March, 2024
www.us-tech.com Nihon Superior “Cracks the Code” Continued from previous page
would be to switch to a solder alloy that can form reliable joints at a peak reflow temperature lower than the 482 to 500°F (250 to 260°C) required by the Sn- 3.0Ag-0.5Cu (SAC305) alloy that has become the standard lead- free alloy for reflow soldering. On the basis of the charac-
teristics of widely used IC pack- ages the requirement for the avoidance of warpage-related defects is that it should be possi- ble to form a solder joint that meets the usual quality criteria with a peak reflow temperature less than 392°F (200°C). Because of its relatively low
melting point and its unique ability to react with most of the termination finishes used in elec- tronics, tin (Sn) is an essential ingredient. Nature offers only two elements that can reduce the melting point of the resulting alloy to a temperature that makes reflow at less than 392°F (200°C) possible without intro- ducing other complications: bis- muth (Bi) and indium (In). The high cost of indium means that its use is likely to be limited to specialized high value applica- tions.
Bismuth, which is typically
cheaper than tin, provides a sig- nificant saving in material cost, and the long history of the use of bismuth salts for the treatment of indigestion means that its non-toxicity is well proven. There is a tin-bismuth
eutectic, Sn-57Bi with a melting point of 282.2°F (139°C) and wet- ting and flow characteristics that suggest that, with its sharp melt- ing point, it might be the ideal solution. However, because of the particular characteristics of the near-pure bismuth phase that forms in Sn-Bi alloys, a con- sideration that is as important as its effect on melting point is its mechanical properties.
Optimizing Bismuth Content Studies of the failure mecha-
nism in Sn-Bi alloys indicate that at strain rates at the higher end of the range to which solder joints can be exposed in service the Bi phase cracks in a brittle manner with the crack propagat- ing into the tin phase in which it is embedded triggering brittle failure of the whole solder joint. The Nihon Superior view
was, therefore, that careful con- sideration had to be given to the volume fraction of the bismuth phase in the solder joint. Recognizing that reality, in their formulation of a solder that would meet the less than 392°F (200°C) peak reflow temperature criterion Nihon Superior reduced the Bi content to the lowest level that would still leave the alloy with melting characteristics that would make it possible to meet
that criterion. That Bi level was identified as 37 wt% and that provided the basis for the alloy that, with further optimization,
was introduced to the electronics manufacturing industry under the brand name TempSave B37.” Nihon Superior acknowl-
edges that by reducing the Bi content to 37 wt% they have increased the melting behavior from the sharp 282.2°F (139°C) of the eutectic to a melting range of 282.2 to 345.2°F (139 to 174°C). There was a view in the
industry that such a melting range would increase the likeli- hood of a phenomenon known as hot tearing,” the result of which is a cavity or crack. However, the behavior of
Two common defects caused by package warpage.
metals in what is sometimes known as “the pasty range” is
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