Die attach failure mechanisms include physical failure of the die bond, physical damage to the die including its surface and adhesive contamination on the die surface. The failure mechanisms can be minimised by identifying their causes and rectifying them.
The following describes the common die attach failure mechanisms and methods to identify and prevent the causes; the causes are described in more details sections.
Die lifting refers to partial separation of the die from the substrate after the die attach process. It can often be identified by visual inspection of the die edges either by naked eye or a microscope.
Die lifting can be caused by either incomplete coverage of the adhesive or formation of voids in the adhesive deposition. Factors that cause incomplete coverage include contamination on the die surface and less than required volume of the adhesive based on its rheology. Adhesive rheology contributes to the spread of an adhesive on a surface. Voids formation is caused by contaminated surface and incomplete coverage of the adhesive.
Die lifting can be prevented by ensuring that an adhesive deposition uniformly wets the substrate and/or die surface. The surface of the die/ substrate should be thoroughly cleaned before adhesive deposition. The adhesive quantity should be carefully controlled and every deposition should be visually inspected using the microscope on the die attach tool. In case of incomplete coverage, a larger volume of adhesive should be dispensed. Volume of dispensed adhesive can be increased by increasing the dispensing time, dispensing pressure, resolution and decreasing the dispensing speed.
Die cracking is partial physical damage to the die in the form of fractures. It can be identified by visual inspection of the die through naked eye or microscope.
Die cracking is caused by application of high die ejection force when separating a die from a wafer on a backing tape.It can also be caused by incomplete adhesive coverage and voids in the deposition. Incomplete coverage and voids can cause any mechanical shock/force on the die to be distributed unevenly leading to cracks or fractures.
Die cracking can be prevented by using an appropriate ejection force for the dies. This is done by increasing the ejection force in small steps till the appropriate force is found. It can also be prevented by ensuring uniform deposition of the die attach adhesive.
Adhesive contamination is the adhesive deposition climbing the die edges to spread on the surface of the die. If the adhesive is conductive, it may cause short circuits between the electrical interconnections on the die surface leading to functional failure. It can be identified by visual inspection of the die.
Adhesive contamination can be caused by dispensing a higher than required volume on the substrate/die surface. When the die is the placed on the substrate surface, the adhesive can climb the die edges and deposit on the surface.
It can be prevented by ensuring that the volume of dispensed adhesive is in an optimum range. Optimum volume can be selected either by using previous experience or by selecting a smaller volume and increasing it in small steps to achieve complete coverage.
Die scratching can be caused by poor die handling during the process. It may lead to functional failure of the die if the scratches disrupt the surface structure of the die such as deformation of the metallization on the die surface. Die scratching can also be identified by visual inspection of the die. It can be prevented by careful handling of the die during the process.
The die attach process can be characterised by carrying out one or more of the shear tests, pull tests and thermal tests. The aim of the characterisation is to replicate real life loads on the bond and test its strength.
Shear tests apply force on the die to try and shift it laterally. The force is applied to the die while the substrate is held in a fixed position. The load applied to the die produces a number of failure mechanisms, which indicate the strength of the die bond in terms of applied force.
Die pull tests are useful for testing dies which are attached directly to a substrate/package surrounded by other electronic components or dies. A die is pulled by a force exerted on its surface by pulling a stud, which is glued to its surface. The layer of glue applied to connect the stud to the die should be thin and even.
The purpose of thermal testing is to determine if the die bond will survive either a specific temperature or thermal cycling. The temperature specifics are determined by the application requirements. Thermal tests can cause die lifting, die bond failure, substrate bond failure and bond failure owing to the thermal stress.
The Advanced Packaging Facility has various equipment to characterise the die attach quality which includes the Nordson dage 4000 bond tester, the Zeiss EVO SEM equipped with Gatan XuM and the Weiss environmental chamber. The Nordson dage 4000 bond tester can be configured to perform die shear tests. The SEM can be used to image the outer edges of the attached die to inspect any micro cracks. The X-ray capability can be used for computed tomography which an imaging technique that provides 2D images of the internal structure of samples. A series of 2D images taken at different depths can then be reconstructed to provide a 3D image of the attached die. This can be used to inspect any void in bond between the die and the package/die/substrate. The environmental chamber can be used for operating conditions tests such as specific temperature and humidity levels.