Authored By:
Emmanuelle Guene, Steven Teh
INVENTEC Performance Chemicals
Bry sur Marne (France), Selangor (Malaysia)
Summary
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 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.
Conclusions
The purpose of the paper was to highlight the differences between water-soluble and no-clean solder pastes in order to guide users in their choice. To achieve this goal, six lead-free solder pastes were extensively studied, three being water-soluble and three being no-clean. In the first part of the paper, water-soluble pastes generally yielded results below the no-clean pastes with a significant sensitivity to temperature and humidity, a tendency to slump during preheat and a narrower printing window.
It was concluded that the WS pastes had to be stored, handled and used with more caution before reflow compared to no-clean pastes. In the second part of the paper, regarding wetting properties, WS pastes rank generally better, especially for oxidized substrates. As far as cleanability is concerned, of course only water-soluble pastes can be cleaned with water only whereas no-clean pastes need detergent to achieve a complete removal of their residues.
However, in case of poor cleaning, as the amount of ionic species usually found in WS residues is high (presence of ionic surfactants especially), the risk of corrosion is very high: this is the major drawback of such pastes. The use of water-soluble pastes generally takes place in high reliability assembly as Medical, Military or Aerospace fields where close attention is paid to the quality of substrates, components, where reflow is done in inert atmosphere (nitrogen, vapor phase or under vacuum ovens) and where cleaning is compulsory for the majority of the products.
The use of aggressive chemistry may not be necessary when using such equipments. Moreover, the risk of tombstoning is increased. In such fields, in case of new solder paste evaluation, it is useful to also consider the option of no-clean solder pastes in comparison with water-soluble solder pastes and even to think about review the whole cleaning process. This paper can then be used as a guide to study the critical aspects of these two types of solder pastes.
Initially Published in the IPC Proceedings
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