Strain Analysis of PQFP Packages
Digital image correlation metrology technology
BY LIAM KEHOE, VINCENT GU??NEBAUT, PATRICK LYNCH, MAURA O'SULLIVAN, PAT KELLY AND DANIEL VANDERSTRAETEN
Product reliability with the customer is the quality goal of all micro-engineering design. The improvement of package and microsystem design methodologies requires a capability to perform deformation and strain analyses as well as visualization of the strain field on the submicron scale. Using advances in digital image correlation technology, deformation and strain characteristics can be measured as soon as an IC package prototype is available, without the need for an extended thermal cycle stressing study on an ensemble of prototypes to induce defects. Deformation strain field determination highlights the high strain localities in a component or package from which voids, cracks and delaminations can initiate during subsystem assembly and component lifetime. The identification (by measurement) and elimination (by improved design) of excessive strain points in a micro-engineered product, such as an IC package, is a key to improving both yield and reliability.
However, improvements in micro-engineered product design are critically dependent on the availability of a fast, programmably driven deformation analysis strain field metrology system robust enough for the industrial production facility. IC package product development cycles extending over a 12- to 18-month period — with repeated product design iterations, extended duration reliability testing cycles and qualification steps — are no longer competitive. The availability of routine deformation analyses and strain field maps on the same day that the first prototypes become available has the potential to significantly cut development costs and time-to-market, providing an essential competitive advantage in this era of shorter product life cycles. Application of the deformation analysis and strain field data in the design provides further benefits in the reduction of customer returns when the product is in the field.
Deformation analysis and strain field determination addresses the next-generation requirements of package and microsystem metrology. Commonly used techniques such as integrated strain gauges are too localized and no longer provide the degree of accuracy and reliability demanded by the designers of advanced microelectronic and microsystem packaging technologies. Moreover, they provide no information on what is happening outside of the silicon die, and only address the types of failure associated with the die itself experiencing strain — especially those associated with analog and mixed-signal devices. Digital device designers have a high tolerance for strain in the die, but share the concerns of all engineers about the integrity of the package assembly during product lifetime.
Older, indirect thermomechanical deformation strain metrology optical methods, such as Moiré interferometry, can sometimes be effective as an indirect deformation measurement method. However, the difficulties of sample preparation, and the requirement to apply gratings with adhesive layers for in-plane deformation analyses, negates their usefulness and decreases uptake in an industrial context. Defect detection techniques such as dye-penetrant testing are costly in sample preparation and analysis time and labor. X-ray analysis and scanning acoustic microscopy have relatively low resolution limits and are only useful in detecting defects that have already occurred, necessitating the performance of a costly thermal stressing cycle to induce defects in a fraction of a set of prototypes. It is recognized by the packaging industry that defect detection by acoustic microscope requires a secondary independent inspection. This has been explicitly recognized in the recent JEDEC standard J-STD-020-B for substrate-based IC packages, which prescribes a second cross-sectional analysis for packages shown to have failed acoustic microscopy inspection.
Digital Image Correlation Technology
A new system* uses advances in digital image correlation technology for in-plane and out-of-plane deformation analysis and strain measurement (Figure 1). Digital image correlation works by recording two or more optical images of a sample under different conditions of an applied thermal stress to the sample. In its simplest form, an image of the specimen under test is taken in two different states of stress (in this example, cold and hot). A small region of the image, called the sub-image, observed in the cold state and its match are then found in the hot image. Using numerically intensive algorithms, the size of the displacement between the two is then calculated to accuracies equal to small fractions of the size of each pixel of the CCD camera used to capture the images. One motion vector per sub-image is calculated. Sub-images can be stepped by fractions to increase the density of vector origins. The correlation of the greyscale pattern of the sub-image in the hot image is also calculated, to determine whether any fundamental deformation of the material surface has also occurred. From the images, the analysis algorithm calculates the vector referencing the sub-image (actually an array of N × N pixels) location in the second image to its location in the reference image. Thus, an array of sub-image deformation vectors is calculated, showing the local relative deformation of different parts of the workpiece within the field of view of the optical system under the thermal stress.
Figure 1. The basic principle of digital image correlation is to calculate the sub-image greyscale matrix deformation between two component images with different externally applied thermal stresses.
Digital image correlation deformation vector maps have two scales, and two associated resolutions. The image scale is depicted on captured video microscope images, and its resolution is limited by the optical system and camera characteristics, ultimately limited by diffraction. The deformation vector scale is that of the deformation vectors superimposed on the reference image, and has a deep subpixel resolution. To display the measurable submicron deformation, overlay vectors are plotted on a scale of typically 5 to 20 times that of the image. The scale and resolution depends on a number of factors, but primarily on the magnification of optics used.
Digital Image Correlation Analysis of PQFP Packages
The following examples illustrate the capability of digital image correlation to analyze both defective and non-defective plastic quad flat pack (PQFP) packages. One set of these had previously been subjected to a popcorn-testing stress cycle. A second set was packaged units that had not experienced board assembly stresses. The packages were cross-sectioned to inspect in-plane deformation on a cross section through the package.
Figure 2. Motion analysis of a PQFP package, referenced to silicon die.
The digital image correlation system has a programmable thermoelectric stressing chuck that permits cooling as well as heating, and allows "room temperature" to be specified and controlled in all stressing cycles. The same thermal stressing cycle was used to test both packages. Samples were heated from 20 to 95°C, with a 5-min. soak at each temperature before imaging. Images of the samples were taken at both temperatures and deformation vector and strain maps were obtained for both.
Figure 3. Shear strain field map of a PQFP package.
Motion analysis of the unit at the edge of the silicon die is shown in Figure 2 as a deformation vector map. There is a difference in relative size of the deformation vectors between the die attach pad and the package epoxy. This motion mismatch arises because of the differential in the coefficients of thermal expansion, and their cumulative effect from the center of the package is shown more clearly by the shear map in Figure 3, and strain in X and Y maps. Strain maps are calculated as a derivative of the deformation vector map.
Figure 4. Motion analysis of popcorn-stressed PQFP package, showing a crack.
Figure 4 shows the motion analysis of the post-popcorn-tested package at the lead frame edge extending into the encapsulant as a deformation vector map. The popcorn crack is evident in the abruptly changing direction of the motion vectors on either side of the crack. This defect was not visible to the scanning acoustic microscope or the high-resolution optical microscope.
Digital image correlation can be a powerful tool for fast thermomechanical deformation and strain analysis in IC packages and microsystems. It can provide valuable early indications of high-strain regions that are the most likely sources of failure, as well as detecting thermally induced failures in packages.
LIAM KEHOE et al. may be contacted at Optical Metrology Innovations Ltd., 2200 Cork Airport Business Park, Cork Airport, Co. Cork, Ireland; 353-21-4316907; e-mail: firstname.lastname@example.org.