Authored By:
Clive Ashmore, Mark Whitmore, Jeff Schake
DEK Printing Machines Ltd.
Dorset, UK
Summary
The next generation miniaturised SMT devices waiting to make their mark will require the assembly community to re think their processes and toolsets. The feature sizes that are involved in this new wave of miniaturisation are sub 200 micron, to put this into context, only a decade ago this would have been considered as Semicon domain.
Of all the process involved within the Surface Mount Assembly process the printer is certainly the most sensitive to these changes. But it's not only about printing miniaturised features - the process engineer has to balance miniaturisation with the requirements of "standard" technology, thus we are experiencing the age of heterogeneous assembly.
Therefore the miniaturisation program is causing the print process to be challenged in new ways especially the impact on the available process window available to achieve high yield heterogeneous assembly.
This paper will investigate the impact of miniaturisation and heterogeneous assembly on the print process and strategies to keep one generation ahead. In latest research work, actual paste deposit volumes and transfer efficiency have been monitored and compared for both square and round apertures with area ratio's ranging from 0.20 thru to 1.35. This covers apertures sizes of between 100 and 550 microns in a nominal 100 micron thick stencil foil. In addition, the effect of ultrasonically activated squeegees (ProActiv) has been assessed as part of the same experiment. A further comparison has also been made between type 4 and type 4.5 solder paste as well.
The data presented here will help provide guidelines for stencil aperture designs and strategies for ultra-fine pitch components such as 0.3CSP's.
Conclusions
The next generation of ultra fine pitch components will place extreme demands on the stencil printing process. The requirement for printing solder paste through stencil apertures with area ratios below 0.5 will become common place. The data presented here indicates that with judicial choice of stencil design and materials it will be possible for designers to work with aperture area ratios down to 0.4.
To optimise a process it is becoming increasingly important that an engineer has a good understanding of stencil aperture design specification, material properties and process options/aids available to him. The interactions between all of these facets is becoming more complex and critical to the successful implementation of a process.
Initially Published in the IPC Proceedings
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