SnAgCu alloys

If the elevated temperatures or other issues make one feel that the SnAgCu alloys may not be appropriate for their facility, Dreezen and Luyckx present information on lead free assembly with conductive adhesives. 

Lower-budget operations and/or those that have a relatively new wave machine that is not depreciated fully may want to continue using their current wave-soldering equipment after lead-free soldering has been implemented. However, if you are using an SnAgCu alloy, baffles, pumps, and... 

Surface-tension Test —The tombstoning performance of solder pastes has been speculated to be related to the surface tension of respective solder alloys. In this study, a wetting balance was used to measure the surface tension of solder alloys following established conditions. A piece of alumina coupon with a dimension of 2.5 X 0.4 X 0.062 cm was used for the testing. The solder pot was maintained at 245°C for the SnPb alloy and 260°C for SnAgCu alloys. The measured force with respect to time was translated to a graph of force vs. dip depth based on dip speed. When the alumina piece immersed into the solder for a certain depth where the meniscus became stable, the measured force was proportional to the displaced volume of solder. This force may be described as follows ... 

Differential Scanning Calorimetry — DSC melting curves of the powder of SnAgCu alloys are shown in Figure 2. The data... 

Figure 4. Relation between surface tension of SnAgCu alloys and tombstoning rate.

Another issue to consider is the effects that solder alloys have on voids in plated through-hole (PTH) barrels. SnAgCu has shown to be more prone to voiding than tin/lead and SnCuNi alloys. If the leads are used for high-power resistance or temperature dissipation, this can be problematic. For many other applications, these voids will not cause... 

The following is a brief comparison of SnAgCu and SnCuNi alloys, which appear to be the most likely lead-free wavesoldering alloy candidates. 

Both alloys had greater difficulties with hole fill and fillet forming during second-pass rework soldering both alloys needed more dwell time for rework, especially the SnAgCu solder. For the SnAgCu wave process, different solder-pot-temperature settings were used in the evaluation 260° and 274°C 300°C at different dwell times with and without board preheat on the 135-mil board. An external BGA rework machine provided top-board preheat of 120°C because the mini-pot machine used did not have board-preheat capabilities. 

---