controls on reflow parameters. Reflow ovens equipped with nitrogen 
reflow atmosphere have shown to improve wettability at lower peak 
temperatures and reduce temperature gradients across the board and have 
proven beneficial for double-sided assemblies (6).
..."

PCB Layout Recommendations for BGA Packages
http://www.latticesemi.com/lit/docs/technotes/tn1074.pdf
http://www.eetasia.com/STATIC/PDF/200906/EEOL_2009JUN16_EDA_AN_01.pdf?SOURCES=DOWNLOAD

Application Note AN-1028
Recommended Design, Integration and Rework Guidelines for International 
Rectifier's BGA and LGA Packages
by Kevin Hu, International Rectifier
http://www.irf.com/technical-info/appnotes/an-1028.pdf

Search:
http://www.google.dk/search?q=solder+bga+recommendation

-

http://www.google.dk/search?q=site%3Axilinx.com+bga+solder

-

http://www.google.dk/search?q=coplanarity+bga+warp

NASA guidelines for ball grid array selection and use:
http://nepp.nasa.gov/docuploads/87ED1BD1-16C1-4F5E-A1227047B50C6CBE/BGA%20Guidelines%20090706.doc
Quote: "...
9.6. Coplanarity
Generally, the coplanarity of the BGA is very small since there are no 
leads to bend and the plane of the solder bumps is flat.  The warp of 
the printed circuit board will generally dictate the coplanarity of the 
BGA assembly.  It is recommended that the allowable warp of a PCB for 
BGA use be held to less than 1%; 1% warp could be greater than 0.010 
inch under a 35 mm square BGA.  An allowable PCB warp of 0.5% max is 
recommended.
...
10.3 Part Moisture Control
Prior to population of flight printed circuit assemblies with PBGA's or 
other plastic IC's, it must be verified that controls for moisture have 
been followed in accordance with IPC/JEDEC J-STD-033A.  Bake-out of 
PBGA's at 125oC for 4 hours is recommended followed by dry storage until 
use. The time between removal from storage and reflow is dependent upon 
the parts sensitivity factor per J-STD-033A and must be strictly 
followed. PCB's should be baked at 120oC for 3.5 hours.
...
Appendix A
Process Evaluation Guideline
..."

Implementing High Temperature Coplanarity Requirements for Components 
and PWBs:
http://thor.inemi.org/webdownload/projects/ba/Board_Coplanarity/Board_Coplanarity_Background.pdf
Quote: "...
* Its clear that numbers don't add up properly, yet packages surface 
mount anyway .Most components and boards do not approach the maximum 
allowable
* However the case above is possible according to the specs, but not 
probable
..."

CSN33: Micron BGA Manufacturer's User Guide:
http://www.micron.com/~/media/Documents/Products/Customer%20Service%20Note/6882csn33_bga_user_guide.pdf

-

br,

Glenn

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