Authored By:
Reza Ghaffarian, Ph.D.
Jet Propulsion Laboratory, California Institute of Technology
Pasadena, CA, USA
Summary
Ceramic column grid arrays (CGAs) are commercial-offthe-shelf (COTS) area array packaging technologies that are now widely used in numerous high-reliability application electronic systems. In the past, there was always a ceramic version of a plastic package, including the plastic BGA(PBGA) which has the analogous CBGA and CGA. Today, there are few, if any, ceramic (high reliability) versions of the latest technologies. Therefore, the choice of daisy-chain CGAs for solder joint reliability evaluation has become scarce. The daisy-chain package is needed to perform solder-joint reliability under thermal cycling conditions that have now become an integral part of the electronic packaging equation for overall reliability risk analysis. Extensive work has been carried out on this subject within the last two decades by industry.
However, there is limited test data presented in literature on the synergistic effects of sequential thermal cycles and mechanical vibration. In fact, there are none to the author's knowledge on the synergistic effect of thermal cycle and drop shock performance, especially for higher than 1,000 column CGAs. This paper presents the effect of a priori thermal cycle on subsequent drop to failure behavior of CGAs with 1272 columns. After successful assembly of these CGAs onto printed circuit boards (PCBs), they were subjected to two thermal cycle regimes. Three samples were subjected to thermal cycling in the range of -55 Celsius /125 Celsius for 300, 400, and 500 cycles and one was subjected to -55 Celsius / 100 Celsius for 500 cycles. Drop tests were performed first from a height of 36'' and then increased to 48''. This paper presents optical and SEM images showing solder damage progression with thermal cycling and optical/ SEM images showing failure mechanisms of CGA1272 assemblies after drop testing with additional thermal cycling.
Conclusions
Within the last decade, the use of CGA for high reliability applications has shown significant growth; however, test data is scarce, especially for newer CGA with larger than thousand I/Os. Reliability test data were presented for CGA1272 with copper-wrapped columns under accelerated thermal cycling conditions along with their failure mechanisms.
Test results are also presented for failures of CGA1272 under sequential thermal cycles, drops, and thermal cycles. Simple analytical models were developed to project stresses/strains for BGA/CGA under thermal cycling and drop testing. Our test data, as well as data presented in literature for CGAs or BGAs, does not show a clear trend as a dominant damaging mechanisms for a synergism effects of sequential or combined environmental exposures.
CGA1272, unexpectedly, "as assembled" package, failed before those with initial thermal cycle exposures. One possible reason for this earlier failure is pad cratering, and stiffer solder joints for "as assembled" test vehicle. More work on these topics should shed light on answering many questions on understanding failure mechanisms under single, as well as sequential or combined thermosmechanical, testing.
Initially Published in the SMTA Proceedings
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