TEST METHODOLOGY CONSIDERATIONS
BY PAUL WALTER
As device manufacturers move from traditional devices interconnected with wirebonds to more advanced bumped devices, along with the process changes required, they also need to consider test methodology. As devices get smaller, the need to check the interconnect integrity becomes ever more demanding. Wire bonds normally are tested with simple wire pull and ball shear techniques, but when it comes to testing bumps several new tests should also be considered. These include traditional and cavity shear-and-pull tests using the cold-bump pull technique. There are also other considerations that affect equipment and test selection — bump structure and metallization, and the handling and tooling for the wafer or chip.
Shear Testing
Shear testing is a method of determining bond integrity that has been in use for many years. A simple tungsten chisel tool is positioned behind the bump and, as the tool moves forward at a predetermined shear height, it performs the test. The peak force reached during the test is recorded, and the sheared pad or interface is observed and graded for analytical purposes.
On advanced bond testers, an array of bumps can be preprogrammed to make the shear test automatic — leaving only the grading for the operator. However, a standard shear tool can deform the bump before a significant force is applied to the bond and, in many cases, the bump fails before the bond interface, limiting the usefulness of the data in relation to the bond interface. To improve this situation, a new method of shear test tooling was recently developed. Cavity shear testing improves the quality of data obtained for a range of samples tested and is useful for bumps that have a circular geometry.
A cavity tool design results in the following advantages: an increase in accuracy and quality of bond strength test data; the cavity tool mimics the shape of the bump, allowing the load to be applied for a larger surface; a reduction in force sensitivity in relation to the step-back height; a substantial increase in the test force applied to the bond results in better test and data quality.
Cavity Shear Testing
The initial ball deformation and subsequent failure mode is significantly affected by step-back height. As the step-back height is increased, the amount of solder residue increases and test reliability decreases. Use of a cavity shear tool with a curved cavity reduces bump deformation and increases the maximum possible force applied to the bond. The cavity tool's curved shape supports the bump, resulting in less deformation. As a result, the test is more effective at testing bond failure and the negative effects of step-back are minimized. Cavity tools are suitable for most bump geometries produced today.
 Figure 1. Bond tester. |
In general, shear testing (traditional and cavity shear) is a good indicator of bond integrity, but in some cases it cannot show the engineer more than the shear force of the solder. If the bond interface needs further examination, then a tensile test such as cold bump pull (CBP) should be considered (Figure 1).
Pull Testing Bumps
There are four possible reasons for considering cold bump pull testing in place of, or along with, the more traditional shear test:
- When the bump is recessed below the wafer surface, it gains support from the sidewalls of the cavity. In a CBP test, the cavity adds little or no support to the bond.
- During a shear test when load is at maximum, the bump becomes deformed above the bond site. With correctly designed tooling, bump deformation above the bond can be significantly reduced using a CBP test.
- Surface irregularities at the bond site often couple together, even though there may be little or no chemical adhesion between them. In shear testing, the locking together of surfaces may produce higher test loads than in a CBP test where the surfaces are separated.
- CBP tests the bond with an application of a simple tensile load, as opposed to shear test that applies a complex load consisting of tensile, shear and compressive forces. This complex mixture of loads may present problems when analyzing test data and trying to interpret failure modes.
Cold Bump Pull Testing
The test force exerted on samples with a good pad design and with high bond strength is limited by solder strength. This sample typically exhibits a single failure mode when the solder fails. When this happens, no data on the pad or bond strength can be collected.
Data collected from several test samples indicated that the limit imposed by solder strength is more common during shear tests than during CBP testing. In both the shear and CBP test, the tool distorts the solder bump under test. However, the distortion in a CBP test is controlled and minimizes the effect on the portion of the bump used during the test. CBP results in a greater variety of out-of-spec pad and bond failures.
A simple force analysis of both the shear and CBP test suggests that the stress on the bond under shear is more complex than that of a bond being pulled. It is considered that the simpler loading of tensile force can be an advantage when trying to analyze failure modes and the relative strengths of the solder ball bond, its mating pad and the substrate. CBP test result comparisons from samples with different bump geometries is considered more meaningful, because the simple tensile load per unit area will not be as affected by any changes in size.
To perform the test properly, CBP requires a gripping tool connected to a conventional pull load cell (Figure 2). This tool should be designed to hold the ball as firmly as possible, without significantly changing the characteristics of the bond and producing an optimal grip on the bump. Once the bump is gripped, it is reformed by the jaw cavities, creating a maximum grip and allowing the test force to reach its potential. However, the CBP test is slower than the shear test and can present a challenge in removing the solder residue from the jaws after the test.
 Figure 2. Cold bump pull jaws. |
The goal is to produce the maximum pull force possible. As is the case with other tests, like shear or wire pull, the sample material and test tool design limit the maximum possible test load. CBP jaws are currently being used with bump geometries down to 80 µm and under development to address the smaller sizes many device manufacturers have listed on their technology roadmaps.
CBP testing now is recognized as a real step forward in being able to evaluate the bond interface of all types of bumps and the work done has resulted in a published standard (test method) for hot and cold ball pull (JEITA EIAJ ET-7407).
Handling the Wafers
A key issue when performing bump testing is to determine if the parts will be presented as single chips or in wafer format. As a single chip, most bond testing equipment with simple fixturing modifications can handle the device allowing bond test using any of the methods explored above. If the devices are to be tested in wafer form, then consideration must be given to how the wafers are loaded, unloaded and handled on the bond tester.
Conclusion
Moving into the production of bumped devices introduces a number of challenges. In testing these devices, process engineers can start by using the simple shear test method. However, they may find that the shear test can only provide part of the picture on the bond interface integrity, and it becomes necessary to move to other test techniques such as cavity shear and, ultimately, CBP.
Device manufacturers are using different bump geometries, metallization and process techniques and, unlike wire bonding, there is no set test regime that fits all requirements. As a result, some degree of experimentation is needed to find the best test for regime specific products.
Another consideration is when to test the bumps, in wafer form or later in the process when they are diced into single chips. Again, this decision will affect equipment choices. A close working relationship established during device design between the device manufacturer and the bond tester supplier is the best way to determine what test regime will be best suited to the devices.
PAUL WALTER, managing director, may be contacted at Dage Precision Industries Ltd., Rabans Lane Aylesbury, Buckinghamshire HP I9 8RG, England; 44 (0) 1296 317800; e-mail: [email protected].