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|Temperature Cycling Induced Warpage of Packages with Embedded Copper Heatspreaders|
|Keywords: Temperature Cycling, Leadfree, Warpage|
|Temperature cycling is an important reliability qualification test given the differences in thermal expansion coefficients for the materials in integrated circuit packages. In this work, leadfree Plastic-Ball-Grid-Array (PBGA) packages with embedded C1100 copper heatspreaders were exposed to standard qualification testing including MSL3 Moisture Preconditioning with leadfree reflows at 245C followed by Temperature Cycling (TC) with ranges of -55/+125C (TC-B) and 0/+125C (TC-K) per JEDEC JESD47. Electrical performance and package warpage were characterized on as-received, post-preconditioning, and post-TC devices. After 200 cycles TC-B, gross electrical open failures were found on a large percentage of devices in some package lots. Physical failure analysis of the open failures revealed severe package warpage, as high as 20mils on a 31mm package. The severe warpage was accompanied by delamination and sheared wires. In contrast other package lots did not show failures nor severe warpage (<6mils) even after 1000 cycles of TC-B. The same package and BOM was qualified with 225C reflows for eutectic lead/tin solder with no warpage or failures after TC. Detailed commonality studies revealed that the copper heatspreader lot used was the only definitive difference between “good” and “warped” package lots. It was found that for “warped” lots exposure to the leadfree reflow at 245C caused a significant reduction in the micro-hardness of the copper heatspreader, while there was minimal change in micro-hardness after exposure to leadfree reflow in the “good” lots. The mechanism for this change is explained by the softening temperature of the C1100 copper heatspreader which is well within the range of leadfree reflows. Above this softening temperature, re-crystallization and grain growth occur, which result in susceptibility to permanent warpage induced by temperature cycling. Control of this warpage is critical to qualifying temperature cycling performance for heatspreader PBGA packages, and this can be achieved through micro-hardness screening of the heatspreaders.|
|Matthew Stahley, Principle Engineer