Reliability of No-clean and Water-soluble Solder Pastes

Emmanuelle Guene, Steven Teh
INVENTEC Performance Chemicals
Bry sur Marne (France), Selangor (Malaysia)

Looking back twenty-five years ago, the solder pastes residues had to be cleaned after reflow due to their corrosive nature: two ways of cleaning were possible, either with solvent or by using water, with or without detergent. Now the assembly world is mainly no-clean: paste formulation is safer in terms of chemical reliability and process costs are reduced without cleaning. However, some applications, i.e. military, aerospace, high frequency, semiconductor require a perfect elimination of the residue after reflow.

There are several options to achieve this result: the use of a no-clean paste which residue can be removed with the most suitable cleaning method or the use of a paste designed to be cleaned, as a water-soluble solder paste.

The water-soluble solder pastes generally show great wettability because of their strong activation but they are also known to have shorter stencil life and to be more sensitive to working conditions as temperature and humidity, compared to the no-clean pastes. Additionally, with the components stand-off getting smaller and smaller, washing residues with water only is more and more challenging due to its high surface tension: the addition of detergent becomes often necessary.

The purpose of this paper is to highlight the differences between these two families of solder pastes to guide users in their choice. This will be achieved through the comparison of several recent water-soluble and no-clean formulations as far as reliability is concerned. First the printing quality will be evaluated (viscosity, tack, cold slump, printing speed according to pressure, stencil life, idle time, printing consistency).

Then the reflow properties will be compared (hot slump, solderballing, reflow process window, wetting ability on different finishes). Finally the residue cleanability will be assessed. The IPC SIR test (method IPC TM 650 2.6.3.7) will be also done to conclude the study. Both standardized tests and production tests will be used to evaluate the performance of these two kinds of solder pastes.

The purpose of the paper was to highlight the reliability differences between water-soluble and no-clean solder pastes to guide users in their choice. To achieve this goal, six lead-free pastes were extensively studied, three being water-soluble and three being no-clean. The first part of the study focused on printing performance.

The pastes were characterized using standardized tests and internally developed tests: dynamic viscosity, tackiness, slump and solderballing. The influence of accelerated storage at elevated temperature, the influence of time and conditions between printing and reflow and the influence of continuous shear according to time were shown.