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The Development of a 0.3 mm Pitch CSP Assembly Process
The Development of a 0.3 mm Pitch CSP Assembly Process
A major study looks at different aspects of the stencil printing process and their impact upon the assembly and reliability of CSP components.
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Authored By:
Mark Whitmore and Jeff Schake
DEK Printing Solutions, ASM Assembly Systems
Weymouth, Dorset, United Kingdom

Summary
With shrinking component technology the stencil printing process is becoming increasingly challenged. Not only do long established design rules need to be broken to accommodate finer pitches, but print quality and consistency of print are becoming even more critical to maintain a high yield assembly process.

A major study has been undertaken looking at several different aspects of the stencil printing process and their impact upon the assembly and reliability of 0.3mm pitch CSP components.

Previously, in Part 1, stencil printing factors such as aperture design (circles, rotated squares), aperture size (140 microns thru 200 microns) and printing technology (standard squeegees, active squeegees) were investigated. Highlights of the experiments are summarised here (whilst full details can be found in the 2015 conference proceedings of SMTA South East Asia [1] and SMTA ICSR [2]).

Key learnings from Part 1 have subsequently been used for a series of assembly trials to determine the impact, (if any) of the stencil printing process on ultimate solder joint reliability of 0.3 mm pitch CSP assemblies.
Assembly yields in terms of electrical continuity and identification of shorted solder bumps are reported here, together with preliminary reliability data from ongoing thermal cycling of assemblies.

Conclusions
When pooling the thermal reliability data together with the assembly yield data (Figure 17), it becomes quite clear that there are benefits to using active squeegees. No failures were experienced with any component assembled using an active squeegee print process, even with apertures down to 170 microns in diameter. In contrast 47 components of 120 assembled with a standard squeegee process were lost to assembly defects and a further 5 thru thermal cycling failure.

This has some significant implications for future, sub 0.5 area ratio assembly challenges. Being able to print using 170 micron circular apertures on a 100 micron thickness stencil (aperture area ratio 0.425) will permit 0.3mm pitch CSP components to be assembled alongside today's standard components (which currently utilize 100 micron thick stencils) in a true heterogeneous assembly process.

It would also appear that the actual print deposit produced from an active squeegee has a positive effect on assembly yield criteria, although the mechanism behind this cannot yet be explained. For those wanting to use a "standard" squeegee print process, a 100 micron thick foil with 200 micron circular apertures could provide an option, but the minimal 100 micron interspacing between apertures is delicate and would necessitate a high under stencil cleaning frequency in production.

A stepped stencil could be a further alternative. By reducing the stencil thickness locally for 0.3mm pitch CSP components to 80 microns, circular apertures of 160 microns can be utilized, providing a favorable printing area ratio of 0.5. This however, adds cost and complexity to the process which is always of major consideration in a production environment.

Overall the best solution for heterogeneous assembly and sub 0.5 area ratio printing would appear to be thru the use of activated squeegees. Not only do they open up the print process window for sub 0.5 area ratio apertures, but potentially they impact the quality of the printed deposit in such a way that assembly yield and productivity criteria are enhanced as well.

These bold statements however need to be tempered; all of the testing was conducted under ideal laboratory conditions, far removed from a production environment, and the experiment was relatively small in terms of sample size. However, the trends were strong and it is clear that the use of activated squeegees warrants further, more detailed investigation.

Initially Published in the SMTA Proceedings

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