Scanning Acoustic Microscopy for FCBGAs
Scanning Acoustic Microscopy (SAM) is one of the pillars of non-destructive failure analysis of packaged integrated circuits, providing an analyst with a comparatively easy method of analysing a device’s structural integrity (SAM). It is possible to produce an accurate, detailed image of a packaged semiconductor device by using tightly focused pulses of ultrasonic waves and analysing the sound reflected by and transmitted through a sample. This image will reveal any air pockets or delamination that may cause early-life failure. SAM has proven to be a crucial technique for analyzing the wire-bonded, plastic-encapsulated parts that have historically dominated the market. The industry may be moving away from these kinds of devices in favour of packaging innovations like flip-chip ball grid arrays (FCBGAs), which make better use of bonding space and have the potential for greater thermal compensation. Despite this, the SAM is still relevant, and with a few modifications, it can offer invaluable information on these cutting-edge technologies.
The substantially smaller size of the features that must be examined is one of the difficulties presented by FCBGAs for scanning acoustic microscopy. Far better precision than what SAM is often known for is needed to find a defect that can be as little as a fractured die bump. Ultra-high frequency transducers, which can produce sound at frequencies between 110 and 250 MHz, are used to boost the spatial resolution (depending on the design of the transducer). Although these high-resolution images necessitate additional interpretation that may not necessarily be necessary on other devices, the narrower wavelengths of these transducers allow them to resolve even the smallest faults that might be hiding on the device.
The scanning acoustic microscope typically has algorithms for automatically finding defects built into it. For example, it may look at the phase of the echo waveform or set amplitude thresholds that signal any locations where the returned sound pulse is too “loud,” flagging them. The difference between good and bad devices is frequently so small as to not be detected by any kind of automated inspection, and the different material composition of the FCBGA is a confusing factor that can prevent direct analysis of a waveform’s phase. While these algorithms are frequently sufficient for the analysis of traditional devices, they may fail when applied to FCBGAs. Though a machine may not be able to detect the subtle signs of a dewetted die bump, a trained eye can pick it out from a lineup of properly formed connections with ease. Fortunately, the well-trained, inquisitive failure analyst has yet to be replaced by a mindless automaton (possibly to the dismay of accountants and science-fiction writers everywhere).
While being most frequently linked to the failure analysis of older semiconductor devices, scanning acoustic microscopy is more than capable of providing information for more contemporary processes. These processes do, in fact, frequently benefit the most from examination with SAM because they are unquestionably much less developed than the conventional packaging approaches.
