This article investigates the challenges and solutions to performing bond shear on overhanging die surfaces. New test software is necessary to achieve virtually zero landing force when testing the bond strength of ball bonds on overhanging die within semiconductor packaging. Advances in semiconductor packaging technology continue to drive bondtesting capability at a rapid rate.
The testing of interconnections within stack die typically found in system-in-package (SIP) devices generates several exciting and new challenges. In particular, overhanging die can be difficult to bondtest due to various complications such as thin spacing between die and low-profile wire bonding. A solution is yet to be perfected or rolled out industry-wide for the shearing of ball bonds underneath the overhanging die. However, the traditional problem of die surface deflection associated with the load tool landing on an overhanging die surface has been solved with the assistance of new software.
Ball Shear on Overhanging Die
One of the major issues in performing ball shear on the overhanging die is the possibility that the die surface will be deflected during load tool landing and setting the required step back. In this instance, step back can be defined as the distance between the tip of the load tool and the surface adjacent to the bond. Other terms include step-up or shear height. This difficulty can be worked around by utilizing an oscillating load tool in combination with new test software, which controls the sequence of operations during bondtesting.
The initial step is the normal touch down, this is where the shear tool lands on and detects the overhanging die surface. (Figure 1 shows a typical alignment of the shear tool during touchdown.)
Figure 1. Alignment of shear tool with ball bond.
Then, following tool landing, the Y-axis is oscillated with a minimum amplitude to induce an oscillating force that the transducer can detect. (Figure 2 shows tool landing and oscillating a small amount to produce a measurable force.)
Figure 2. Shear tool lands and oscillates producing a measurable force on the transducer.
Raising the Z-axis, a small step for each oscillation is then carried out until no output is measured by the transducer. This position represents the point of contact between the shear tool and the overhanging die surface at which there is zero or minimal landing force. (Figure 3 shows shear tool being raised until the oscillating force is reduced to zero.)
Figure 3. Shear tool being slowly raised until oscillating force is reduced to zero.
Normal step back is then applied to the shear tool at this point. Shear height may vary if vertical thrusts occur during the actual bondtest, even though step back is controlled. This kind of thrust can be significantly reduced using a cavity shear tool. This will be described later. (Figure 4 shows the step back of the shear tool followed by bond shear.)
Figure 4. Step back of the shear tool followed by bond shear.
This new process for testing has a slightly longer duration as the system finds the zero-landing force condition prior to actual bondtesting. The additional time is usually reliant upon the amount of deflection exhibited by the overhanging die surface.
One aspect of bondtesting is dealing with the problem of overhanging die deflection in these types of packages. Other requirements are considered in the following.
Bonds on overhanging die tend to be small and pitches are fine, so the bondtester must be really precise and repeatable and this can only be achieved with advanced technology. This necessitates that the system has frictionless, air-bearing, ball shear load cartridges with sub-micron step back and +/- 0.25 micron step back accuracy.
Cavity shear for small bonds with passivation layers and other new load tool designs need to be optimized.
A high-grade magnification optical system is a must to achieve tool alignment and bond failure mode grading; for example, using a Borescope imaging system.
Furthermore, it is essential that the bondtester is equipped with a built-in self-leveling, self-damping, anti-vibration system, due to the low forces involved. Precision control for accurate sample manipulation is also vital.
Borescope Imaging System
Traditional stereomicroscopes come equipped with large diameter object lenses which allow for a long working distance (typically 140 mm) but an inadequate depth of focus for small geometry ball or bump shear applications. (Figure 5 shows a typical view through a stereomicroscope.)
Figure 5. Typical view as seen through a stereomicroscope with 128X optics and 0.2 mm FOV.
Operators carrying out ball shear tend to find it easier to align the load tool to the ball bond by viewing the package on a screen with a monocular image provided by a Borescope. The reduced working distance of the Borescope provides the higher magnification that would not be possible with conventional stereomicroscopes. The higher magnification of grading process is greatly improved following a shear test. (Figure 6 shows a view as seen through a Borescope.)
Figure 6. Typical view as seen through a Borescope with 330 micron FOV.
There are many advantages to using cavity shear tools for the testing of ultra-fine pitch bonds on overhanging die. Overall, implementing the use of cavity shear tools improves the accuracy and quality of the bond strength test data. This advanced development in several ways. A cavity shear tool acts as an aid to bypass the sensitivity to minimum step back height which occurs when the load tool lands on a passivation layer. Contrasting this with a conventional shear tool - a cavity shear tool distributes the force over the surface of the bond equally, thus, substantially increasing the load applied to the bond area.
The influence of ambient vibration on accurate bondtesting becomes more significant as geometries reduce. This also depends on the location of the bondtester within a test laboratory or manufacturing facility. Therefore, preparations should be made to ensure the machine is isolated from vibration. Unchecked vibration will greatly affect test results, instrument calibration, and the optical image quality at high levels of magnification. As described earlier, an oscillating load tool method is implemented during the overhanging die ball shear cycle and, for this technique to be effective, the machine must be free from unwanted ambient vibration. This may be achieved using an antivibration system of mounts that are self-leveling and self-damping.
The final feature required for bondtesting on overhanging die is sample manipulation with considerable accuracy. It is critical that precise alignment of the shear tool to the ball bond is carried out during setup.
Precise X and Y-axis movements are important for effortless manipulation through coarse and fine alignments by the operator. The Z-axis motion control of the bondtester needs a sub-micron resolution. The landing force of the shear tool must be proportionally reduced to the smaller landing area of the tool tip. For example, the landing force must be reduced by a factor of four if the width of the load tool face is halved. Sequential testing of bonds may at the exact shear height be accomplished in two ways; joystick step and repeat automation, where the co-ordinates of the first and last bonds are entered in conjunction with the pitch, or auto pitch testing where the pitch is set down and the tool pauses before shearing the next bond to enable grading. Automation at this level in ultra-fine pitch applications is only feasible with the appropriate precision in XY table manipulation.
Step Down Shear
There is an immediate problem faced when shearing the overhanging die due to inconsistent and non-planar surfaces. In order to prevail over this problem, the step-down shear sequence can be put into action in conjunction with soft land.
Further progress in semiconductor packaging will harbor the use of overhanging die for the foreseeable future. Testing the bond strength integrity on the various surfaces of these complex packages remains a challenge across industry. Implementing the use of a proper test sequence, combined with a special toolset, new test software, and a self-damping anti-vibration system permits the effective bondtesting on overhanging die surfaces.
This information has been sourced, reviewed and adapted from materials provided by Nordson DAGE.
For more information on this source, please visit Nordson DAGE.