Electronics Assembly Knowledge, Vision & Wisdom
Estimating Stencil Life
Estimating Stencil Life
This paper discusses the technique used to predict how long solder paste can be left on the stencil before it dries out.
Production Floor

Authored By:
Gerjan Diepstraten, Dr. Di Wu
Cobar Europe B.V.
Breda, The Netherlands
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Transcript
Thermal gravimetric analysis measures weight changes in a material as a function of temperature or time under a controlled atmosphere. This technique is currently used in the design phase of lead free solder paste.

In many SMT production lines the solder paste lies exposed on the stencil for an extended period. Some chemistry of the paste slowly evaporates even at room temperature. This affects the properties of the solder paste and may result in poor printing or aperture clogging of the stencil.

Advanced thermal gravimetric analysis instruments have the ability to simulate a reflow process under different atmospheres. Thermal gravimetric analysis shows that the chemistry of a solder paste responds differently when the atmospheric conditions change.

This paper discusses the technique used to predict how long solder paste can be left on the stencil before it dries out and it is no longer suitable for further use. 
Summary
Thermo-gravimetric Analysis (TGA) measures weight changes in a material as a function of temperature (or time) under a controlled atmosphere. This technique is currently used in the design phase of lead-free solder paste. In this study, the same technique is used to predict how long solder paste can be left on the stencil before it dries out and is no longer suitable for further use.

In many SMT production lines, the solder paste lies exposed on the stencil for an extended period without being used. This can be due to many reasons: product change-over, machine downtime in the SMT line or just not enough production volume for the line to run continuously. Some chemistry in the paste slowly evaporates even at room temperature. This affects the rheological properties of the solder paste and may result in poor printing, or aperture clogging of the stencil.

Advanced TGA instruments have the ability to simulate a reflow process under different atmospheres. A solder paste can be heated in nitrogen or air with different gas flow speeds over the sample. The TGA shows that the chemistry of a solder paste responds differently when the atmospheric conditions change. Knowing this, an optimal heating profile under nitrogen may be different from one when soldering in air. Heating solder paste and determining its characteristics will help reduce the number of solder defects in SMT production.
Conclusions
It has already been mentioned that a solder paste flux has to fulfill a large number requirements, many of which are not necessarily completely compatible or optimally suitable for a single formulation. Compromises must be made, and this somewhat explains the great variety of lead-free solder pastes.

From this study a number of conclusions with respect to stencil-life can be made:
  • Weight loss is different for pastes with different flux chemistry.
  • Even with the same flux chemistry, the evaporation rate can be different due to the oxidation level of the powder.
  • Some fluxes are more sensitive for the area exposed to air than others. Evaporation depends on the mass of the solder paste.
With respect to soldering, the experiments did show that spreading (wetting) decreases when the paste is dry (see Table 6). It also shows that there are significant differences in spread, which make some pastes only suitable for nitrogen atmospheres.

Thermo-gravimetric Analysis is a nice instrument that can be used to study performance of a solder paste in a reflow process. When used in combination with a Rheometer, printing and soldering characteristics of solder pastes can be monitored and used for statistic process control of solder paste manufacturing.
Initially Published in the IPC Proceedings
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