Electronics Assembly Knowledge, Vision & Wisdom
Revolutionary Printing Solution for SMT Assembly
Revolutionary Printing Solution for SMT Assembly
This paper presents new techniques in the screen print process with the aim of increasing solder paste transfer efficiency.
Production Floor

Authored By:
Clive Ashmore, Mark Whitmore
DEK Printing Machines Ltd
Weymouth, Dorset, UK
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Transcript

As consumers, the expectation of increased functionality within new products is a given. However there comes a time where this tireless demand for product efficiency starts to stretch the design for manufacture rules.

Fabricating products with decreasing feature size and increasing complexity is not the issue, nor is producing products that have larger components; the dilemma is when products require both. This predicament is now upon the surface mount assembly community. The imminent role out of zero point 3 millimeter C.S.P. looks to be pushing the feature size below 200 micron.

The main issue surrounding the stencil printing process when dealing with heterogeneous assembly is area ratio, that being the ratio between stencil aperture open area and aperture wall area. When complying with traditional design rules and maintaining area ratios greater than zero point 66, it becomes near impossible to design a print process for a wide mix of fine and large pitch components.

While developments in solder pastes and stencil manufacturing techniques in recent years have allowed skilled operators to push the area ratio rule of thumb to zero point 5 to zero point 6 to accommodate zero point 4 C.S.P. assembly, the next generation of component technology, zero point 3 C.S.P's, is one step beyond this capability.

To address this, new techniques have been investigated with the aim of increasing solder paste transfer efficiency in the screen printing process. The results of this investigation have shown that existing area ratio rules can be seriously challenged and broken to permit zero point 3 millimeter pitch C.S.P. assembly within a traditional S.M.T. process.

The solution was to take a fresh look at the mechanics of the printing process. The filling of the aperture is vital to the print process; if you don't fill the aperture then the print deposit is never going to be volumetrically correct. However the filling process also influences the aperture release characteristics, the bond force of the material to the pad significantly affects the overall release process.

As a result if the filling process fails to pack the material into the aperture such that the material does not make a chemical bond, then poor release will result. So how do we improve the materials ability to pack into small apertures?

The answer can be found in the material properties that are currently being used in surface mount assembly production. If the shear energy imparted into the solder paste can be increased, the paste will becomes less viscous and its ability to flow into small apertures will be increased, thus the packing of the material in challenging area ratio apertures will be improved.

As a result the aperture is not only filled with the desired volume of material, but the chemical bond onto the pad is also made such that during the release process the bond force will overcome the wall surface bond tension, therefore allowing the material to be released more efficiently.

The traditional method of increasing the aperture fill for challenging area ratio has been to increase the squeegee (attack) angle, typically moving from 60 degrees to a 45 degree angle but this increase in attack angle is not free from pitfalls; therefore a different method of improving aperture fill is needed. One in which the improved filling of apertures can fit alongside the day to day process engineers requirements.

This paper examines new squeegee technology that uses proprietary technology to energize the print medium such that it locally modifies the rheology of the solder paste in order to increase fluidity; this action improves the packing density of particles into apertures, enhances the cohesive bond between solder paste particles and allows material to freely drop from the squeegee blades, leading to paste management benefits. This technology has the capability to deliver to the surface mount assembly community a remedy to the miniaturization headache.

Summary
As consumers the expectation of increased functionality within new products is a given. However there comes a time where this tireless demand for product efficiency starts to stretch the design for manufacture (DFM) rules. Fabricating products with decreasing feature size and increasing complexity is not the issue nor is producing products that have larger components; the dilemma is when products require both.

This predicament is now upon the Surface Mount Assembly (SMA) community, the imminent role out of 0.3mm CSP looks to be pushing the feature size below 200 micron but still RF shields and connectors are required - or put another way heterogeneous assembly is looming upon us.

The main issue surrounding the stencil printing process when dealing with heterogeneous assembly is area ratio (the ratio between stencil aperture open area and aperture wall area). When complying to traditional design rules and maintaining area ratios greater than 0.66 then it becomes near impossible to design a print process for a wide mix of fine and large pitch components.

Whilst developments in solder pastes and stencil manufacturing techniques in recent years have allowed skilled operators to push the area ratio rule of thumb to 0.5-0.6 to accommodate 0.4CSP assembly, the next generation of component technology (0.3CSP's) is one step beyond this capability.

To address this, new techniques have been investigated with the aim of increasing solder paste transfer efficiency in the screen printing process. Of several techniques investigated one has stood out and has been the subject of intense laboratory trials. The results of this investigation have shown that existing area ratio rules can be seriously challenged and broken to permit 0.3mm pitch CSP assembly within a traditional SMT process. Details of these new developments together with substantive paste transfer efficiency data will be presented.
Conclusions
At the beginning of this paper we discussed the ongoing battle between miniaturization and the constraints that are present within today's manufacturing processes. We know that there are ways around these constraints but they are a half way houses at best, in fact they can lead to increased costs, decreased throughput and lower yields ; but because the miniaturization program is not waiting nor slowing these deficient solutions are been seriously considered.

The activation of solder paste through an energized squeegee assembly has been tested with 3 materials within this paper and although the test and material choice is not totally inclusive, the work outlined in this paper has shown that the squeegee system has been able to locally modify the print medium such that it is now possible to print extremely challenging Area Ratios using standard stencils and solder paste.

This technology has the capability to deliver to the SMA community a remedy to the miniaturization headache.
Initially Published in the IPC Proceedings
Reader Comment

Yes we've proven these techniques with our solder transfer stencil technology and been using it in our products for years. They are right it works so well you can see visually if there is a problem with the solder connection.

Gary Whittaker, Spheretek Div of MVM Technologies Inc., USA
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