Power system applications, such as street lighting, typically have a 10 year warranty. When these systems include electrolytic capacitors, it is important to choose a supplier that meets these requirements. Traditional lifetime testing of electrolytic capacitors to ascertain their life expectancy requires specialized equipment, is time consuming, labor intensive, and for most OEMs, is ultimately cost prohibitive. Electrolytic capacitors with the same capacitance and voltage ratings from different suppliers may be rated to the same lifetime, but historical data confirms that they can have significantly different operational expected lives. An accelerated testing methodology is needed to compare the reliability of electrolytic capacitors from different suppliers. DfR has developed an approach that reduces test times from thousands of hours to several weeks by taking advantage of two key behaviors of electrolytics. The first involves the rate at which capacitors lose electrolyte, which is fairly predictable at a given temperature and electrical stress. The second key behavior is the dependence of the equivalent series resistance (ESR) of electrolytic capacitors on the volume of liquid electrolyte. The approach that will be described in this paper will demonstrate a means of comparing the time to failure for comparable capacitors from different suppliers under the same conditions. Case studies will demonstrate how this method avoids the extended testing that is typically required.
This research compared the lifetime of similar aluminum electrolytic capacitors from different manufacturers using an accelerated life test, which consisted of critical weight loss testing and rate of weight loss testing. In critical weight loss testing, capacitors are perforated to speed up electrolyte evaporation and the equivalent series resistance (ESR) and weight are measured periodically to determine their relationship. In rate of weight loss testing, capacitors are subjected to final operating conditions (i.e. voltage and ripple current are applied) and the weight is periodically measured over the course of 500 hours. After test completion the relationship between ESR and weight loss is used to calculate the critical weight loss that occurs at datasheet-defined failure, which is typically a 200% increase in ESR. The rate of weight loss is extrapolated to the critical weight to estimate a time to failure that can be compared to other capacitors tested using the same accelerated approach. In this research, testing compared 450 V, 68 μF capacitors from Manufacturer A and Manufacturer B, and results indicated Manufacturer A had a significantly longer lifetime. Therefore, capacitors from Manufacturer A are more reliable than capacitors from Manufacturer B.
The electronics assembly market has experienced a material shift from lead (Pb) based solders to Pb-free solders. This is a result of the widespread adoption of Reduction of Hazardous Substances (RoHS) legislation and practices in commercial industry. As a result, it is becoming increasingly difficult to procure commercial off-the-shelf (COTS) components with tin-lead (SnPb) solder balls or finish. There are essentially three responses to the scarcity of acceptable SnPb parts: custom order, post process or adapt. Custom ordering parts with SnPb finishes negates the benefits of COTS based acquisition; however, has a reduced reliability risk because the material and processes are known. Reprocessing parts once in house saves money because the parts are COTS, but expends money and resources by performing post processing on them. Also, the additional touch labor and handling increases the risk of damaging the part. Finally, adapting to Pb-free finishes is the preferred long term approach because it preserves the cost benefits of using COTS parts and does not require post processing. It is the riskiest approach due to the lack of historical data in the DoD environment. This paper presents results regarding reballing 208 I/O Ball Grid Array (BGA) parts from tin-silver-copper (SAC305) solder to SnPb eutectic solder. It is important to understand the reliability risks associated with the reballing procedure, particularly as it relates to thermal cycling, shock and vibration environments. Three major efforts will be presented to answer these concerns. First, a survey of reballing vendors was performed to better understand the processes and variables associated with that industry. The results of that survey were used to down-select to five vendors that were used for the physical testing portion of the effort. Finally, physical testing consisting of thermal cycling, shock, and vibration was performed. The physical testing was performed on parts from the five different reballing vendors as well as native SnPb parts and native SAC305 parts. The results of these activities will be presented.
The risk of damage caused by reballing SnPb eutectic solder balls onto a commercial off-the-shelf (COTS) active flip chip with a ball grid array (BGA) of SAC305 was studied. The effects of reballing performed by five different reballers were examined and compared. The active flip chip device selected included manufacturer specified resistance between eight (8) differential port pairs. The path resistance between these pins following reballing, as compared to an unreballed device, was used to assess damage accumulation in the package. 2-dimensional x-ray microscopy, acoustic microscopy, and x-ray computer tomography were also used to characterize the effects of reballing. These studies indicated that no measureable damage was incurred by the reballing process, implying that reballed devices should function as well as non-reballed devices in the same application.
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