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Double-zone acoustic scanning of plastic-encapsulated ICs

Posted: 19 Apr 2011 ?? ?Print Version ?Bookmark and Share

Keywords:acoustic microscopes? scanning? transducer?

Prior to assembly of electronics systems, plastic-encapsulated integrated circuits (ICs) are often imaged by acoustic microscopes in order to identify internal packaging defects so that defective components can be removed from production. To perform acoustic imaging, the components are typically loaded into JEDEC-style trays or placed onto carrier plates.

The defects of interest are gap-type structural defects, including delaminations, voids and cracks. They vary in threat level by location and dimensions. Even a small die face delamination poses a considerable threat to reliability because delaminations between the mold compound and the die face tend to expand across the face of the chip and break wires. Small delaminations on lead fingers are ordinarily not much of a threat unless the lead fingers are very short or the delaminations are numerous. Small voids isolated in the mold compound are generally considered harmless anomalies, not defects.

When using acoustic imaging to screen incoming components, the user defines the accept/reject criteria for each part type. The user might specify as reject any plastic part of a given type that has a delamination whose greatest x-y dimension exceeds a benchmark such as 50, 100 or 200 microns.

The transducer that pulses ultrasound into the parts in the carrier and receives the return echoes is driven by software; other software analyzes the acoustic data from each part and identifies it as accept or reject.

There is considerable variation among the part types, the carrier they travel in, and the scanning and imaging methods. Acoustic imaging may be reflection-mode, in which echoes are used from a defined ("gated") depth where defects are most likely to reside. A plastic part where die face delaminations have been known to cause failures may be gated acoustically on a narrow depth just above and just below the top of the die. Or imaging may use the transmission mode, where no gating is used and where ultrasound is pulsed through the entire package to image the acoustic shadows cast by defects at any depth.

The JEDEC-style tray or glass plate may hold anywhere from a dozen to hundreds of parts, and may be designed for a specific function. For widely-used transmission imaging, the opening is just above a sensor that collects ultrasound that has been transmitted through the part by the transducer above the part. In reflection-mode imaging, only the transducer above the part is needed, since it performs both pulsing of the ultrasound and receiving of the return echoes.

Most of the time the operator of the acoustic system needs to transfer the incoming parts into a new carrier for acoustic imaging. There are many factors that make re-loading necessary. The parts are often tiny, and will move around in the JEDEC-style tray during scanning. Even if the parts are larger, and originally in a tray that has a hole iin the bottom of each cavity, there is often a cross member in the hole to provide supportand the cross member makes through-transmission acoustic imaging impossible. If there is no cross member, the hole may be to narrow for ultrasound to reach the sensor below. On parts of any size there are likely to be air bubbles that cling to the bottom or top of the part and block ultrasound.

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