...this is page 3; click here to go to page 2.
For many years solder bumps have been sheared at speeds of less than 1mm/second. It has been found that testing at these speeds almost always produces failures in the bulk solder but manufacturing and end use failures occur in the bond interface. Recent developments have shown that testing at speeds in the range of hundreds or thousands of millimeters per second more often produce the ‘failure mode of interest’ at the bond interface which is therefore much more meaningful (Figure 3).
Holding the sample is of equal importance. In many bond test applications sub-micron precision is required while in other cases forces of up to hundreds of kilograms of force are often a requirement. High force applications of up to 500 kilograms (kgf) means that great attention be applied to this detail together with the contribution of a skilled or knowledgeable technician as one of the keys to ensure quality results. Standard fixtures are often used but this sometimes requires modification or custom tooling has to be designed to hold the sample. Clamping is normally required to fully support the sample when it is subjected to the test load but in such a fashion that the bond is not stressed and negligible sample deflection occurs.
Die pull also referred to as stud pull, has been used for many years as an alternative loading condition to the more frequently used shear test. It is generally a more difficult test since a stud has to be glued to the die which can then be pulled to test the bond (Figure 4). The strength of the adhesive bond between the die and the stud will limit the maximum load that can be applied to the bond. Adhesive strengths of around 20 to 30 megapascal (MPa) are possible and current research is trying to increase this limit to 45 MPa. The increase in stacked die and wafer-to-wafer applications using multiple layers of silicon is driving a corresponding increase in this type of test. It also has relevance when a layer or die is very thin and the stress concentration on the side of the sample exceeds its yield stress in a shear test. In these cases the stud pull test distributes the load over the larger top surface.
Ease of use is another fundamental requirement of any bond testing system. Any complex measurement system potentially runs the risk of producing errors due to operator misunderstandings and or fatigue. Ease of use also directly affects throughput and cost of ownership. The most common interface with the operator or engineer is that the software must be straightforward, intuitive and yet comprehensive and flexible for the very wide range of bond testing applications it must serve. Hardware is also important, starting with the loading of a sample through to continuous and easy testing. Controls and optics, usually a stereo zoom microscope, must be conveniently and ergonomically placed.
A final ingredient in a bond tester is flexibility. There are many differing demands made upon a modern bond testing system. To serve this range of applications the machine must be flexible for the wide variety of tests and samples. Some requirements are narrow and so the system must be configurable to a cost effective solution. Other requirements can be varied and so the same configurability can be used to enable on-site changeover for a wide variety of applications. To achieve this, a range of various frame sizes are needed that have an open layout with accurate and long travel axes coupled to an easy way of changing between test sensors. In effect, a multifunctional bond tester equipped with automatic selection of pull, shear and impact tools and interchangeable revolving measurement unit and impact measurement unit sensors has a high degree of versatility and is capable of bond testing a wide variety of semiconductor and printed circuit board assembly applications.