A cost-effective, fast method measures warpage in electronic devices.
BY MIKYOUNG LEE, MICHAEL PECHT, JOHN TYSON AND TIMOTHY SCHMIDT
Ball grid array (BGA) package warpage caused by the mismatch of coefficients of thermal expansion and asymmetric package geometry creates some solder joint reliability problems — especially for chip-scale packages. Methods to assess the design and process factors influencing warpage include: electronic speckle pattern interferometry (ESPI), shearography and moiré. However, these methods are generally time-consuming and expensive.
 Figure 1. Vertical structure of a FBGA package is shown with a two-layer PCB. (Courtesy of Samsung) |
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A non-contact and material-independent determination of deformation and strain using a combination of the three-dimensional (3-D) image correlation method with high-resolution digital CCD cameras has been used successfully to measure package warpage. This method significantly reduces the cost and time-consuming preparation. For fast measurements, standard cameras can gather data at up to 20 fps. For high-speed manufacturing tests, high-speed cameras can gather data at 485 fps, to rates of up to 10M fps, or in real-time. Therefore, this technology can collect data 30 times faster. It also provides a much higher dynamic range of deformation measurements and is hundreds of times less sensitive to vibration interference than ESPI.
3-D Image Correlation Method
The 3-D image correlation method features a static or dynamic non-contact, full-field, 3-D strain measurement. To implement, a pattern that can be random or regular with good contrast is applied to the surface of the test object. The pattern deforms along with the object. The deformation of the object under different load conditions is recorded by two CCD cameras and evaluated using digital image processing. An initial image prior to loading defines a set of unique correlation areas across the entire area of interest. These areas are known as macro-image facets, which are typically 5-20 pixels across. Macro-image facets are tracked in each successive image during loading with sub-pixel accuracy. Using photogrammetric principles, 3-D coordinates of the surface of the specimen can be calculated precisely. The results are 3-D coordinates of every point in the area of interest, contours of components, any displacements during the test, as well as the plane strain tensor.1 Although only two image sets are required to measure the change from minimum to maximum loads, multiple image sets provide a progressive measurement of the deformations and strains.
Ball Grid Array Warpage
Fine-pitch BGAs (FBGA) have been developed for high-end logic devices. The vertical structure of a FBGA package with a two-layer PCB is shown in Figure 1. The package is constructed of a substrate (BT), onto which a die is mounted and an array of solder balls is attached. This die is encapsulated with epoxy molding compound (EMC) for protection. A basic FBGA uses the same construction, but reduces the size of the substrate as well as the pitch and size of the balls.2
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 Figure 2. This series of images, with a fixed scale, show the progression of warping during the cool-down process. |
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Warpage in the FBGA is a serious problem affecting solder joint reliability. Researchers have tried to measure warpage level directly for better understanding of packages.3 Some efforts have been made to evaluate package warpage in terms of design and manufacturing factors by using simulation.4 Simulation, however, requires verification including adjustments for assumptions made during analysis. Three-dimensional measuring of the total deformation of complex objects, rather than relative deformation, is necessary for that reason.
 Figure 3. Displacement along the diagonal section line indicates warpage. |
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Color plots of out-of-plane displacements using a 3-D image correlation measurement system, as shown in Figure 2, exhibit a generally symmetrical behavior that is expected under the applied thermal conditions. A slight asymmetric bulging in that area may indicate a defect such as internal debonding or asymmetric surface mount bonding. Also shown in Figure 2, is the progression of warping during the cool-down process. The bulge at the upper right edge could be a focus of failure analysis efforts. Direct measurement of warpage, or capturing abnormal deformation, can help design and reliability engineers gain an understanding of what is going on under a package's surface. Figure 3 plots out-of-plane displacements along the diagonal section line shown in Figure 2.
Conclusion
Package warpage caused by mismatch of CTEs among construction materials affects the reliability of high pin-count BGA devices. This study demonstrated a cost-effective, fast method to measure warpage and identify anomalies in electronic devices.
The authors will furnish references upon request.
MIKYOUNG LEE, research scientist, and MICHAEL PECHT, professor, may be contacted at University of Maryland, CALCE Electronic Products and Systems Center, College Park, MD 20742; LEE may be contacted via e-mail at [email protected]. JOHN TYSON and TIMOTHY SCHMIDT may be contacted at Trilion Quality Systems, Four Tower Bridge, 200 Bar Harbor Dr., Suite 400, West Conshohocken, PA. 19428.