A1. Silicon is selected after a systematic R&D research on various possible dopants and it was found that silicon provided excellent strengthening effect to SnCu matrix without affecting the electrical properties of the solder when added in very small quantity. It also reduces the rate of copper dissolution when exist in the intermetallics by acting as a barrier between solder and copper pads.

In addition, Silicon is low in toxicity; the cost of Silicon is low and also stable as it is very abundant in the earth crust.

 

A2. Although SCS7’s liquidus temperature is 10 oC higher as compared to SAC305, the wettability and fluidity of SCS7 at the normal process temperatures of SAC305 at 255 oC – 260 oC are excellent and from the numerous implementation data we had in various applications, the solderability results is good with SCS7 using the same process temperature as their previous SAC305 parameters or even lower when running the same type of boards.

 

A3. SCS7 has a eutectic alloy composition, like 63SnPb37 and other eutectic solder alloys such as 99.3Sn0.7Cu and 96.5Sn3.5Ag, melts and solidify at a single melting temperature and thus the resultant joints would be shiner as compared to joints formed by SAC305 solder, which is a non-eutectic alloy. The final appearance of the solder joints would also depends on the cooling rate of the soldering and also on the type of fluxes used, as there are fluxes which are especially formulated to create a matte finish.

 

A4. Besides process parameters such as solder pot temperature and degree of wave agitation, the amount of dross is also affected by the inherent properties of the solder. SCS7 do contain additives to reduce surface oxidation rate and thus resulted in minimal dross generation during soldering.

 

A5. It is recommended that Asahi soldering fluxes be used with SCS7 solder as they are specially formulated for SCS7. However, there are many customers that use our SCS7 solder together with liquid fluxes from other suppliers of various types (rosin-based, water-soluble, VOC-free, etc) with good results. Asahi is committed to provide total solution to the industry.

 

A6. During the initial transition period, there are customers that need to use up their current stock of lead free wires (SAC305, SCN, 96.5Sn3.5Ag) to rework on SCS7 wave-soldered joints and after testings, it was found that there are no compatibility or reliability issues. However, it is recommended to use SCS7 core wires for best results.

 

A7. Cross-sectioning analyses and pull / shear test results after reliability testings (high temperature ageing, thermal cycling, and thermal shock) show that the reliability of SCS7 solder joints to be comparable to SAC305.

 

A8. Different dopants have different optimized levels and for SCS7, it was found after systematic research that a 0.02 wt% of Si in SnCu matrix is the optimal quantity to provide good strengthening of the SnCu matrix. Mechanical properties comparison of as-cast and thermal aged SCS7 with eutectic SnCu, SCN and SAC0307 show that it has better mechanical properties as compared to the rest, even though SCS7 has the lowest dopant quantity.

 

A9. This is due to intrinsic properties of SCS7, as it has a lower surface tension and higher fluidity than SAC305 at normal wave soldering process temperatures. Although SCS7 joints are thinner as compared to SAC305 joints, pull / shear tests comparison data have shown SCS7 joints to have comparable strengths as compared to SAC305 solder joints.

 

A10. Buying new machine is not needed for low temperature alloy. The existing equipment may have served its fully service life potential, hence it is a waste to throw away
functional equipment.