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Effect of Fine Lead-Free Solder Powder on the Reflow Property Pastes
Effect of Fine Lead-Free Solder Powder on the Reflow Property Pastes
This paper evaluates the effect of particle size distribution and oxidation on the reflow performance of solder paste.
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Authored By:
Amir H. Nobari, Ph.D. and Sylvain St-Laurent, Ph.D.
5N Plus Micro Powders Inc.
Montreal, QC, Canada

Summary
Miniaturization of components and printed circuit boards (PCB) has increased demand for finer pitches both on PCBs and on chip level. In order to meet this demand, more advanced solder pastes with finer particle sizes are needed. However, determination of the real effect of particle size distribution on oxidation, reflow performance and paste properties is still required. It is the object of this paper to evaluate the effect of particle size distribution and oxidation on the reflow performance of solder paste. The powder investigated here was produced with a proprietary atomising technology particularly effective in producing powder ranging from 1 to 25 μm. Solder powder was classified into the different size categories. Powders were then admixed with fluxes and reflow characteristics were evaluated. The reflow property of pastes is then discussed as a function of the particle size, and oxidation level.

Conclusions
Surface characterization of SAC305 solder powder was performed and effect of powder oxidation on the reflow performance of solder paste was studied. The following conclusions were obtained:

(i) Oxide thickness is the main factor that affect the solder balling; however, some other powder characteristics and/or some external factors play role in reflow performance of solder pastes.

(ii) The oxide thickness formed at the powder surface is the range of 3-5 nm.

(iii) The thickness of tin oxide at the surface powder is not dependent on the particle size.

(iv) When the oxide thickness of powder is thin enough, powder with difference sizes exhibit the same reflow performance. Therefore, finer powders can have same reflow performance as coarser powders.

(v) Finer powder is more sensitive than coarser power to oxidation, i.e. reflow performance is more affected by higher level of oxidation for fine powder.

Initially Published in the SMTA Proceedings

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