Authored By:
Omar Garcia, Enrique Avelar, Manuel Dominguez
Francisco Barajas, Jaime Medina, Dason Cheung
Juan Coronado, Zhen (Jane) Feng, Murad Kurwa
FLEXTRONICS International Inc.
Summary
The challenges of successful solder printing in the High Volume / Low Mix cell phone environment, which is linked with the continuing trend to miniaturize electronic assemblies, requires a new approach to improve the printing capabilities and process repeatability. Actual stencil technologies such as electroform or laser-cut limit the stencil opening due to aspect and area ratios at the smallest devices producing a very tight process window.
It's here that Nano Technology will assist in the printing process. Using Nano Coating over the stencil openings to smooth the surface and improving the paste release, helps reducing aperture openings, and creates a wider process window. Additionally, panel stretch and PCB fabrication tolerances produce a silent non constant variable that moves the process outside the quality printing window, without obvious signs of variation.
To obtain an advantage, and successfully implement this technology, the process requires new controls of chemical and parameter settings. We will discuss some aspects of process optimization and how this very tight process window is affected, by identifying the challenging process parameters, including circuit board fabrication, component pad design, and printing parameters (speed, separation, pressure, etc.).
This printing study will consider the effects of print speed, print pressure, and separation speed, to optimize solder paste transfer efficiency (TE) to establish an statistical process control that gives real time warnings of an out of control printing process. We will discuss our data results which will include the advantage of using Nano stencil vs. E-Fab and Laser NiEX. TE improvement is 5% at the smallest stencil aperture across a panel of 4 images. The cleaning speed significantly reduces defects from 2% with a 50mm/sec, to zero defects using 20mm/sec. By improving the TE by 5% will increase the number of prints without a paste bridge on any board, even up to10 prints between cleaning.
Conclusions
- More stable volume & less paste height variation were obtained using the Nano stencil in comparison with E-Fab & Nickel laser cut.
- Transfer efficiency is significantly better using Nano coated when compared with the other type of stencil technologies.
- Greater transfer efficiencies are obtained using the Nano stencil with water base cleaner rather than IPA with identical settings.
- Interaction between panel stretches with the type of pad affecting the print stability. The pad with the poorest repeatability was the NSMD, which affect more the gasketing.
- From the DoE the best combinations that reduce variability between print settings vs. clean settings were as follow:
* Print speed and print pressure result as main variables in the print repeatability: speed of 40 mm/sec with print pressure of 7 kg/cm2 produce the less variation in paste height between each pad printed over the 0.4 mm pitch CSP's.
*The cleaning settings play a key role to process stability. The combination WD works better than
the WVD with a Dry speed of 20 mm/sec and wet speed of 40 mm/sec.
- Transfer efficiency is greatly affected by the screen printer temperature; best transfer efficiencies were obtained at 22°C; however further experiments are required to demonstrate stability in a long period of time.
- It is recommended to use the Nano coated stencil for the fine pitch device and the smaller aperture for the better paste release to avoid insufficient solder (better TE) as well as the solder bridging (small aperture).
Initially Published in the IPC Proceedings
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