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The solder-joint interconnect between an IC component and the PCB (printed circuit board) is a critical link in the system overall reliability. Trends in the automotive market are driving increased focus on solder-joint performance: (1) increasing electronics content for new functions, especially for ADAS (advanced driver-assistance systems), (2) use in safety critical systems and sub-systems, (3) decreasing interconnect pitches which reduces the stand-off and available solder, (4) increasing industry reliability expectations, and (5) package variations (ex. multi-die). In particular, BGA (Ball Grid Array) packages are used throughout the vehicle across various systems including engine control, braking, communication, infotainment, and radar to name only a few. Among these, under-the-hood applications often require high sustained operating temperatures and many heating/cooling cycles during the vehicle lifetime. The reliability of these interconnects is routinely assessed by cyclical thermal stress (temperate cycling) of components mounted to boards. While AEC (Automotive Electronics Council) offers no standards for solder-joint testing (for example, board level reliability criteria is not included in the AEC Q100 “Failure Mechanism Based Stress Test Qualification for Integrated Circuits”), IPC 9701A “Performance Test Methods and Qualification Requirements for Surface Mount Solder Attachments” can be followed. For automotive under-the-hood the specified cycle range is 40°C to 125°C (TC3). This paper summarizes the BL-TC (board level temperature cycle) performance of various BGA packages used in automotive applications. In all cases the test vehicle packages were daisy-chain versions of production devices, while maintaining critical features such as BGA footprint, physical dimensions, BOM (bill of materials), die size and thickness and substrate layer metal densities. All used Pb-free solders for both the BGA solder ball and the paste printed onto the PCB. The PCB designs were complementary to the packages establishing daisy-chain connections winding through the PCB, the solder-joint and package substrate. Each chain (net) was continuously monitored in situ during cycling. An event detector logged a failure when a net resistance exceeded 300 ohms. Wirebonded and flip chip packages were studied, ranging in size from 10mm to 29mm with BGA pitches including 0.65mm, 0.80mm and 1.00mm. In addition to these primary attributes, various other factors were found to alter the solder-joint lifetimes. For example, increasing BGA pad and solder sphere diameters improved solder-joint lifetime, but increasing the PCB pad diameter often did not. Among solder materials, eutectic SnAg typically showed longer lifetimes than other high Ag SAC alloys such as SAC305 and SAC405. The addition of Bi to the SAC alloy showed promise for further improvements. Other factors that were studied include die thickness, die size, and BGA pad finish. Both mechanical cross-section and dye penetrant analysis (dye-and-pry) were employed for failure analysis, enabling study of crack propagation and crack location within the solder-joint. Additionally, failure location (failing solder-joint) was identified for each as package corner, under the die edge, or package center in a predictable pattern depending on the package type. Examined in total, two opposing trends will force future innovation. Industry reliability requirements continue to drive expectations (i.e. cycles to failure) higher, while increasing package size and decreasing pitch will naturally reduce the solder-joint lifetimes. Solutions will be found in package design, package material and solder selections. |
