Introduction
Since introduction of scanning acoustic tomography (SAT) also known as C-Mode scanning acoustic microscopy (CSAM) technology to the semiconductor package manufacturing for more than two decades, several thousand pieces of equipment have been serving the industry as essential quality assurance tools. Acoustic imaging offers inspection of imperfect material joints containing non-metal structures, such as delamination between silicon-metal joint glues, which in turn are difficult if not impossible to detect with X-ray imaging approach. Therefore, the device package failure analysis engineers routinely utilize both x-ray and acoustic imaging technologies as their complimentary nondestructive analysis tools.
The SAT technology does have its own limitations derived from the physical nature of acoustical wave: requirement of liquid medium to transfer ultrasound energy, requirement of flat and smooth package surface, difficulty in designing transducers, low resolution at lower ultrasound frequencies, less penetration at higher ultrasound frequencies, and slow acquisition speeds, etc.
To extend these limitations to their ends, development of new transducers or probes with applicable frequency range from 50MHz to 300MHz are very crucial, along with other signal handling advancements.
Recently, Kitami, et al., reported development of a specially designed signal processing unit and high resolution probes that can image 1µm features engraved in silicon material and an echo gating technique that intelligently tracks the surface plane so that it drastically reduces invisible area due to rough exterior surfaces of the package.
Encouraged by these new developments, we conducted a case study of SAT and X-ray CT imaging for a multilayer package consisting of two ceramics with flip-chip packages on organics substrates in between. The SAT system available for this study is also equipped with high resolution unit that generates well compressed pulses with excellent signal to noise ratio in a wide range of probe frequencies. In this report, we describe how flaws around flip-chip substrates embedded in a thick stack of ceramics mounted with surface components can be nondestructively inspected layer by layer to pinpoint the manufacturing defects hidden in them.
Experimental
On the manufacturing floor, we noticed that some of the multilayer devices electrically failed but they were also unable to find out the root cause using existing analytical equipment. Cross sectioning the sample is the only option which is not only destructive but also time consuming just to find out about the flaws along one line out of entire surface. Therefore, we selected to study the most acoustically complex device to investigate the capability with state of the art X-ray and ultrasound imaging technologies.
Sample Descriptions and Preview
The sample consists of high temperature co-fired ceramics (HTCC) substrates as top and bottom layers embedded with chip packages on polymer substrates in between as illustrated in Figure 1. The dimensions are 18 mm width, 35 mm length, and 3.3 mm height. The regions of interest for possible delamination or voids are the joints to each interfaces between deep layers. Of course, these HTCC layers themselves are multi-layered substrates as well.
Image: Illustration of material structure of the sample and their thickness in micrometer. Outer layers are high-temperature, co-fired ceramics.
To read this entire article, which appeared in the August 2016 issue of SMT Magazine, visit:
