New Patent-Pending 'Cavity' Shear Test Method
Leading test and inspection equipment manufacturer in the semiconductor and PCB manufacturing markets, Dage Precision Industries, has developed a new patent-pending shear test technology that dramatically improves the quality and accuracy of data obtained during gold ball bond shear testing.
Named ‘cavity’ shear testing (patent applications 09/564171 in the US and 00-135142 in Japan), this new technique is said to reduce sensitivity to step-back height, and allow a substantial increase in the test force applied to the bond.
Traditional testing of gold ball bonds in the semiconductor and microelectronics manufacturing industries has used shear force methods that use chisel (i.e. flat) tools. Although the use of chisel tools for measuring the integrity of bonds between gold balls and their mating pad is generally considered reliable and well-proven, it is not without its limitations.
Namely the accuracy and quality of the bond strength test data can degrade when attempting to measure higher bond strengths, and is also highly sensitive to step-back height. Moreover, the maximum test force that can be successfully applied to a sample is often too small to produce a sufficient test resolution and therefore an accurate measurement of bond strength and failure mode behaviour.
The reason for this is that shear tools tend to deform the ball before a significant force is applied to the bond, and in many cases the ball fails before the bond. Cavity shear tools, however, have a curved cavity (as opposed to a conventional flat chisel plate) that fits more naturally and snugly around the ball to reduce deformation during shear testing. This allows the maximum possible shear force to be applied yielding far higher quality and more reliable bond strength and failure mode data than conventional chisel tool-based techniques.
In addition, when a chisel tool is used, ball deformation often affects the failure mode: At normal step-back heights the tensile strength of the gold behind the tool is less than its bond strength, causing the gold to tear. This part of the ball remains on the pad with its portion of the bond strength untested.
As the step-back is increased, the amount of gold left increases and the quality of test result decreases rapidly. With the cavity tool, its curved shape supports the ball and the initial deformation is less. Consequently, more total (i.e. ‘good’) bond failures occur and the affects of step-back are much less.
Furthermore, as ball diameters shrink in accordance with increasingly popular fine pitch and ultra fine pitch bonding geometries, bond strengths will only continue to increase in the future. Therefore, the cavity shear test could quickly supersede existing shear test techniques as they increasingly fail to deliver an acceptable level of test accuracy and reliability in gold ball bond testing.