Anti-tombstoning lead free alloys for surface mount reflow soldering

Abstract

A lead-free solder alloy composition comprising tin, silver and copper, and a process for reflow soldering for minimizing tombstoning frequency are disclosed. In one particular exemplary embodiment, the lead-free Sn—Ag—Cu solder alloys for minimizing the tombstoning effect of the present disclosure display high mass fraction during melting and prolonged melting as shown by a widened DSC peaks, that allows for a balanced surface tension on both ends of the chip component to develop. In accordance with further aspects of this exemplary embodiment, the alloys display a mass fraction of solid during melting greater than 20% and a DSC peak width greater than 8° C. using a 5° C. / min scan rate. In accordance with further aspects of this exemplary embodiment, the alloy comprises on a weight basis Ag 1-4.5%, Cu 0.3-1% balanced with Sn.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This patent application claims priority to U.S. Provisional Patent Application No. 60 / 517,404 (Attorney Docket No. 64470.000005); filed Nov. 6, 2003, which is hereby incorporated by reference herein in its entirety.FIELD OF THE DISCLOSURE [0002] The present disclosure relates generally to lead free alloys for use in soldering and, more particularly, to anti-tombstoning alloy compositions comprising tin, silver and copper. BACKGROUND OF THE DISCLOSURE [0003] As the electronics industries continue to pursue miniaturization of the electronic devices, increasing utilization of the small leadless components such as 0402 (this terminology means that the components are 40 mil×20 mil in size) and 0201 has resulted in a rapid increase in tombstoning defects. Such tombstoning defects, which are one of the most common defects observed in surface mount reflow soldering of small leadless components such as resistors and capacitors are caused by a to...

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Benefits of technology

[0021] A process of reflow soldering in electronic assemblies using various lead-free anti-tombstoning Sn—Ag—Cu solder alloy compositions is also disclosed. In one particular exemplary embodiment, the process of reducing tombstoning effect ...

Problems solved by technology

As the electronics industries continue to pursue miniaturization of the electronic devices, increasing utilization of the small leadless components such as 0402 (this terminology means that the components are 40 mil×20 mil in size) and 0201 has resulted in a rapid increase in tombstoning defects.

The tombstoning effect is attributed to the imbalance of a pulling force caused by surface tension of molten solders at both ends of a component during reflow soldering.

The intricate balance of the surface tension of the molten solder on the component may be easily disturbed by either the change of the solderability of the component or by differences in melting at the moment solder paste at each end of the component begins to reflow.

Lead is known to have toxic effects and poses environmental and public health risks.

For this reason, federal legislation has imposed strict limitations upon the use of lead and lead-containing compositions.

However, the main problem with the present state of the art in reflow soldering based on the lead-free Sn—Ag—Cu alloys is that usable anti-tombstoning solder alloys have not yet been discovered.

Although Katoh et al. in U.S. Pat. No. 6,554,180 B1 teaches using “twin-peak” alloys with 0.2-1 mass % Ag, balanced with Sn as well as a flux to reduce the tombstoning defects, this alloy range deviates too much from the commonly acceptable Sn—Ag—Cu alloy compositions, and therefore is considered an impractical solution.

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