Equipment feature: Scanning Acoustic Microscope (SAM) This page is also available in printer friendly format (pdf) - click here

Tony Compagno (Tyndall) and Louise Cannon (DIT) analysing samples for a recent NAP project on the SAM

Introduction
Tyndall National Institute has been operating a Scanning Acoustic Microscope for over ten years. The current system is a HS1000 Sonix HiSpeed Scanning Acoustic Microscope. The inspection system is a PC based Ultrasonic Scanning system that is 100% digital. The system typically detects disbonds or delaminations between the plastic resin package material and the die, die pad or lead frame. The system was originally designed for the inspection of microelectronic devices including plastic and ceramic packages, BGA, DCA, Flip Chip, C4, Lid seals, Gallium Arsenide and solder joint inspection etc. However over the last few years investigations using the scanning acoustic microscope have been carried out on a range of materials such as silicon on silicon, silicon on germanium, sheep bone, mussels, sea weed and QA phantoms. The HS1000 uses the pulse echo and through transmission techniques of ultrasonic inspection and operates over a frequency range of 5 to 150MHz.

Applicable Frequencies 50-500kHz – Flaw Detection of concrete 0.5 to 12MHz - Flaw Detection of metal & welding 10 to 100MHz – Ceramics, Electronic Components 100 to 1000MHz – Surfaces of materials

HS1000 Sonix High Speed SAM

Analysis of Tissue Mimicing Material (TMM) using 75MHz lens on SAM

Principles of Scanning Acoustic Microscopy Scanning Acoustic Microscopy analysis is primarily used in the context of plastic packaged devices for the detection of cracks and delaminations. For equipment operating in the 20 MHz to 100 MHz frequency range, the technique is often referred to as Pulse Echo SAM or reflection mode SAM. Recently the trend has been towards both higher ultrasonic frequencies and higher resolution to inspect flip-chips and bonded wafers and today frequencies above 200 MHz are achievable depending on the type of acoustic microscope used. The operating principles of the equipment are as follows:- A regular series of short, high voltage ‘spikes’ are applied to a focusable piezoelectric transducer which produces a series of ultrasound pulses. These are transmitted into the sample under test via a coupling fluid (usually water).At any interface at the surface or within the sample under test, a proportion of the sound energy is reflected back to the transducer which then acts as a receiver converting the energy back into electrical pulses which can be amplified. The acoustic impedance of a material is defined as the product of the density and sound velocity of that material.  The greater the mismatch of acoustic impedance’s, the greater will be the amplitude of the reflected signal from the interface. As the acoustic impedance of air is very low (practically zero) compared with the various materials comprising a package, if a delamination occurs at an interface, the reflected signal amplitude will be relatively high.

The polarity of the phase of the reflected ultrasound signal depends on whether the ultra sound is passing from high-to-low or low-to-high acoustic impedance material at the interface been examined. For example the properties of the materials used in plastic packages are such that a plastic/metal or plastic/silicon interface is low-to-high acoustic impedance where as plastic/air is high-to-low. This means that a delamination can be readily confirmed by observing the phase of the reflected signals. Areas of an interface which are delaminated are separated from the non-delaminated areas by a low amplitude reflected signal which occurs at the phase inversion. Through transmission scanning is used for evaluating the entire volume of the sample in one scan. This approach requires acoustic access to both sides of the sample. It is useful for fast determination of internal structure, including defects, but it does not generally provide depth information about a particular feature or defect. i.e it will tell you if you have a problem/delamination in your sample but not where it is in the sample. Examples of Specific Projects. As well as looking at defects in plastic packaged devices the scanning acoustic microscope has been used in various projects ranging from the “Investigation of acoustic properties and homogeneity of a range of materials for ultrasound test phantoms” to “Micromechanical modelling of normal, drug treated and osteoporotic bone using Scanning Acoustic Microscopy”

SAM analysis C-scan image shows plastic to silicon die delamination (red area)

For further information and to discuss your requirements please contact:
Tony Compagno at +353 (0)21 4904226 and tony.compagno@tyndall.ie

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