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September 25, 2006

0201 capacitors creating process problem

We are having process problems with 0201 devices in the new lead free-line. We did not experience these problems with tin lead process. Could this be a problem with too much or too little paste, we are using the same stencils we had for tin lead process?  Or could this be an issue with the reflow profiles?



P.D.

Experts Comments

What is the issue? Tomb stoning? Or not wetting? Tomb stoning can be profiled out to some extent with a hot soak at 200C. Then a slow ramp to 220C. Alloys also have an effect, SN100C eliminates tomb stoning as compared to SAC305. Stencil apertures can also be modified. Home plate or reverse home plate will also show some differences.

If it is wetting check to see what finish you are receiving, sometimes if there are thick film terminations there is a high glass frit in them or a Palladium oxide content. This would have to be investigated. Then a proper profile would need to be implemented to over come some of the termination issue, however some may not be capable of profiling out.

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Karl Seelig
Vice President of Technology
AIM
In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.

We have run several comparison tests between different stencil technologies (AMTX Electroform and Laser-Cut) for Lead-Free solder paste. We fine a large difference in print performance for small apertures (AMTX provides better paste transfer for small apertures like 0201's than Laser-Cut stencils).

One good reference is "Choosing a Stencil" which appeared in July issue of SMT Magazine. I have attached an example of the performance difference for 0201's, 0210_Electroform_vs_Laser.pdf. Please call and I will be happy to discuss aperture and stencil design.

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Bill Coleman
Vice President Technology
Photo Stencil
For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.

The types of process problems that you are having aren't clear, but I will guess that you are experiencing problems with poor wetting and/or poor coalescence as these are the most common issues with 0201s in general.

It isn't surprising that these issues would become more prominent with the conversion to lead-free. It is mostly a function of a few factors:

  • Slower wetting behavior of lead-free alloys
  • Poorer coalescence of lead-free alloys
  • Greater oxidation rate of lead-free alloys

The poor wetting / poor coalescence issues become most obvious on the smallest paste deposits because the proportion of surface area of powder exposed versus the volume of flux is increased with smaller deposits. This means that powder gets more oxidation (proportionally) on a small deposit than a larger one. The increased oxidation rate of the powder and the slower wetting / coalescence combine to yield a solder joint that appears rough on the surface.

This can usually be corrected by opening up the stencil apertures slightly such that the ratio of surface area of exposed powder to the quantity of flux increases enough to push the activity of the system high enough to achieve complete coalescence.

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Brian Smith
General Manager - Electronic Assembly Americas
DEK International
Mr. Smith has been supporting customers in the electronics assembly industry since 1994. His expertise is focused on solder paste printing and reducing soldering defects. He holds a BS in Chemical Engineering and an MBA in Marketing. He has authored several papers in trade magazines and at industry conferences. He is an SMTA Certified Process Engineer.
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