Optical Leak Testing of Hermetic Packages

A production leak test system using digital holography, Optical Leak Testing (OLT), has been developed for simultaneous gross and fine leak testing of hermetic semiconductor, MEMS and Optoelectronic devices. The technique has also shown the unique capability to leak test ceramic SMC’s on PCBs, even if conformal coated. Many devices are manufactured using welded, brazed or soldered metal lids with metal or ceramic packages. The most common conventional leak test methods used in the semiconductor industry include gross leak testing by the bubble leak method and fine leak testing with a helium mass spectrometer. The application of these techniques is highly problematic for many OLED display, Optoelectronic and MEMS devices. For maximum sensitivity, bubble leak testing requires the package be immersed in a perfluorocarbon liquid at a temperature of 125°C, exceeding the 90°C limit for most of these devices. In addition, helium absorption by the fiber optic pigtail causes fine leak testing with mass spectroscopy to be highly inaccurate when the helium degasses during testing. Further, neither method can be applied to SMC mounted to PCBs to locate leaking devices cracked during soldering. The Optical Leak Test method overcomes these concerns, and other problems with conventional leak test methods and reports the leak rate for all devices tested at one time. The hermetic devices are placed in the test chamber and exposed to a pressurized low molecular weight gas such as helium. If the package is leaking, the lid responds to changes in pressure differences as the device cavity pressure and test chamber pressure come to equilibrium. Precision chamber pressure measurements combined with lid stiffness and velocity data, obtained with digital holography are used to determine package leak rates in helium cc-atm/sec. Leaks in the range from the “no lid condition” to 1x10E-8 cc-atm/sec. have been measured. The method has demonstrated a very high level of accuracy and repeatability. Throughput is determined by the number of devices that can be placed on the boat for simultaneous testing. Cycle times vary from 2 to 20 minutes depending on package size and internal volume. For hermetic optoelectronic devices the problems of high temperature exposure, contamination and helium absorption/release experienced with conventional leak test methods are overcome. Finally, automated Optical Leak Testers provide near real-time leak test data for process control of metal lid seam sealing operations, minimizing lost production time, rework and scrap. The Optical Leak Test Method has been included in MIL STD 883E since 1995 for conditions C4 and C5.