Characterization of Solder Pastes Based on Two Alternative Alloys

Emmanuelle Guene
Inventec
Bry-sur-Marne, France

The Tin-silver copper alloy SAC305 is the most commonly used in lead-free electronic assembly, especially in SMT process. Due to the evolution and the diversification of assembly technologies, needs for suitable lead-free alternative alloys have appeared.

While remaining a relatively small market, the demand for alloys with low melting temperature, to allow soldering of temperature sensitive components or substrates, has grown significantly in the last few years. On the other hand, alloys with a significantly higher melting temperature than SAC305 are required in the first soldering process of the daughter boards, to avoid the reflow during the assembly of the main board.

After outlining the state of the art, the paper will describe the characteristics of the alloys with the expected properties, the development of suitable flux media according to their specificities, the solder pastes performance and the reliability of their solder joints.

A high temperature solder paste based on SnSb8.5 was developed for the study. This paste showed good results in terms of printing, solderballing and wetting. In fact, its performance was very similar to SAC305 solder paste with a slightly lower wettability.

The low temperature pastes based on "C" chemistry were sensitive to cold and shear slump. The same alloys combined with "D" chemistry were not sensitive to slump and allowed higher printing speed thanks to the lower minimum pressure needed.

The solderballing results were relatively poor for all the low temperature pastes compared to SAC305, this trend is known and is due to the type of alloy itself (higher oxide content, sensitivity to oxidation, lower surface tension). However, only a few solderballs were detected on the boards when using standard reflow conditions. The wettability of low temperature pastes was excellent on ENIG finish, good on tin finish and relatively good on fresh copper OSP.

Neither "C" nor "D" chemistries were sensitive to humidity when stored 18 hours at room temperature, leading to the same wetting performance with and without idle time. Some dewetting was clearly observed on OSP and on chemical tin when the boards were submitted to a prior reflow. The use of a harsh profile prior to the test totally prevented the alloy to wet the copper OSP.

It was not possible to rank the pastes neither according to the alloy type (SnBi58, SnBiAg57.60.4, SnBi57Ag1) nor according to the flux chemistry (C or D) in terms of solderability as there was no clear trend. Some investigations to understand the wetting issues encountered on low temperature solder pastes in specific conditions have already started.