Whisker-free lead frames

Abstract

The electrical and mechanical properties of structures such as lead frames and other electrical / electronic devices containing, during processing, copper / tin interfaces are improved by introduction of nickel to such interface. Typically, a weight percentage of nickel to tin in the range 1 to 12 weight percent yields upon melting of the tin, an intermetallic compound with essentially no occluded, unbound tin. Thus undesirable anomalous structures such as tin needles and substantially non-planar interface compositions are avoided. Advantageously a nickel / tin / copper intermetallic interface that is substantially planar is formed in the substantial absence of needle-like tin structures.

Description

TECHNICAL FIELD [0001] This invention relates to tin containing interconnects for electronic and electrical devices and in particular to tin containing interconnects that avoid formation of tin whiskers. BACKGROUND OF THE INVENTION [0002] For electrical and electronic devices, typically electrical connections are made through the expedient of a lead frame or other copper containing structures. For example, in the fabrication of integrated circuits, a silicon body having electronic circuitry is connected to a metal e.g. copper, lead frame such as shown in FIG. 1 at 2 with the chip positioned at 3 and connections between bonding pads on the chip and the lead frame shown at 4. After the chip is bonded to the lead frame, the chip is encapsulated typically in a polymer composition. The strip, 6, in FIG. 1 is removed from the lead frame in a process generally denominated trimming. Thus, after trimming, the individual leads, 7, are no longer mechanically connected on one end. The leads are...

AI Technical Summary

Benefits of technology

[0006] Surprisingly it has been found that tin whiskers are present even after soldering with materials having been subjected to soldering temperatures above 232° C. Thus the issues associated with such whiskers remain with lead-free solders. By practice of the invention, such whiskers are essentially totally avoided without the expedient of adding lead to the tin coating of the lead frame. Such results are achieved in one embodiment by forming on the lead frame a nickel / tin composition having from 1 to 12 weight percent nickel relative to tin. Such composition results in formation of an intermetallic compound with copper such that essentially no free tin remains. Additionally, this copper / nickel / tin intermetallic has an essentially planar surface and thus is mechanically quite stable. As a result, tin whiskers are avoided and the resultant solder connection is mechanically robust.

[0008] The interaction between the tin of the solder and the copper from any of the solder pad metal interfaces (die, substrate, or board) has the potential to produce mechanical instabilities with concomitant reliability issues often characterized by very non-planar interfaces. Through the use of nickel in the solder in the range of 1 to 12 weight percent relative to tin, a robust planar intermetallic compound is formed that substantially reduces the proclivity to induce mechanical instabilities often arising when such an interface is not present. Thus, even in applications where whiskers are not a consideration, but where tin materials promote mechanical instabilities the invention is nevertheless advantageously employed.

Problems solved by technology

Surprisingly it has been found that tin whiskers are present even after soldering with materials having been subjected to soldering temperatures above 232° C. Thus the issues associated with such whiskers remain with lead-free solders.

The interaction between the tin of the solder and the copper from any of the solder pad metal interfaces (die, substrate, or board) has the potential to produce mechanical instabilities with concomitant reliability issues often characterized by very non-planar interfaces.

Images