Effects of Lead-Free Solders on Flex Performance

Ceramic capacitors have proven themselves very reliable with extremely low failure rates. As processes continue to improve, reliability of the dielectric also continues to increase. This increase in the reliability of the dielectric has caused other categories to emerge as the main contributors to the overall failure rate. A recent review of field failure analysis of surface mount MLCCs, has shown Flex Cracks account for as much as 40% of ceramic capacitors failures. Flex Cracks are created when a physical displacement of the board mounted MLCC generates sufficient stress within the ceramic body to fracture the ceramic material. As the industry moves to lead free soldering systems, which are less ductile, there is a concern about the effects on flex performance. This paper was written in response to customer concerns of flex performance as they switch from lead based to lead free soldering processes. In this paper we present the results of flex testing, comparing standard lead based solders to lead free solders. Preparation of samples The objective of this study is to determine the effects of lead free solders and solder process on the flex failure rate. To get an accurate picture of the effects across a variety of sizes, the following case sizes were selected for this study: 0603, 0805, 1206, and 1812. The lead based solder used as a control for this study was a 63-37 Tin Lead (SnPb) solder, and the Lead Free solder (Pb-Free) was a SAC 305 (SN 96.5%, Ag 3% Cu .5%). The solder was dispensed with a metering pump to ensure constant solder amounts. The amount of solder dispensed for each case size was validated before during and after the parts were mounted. The soldering profiles followed the solder paste manufactures recommendation. KEMET Electronics Corp. Test Fixture