Issue link: https://iconnect007.uberflip.com/i/408601
72 SMT Magazine • November 2014 ules for high-power applications. Before surface mounting, the components are screened on a laboratory acoustic system if the numbers are small or by a semi-automated or fully auto- mated system if the numbers are large. The idea is to remove from assembly those component packages whose delaminations, voids or other anomalies are likely to cause eventual electrical failure. Electrical testing can't find these struc- tural defects unless they have already caused a failure. An engineer examining acoustic images will routinely see component packages having single defects (a die face delamination, for ex- ample) or having multiple defects (many de- laminations along the lead finger and on the die paddle). But if no electrical connection has yet been broken, all of the defective components would pass electrical testing. Below are details of three areas in which new acoustic techniques or technologies are being used to image and analyze advanced compo- nents. MEMS (microelectromechanical systems) are sensors built with semiconductor technology. In many applications, they replace much larger systems and do so with significant cost savings. An early and still important MEMS application was the accelerometer used to activate air bags in cars. MEMS devices are widely used in medi- cine to measure blood pressure inside a patient's intravenous line. Among the numerous other medical applications are measurement of fluid pressure during eye surgery, measuring a baby's blood pressure during birth, and monitoring a respiration when a ventilator is in use. A MEMS device typically has a cavity that contains a mechanical element, usually in a vac- uum, and that is connected to electronics that pick up the signal from the device. The cavity is etched into a silicon die. During assembly of the MEMS device, a seal is typically placed around the cavity to preserve the vacuum (or in some cases a gas) that is necessary for its functioning. A cover plate, which may be silicon or glass, is then placed on top of the die, thus forming the seal. During acoustic imaging, the cavity behaves as a large gap, and the interface between the gap and the silicon above it will be bright white. The contents of th e cavity are not usually of interest acoustically, but the condition of the seal around the cavity is. The long-term reliabil- ity of the MEMS device depends largely on the integrity of this seal. As MEMS dimensions have shrunk, acoustic imaging has adapted to image seals that may be as narrow as six microns. The seal material needs to be well bonded to both the cavity die and the cover plate. It also needs to be free from any voids or other gap- type defects that could cause it to leak at some time in the future. An acoustic micro imaging tool is used to look for these defects. Ultrasound pulsed into one side of the device is gated on the full thickness of the seal, including the in- terfaces at the top and bottom of the seal of the seal, and the bulk material of the seal. These interfaces should be displayed as some shade of gray in the acoustic image. But any voids or other gaps in the seal itself will appear as bright white areas—a circular void, for example. In some instances, the defect takes the form of missing material. Insulated gate bipolar transistors (IGBT) are high-power semiconductor switches used in MRI systems and other high-power applica- tions. The switching element can be a silicon (Si), silicon carbide (SiC) or gallium nitride arTiCle aDvances in acoUstic imaging oF meDicaL eLectronics continues figure 1: an acoustic image encompassing the seals around several MeMS devices. The sample was a MeMS wafer before dicing. The arrows point out gaps in the seals.