Electronics Assembly Knowledge, Vision & Wisdom
The Digital Solder Paste
The Digital Solder Paste
This paper discusses a process for creating a digital solder paste using quantitative benchmarking techniques.
Materials Tech

Authored By:
Rick Lathrop
Heraeus Contact Materials Division
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Transcript
Historically, solder paste design and formulations have been based on previous knowledge bases, honed over the years by tweaking and trial and error.

Paste testing followed suit with virtually all performance tests based on subjective expert inspection and critique.

Although statistical design elements have been appending these development methods in recent years, complete digital paste design has been confounded by the lack of quantitative paste tests and the complexity of mixture design technology.

Today digital solder paste has been developed by combining "Design for Six Sigma" techniques, quantitative testing methods for all major attributes and specific performance targets derived from a close customer interface.

The design outcome forms a mathematical basis for a chemical formula with the interactive effects on performance attributes for each constituent fully understood. Shorter development cycles for formula variations to target key paste attributes are one immediate benefit.

This paper discusses process of creating this design system, beginning with the building and refining of the "House of Quality" and then coupling this with previously developed quantitative benchmarking techniques.
Summary
Since the beginning of Surface Mount Technology over 40 years ago solder paste has been an integral element in electronics assembly. Historically solder paste design and development has been, in a sense, analog in nature. Some users have considered it "black magic". Formulations were based on previous knowledge bases, honed over the years by tweaking and trial and error science. Paste testing followed suit with virtually all performance tests based on subjective expert inspection and critique. Although statistical design elements have been appending these development methods in recent years, complete digital paste design has been confounded by the lack of quantitative paste tests and the complexity of mixture design technology.

Today we can state that the digital solder paste has been developed by combining "Design for Six Sigma" techniques, quantitative testing methods for all major attributes and specific performance targets derived from a close customer interface. This paper will walk the process of creating this design system, beginning with the building and refining of the "House of Quality" and then coupling this with previously developed quantitative benchmarking techniques, testing and perfecting these techniques for statistical validity, pre-screening potential ingredients, running the mixture Design of Experiments (DOE) and finally verifying the formula. This system has yielded a comprehensive knowledge of interactions for every constituent in the formula, as well as statistically predicted the optimum formula based on desired properties and their relative importance.

There are many obvious benefits to the digital solder paste over its analog predecessors. The design outcome forms a mathematical basis for a chemical formula with the interactive effects on performance attributes for each constituent fully understood. This permits performance simulations of constituent changes, intuitive troubleshooting and other "what if" scenario exercises. Shorter development cycles for formula variations to target key paste attributes are one immediate benefit.
Conclusions
By using a series of defined measurable tests, selected for relevance and used in DOE's, it is possible to develop a solder paste tailored to specific needs. The result is a formula that is more stable over raw material lots, time, process and environmental variations. This method was applied to a no clean Sn/Pb paste and resulted in the first reformulated paste of this type in the 21st century. Factors in this success include:

Only with quantitative paste attribute tests that are repeatable and reveal formulation performance contrast can a DFSS solder paste development project be successful.

The OEM and its solder paste supplier have developed a more synergistic relationship with a greater level of solder paste formulation comprehension as a direct result of applying the DFSS tool set.

In the application of the DFSS methods, Applications benchmark testing has been both improved and verified. The knowledge and skills have facilitated a new lead free DFSS project which is currently underway.

Analog trial and error development methods have been replaced by DFSS tools. Qualitative perception based testing methods have been replaced by statistically significant quantitative ones. The merging of both of these improvements is required to develop the truly digital solder paste
Initially Published in the IPC Proceedings
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