J. M. Patterson, Applied Micro Circuits Corp, san diego, CA

Summary: A new cause for failure of integrated circuits was reported in a paper by Intel at the 2007 ISTFA conference wherein damage to the encapsulated integrated circuit was induced during laser package marking due to local heating. A path is provided through the epoxy mold compound to the die surface by the spheres of glass filler for thin packages. This paper discusses a method to examine packages that may be susceptible to this failure mechanism. The method uses Optical Beam Induced Current (OBIC) and Thermally Induced Voltage Alteration (TIVA) laser imaging techniques to electrically detect the optical transmission of thin packages. The enclosed integrated circuit is the detector and the laser is scanned on the outside of the package. Example: A good sample (passing ATE) was examined using the OBIC imaging technique on a completed device. This imaging was performed through the top of the package with the mold compound intact. Electrical connection is made to two power supply pins of the device and those connections go to a current amplifier for video imaging as the OBIC laser is scanned. The OBIC laser is 1065 nanometers wavelength and does not ablate the epoxy mold compound. But the glass spheres in the epoxy will pass that wavelength. Also, this wavelength will generate electron hole pairs in the semiconductor die and create a current if the laser light reaches the die surface (and the junctions are not covered by metal). When the first sample was examined this way, at least eleven sites were detected where the laser reached the die through a path of glass spheres in the mold compound covering the die. The package marking laser is more powerful than the OBIC laser. It ablates the mold compound and can cause damage to the die. Reference picture in figure 1 below. Figure 1: OBIC image through the top of the package over the die area. The white spots are the glass spheres in the epoxy mold compound in the reflected mode. The green spots are locations where the OBIC laser reached the die surface and caused a current in the semiconductor junctions through a path of glass spheres. About eleven sites are detected indicating a path to the I.C. Figure 2: TIVA image of the same device type as in figure 1 using a 1340 nanometer wavelength laser. The top of the package was polished to a depth that just removed the laser marking. The red and green spots are locations where the laser goes through glass filler particles that are lined up to the die surface and is detected electrically by the circuit. The white spots are the glass filler particles on the surface showing up in the reflected image.