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Improved reliability of leadfree flip chip assemblies using direct underfilling by transfer molding

: Braun, T.; Wunderle, B.; Becker, K.-F.; Koch, M.; Bader, V.; Aschenbrenner, R.; Reichl, H.


Aripin, A. ; IEEE Components, Packaging, and Manufacturing Technology Society:
IEMT 2006, 31st International Conference on Electronics Manufacturing and Technology : 8 - 10 November 2006, Sunway Resort Hotel, Petaling Jaya, Malaysia
New York, NY: IEEE, 2006
ISBN: 1-4244-0729-X
ISBN: 978-1-4244-0729-3
ISBN: 978-1-4244-0730-9
International Conference on Electronics Manufacturing and Technology (IEMT) <31, 2006, Petaling Jaya/Malaysia>
Fraunhofer IZM ()

Flip Chip technology has been widely accepted within microelectronics as a technology for maximum miniaturization. Typical applications today are mobile products as cellular phones or GPS devices. For these consumer applications upper temperature limits and reliability demands are not very harsh. To widen the application range of Flip Chip technology and to address the volume market of automotive and harsh environment industrial electronics the development of highly reliable assemblies with high temperature capabilities is as crucial as being able to qualify and predict this very reliability. Transfer molding is the standard process for a highly reliable encapsulation of leaded and area array packages as BGAs or CSPs. Advanced materials and process developments now allow the use of transfer molding technology for direct underfilling and / or overmolding of Flip Chip assemblies. Existing standard equipment for encapsulation can be used and no additional process step for underfill dispensing or jetting is required. Molded Flip Chips have the potential of high reliability as the low CTE of the flip chip molding compound reduces the thermal mismatch. Trends of the market drive towards SIPs with an integration of different devices as e.g. SMD and Flip Chip. Therefore the highly reliable encapsulation of these hybrid packages with inhomogeneous topography is the goal. For testing the reliability limits and the determination of failure mechanism of molded Flip Chips a test vehicle has been designed at Fraunhofer IZM. This test vehicle for process evaluation and reliability testing allows the encapsulation and underfilling of a single Flip Chip. Test vehicles were built using lead-free SnAgCu solder on standard FR4 substrates and different state-of-the-art Epoxy Molding Compounds, Reliability results of molded Flip Chips were compared to results from identical assemblies with standard capillary underfill. Molded Flip Chips showed much higher reliability than packages with capillary underfill. All molded devices passed Moisture Sensitivity Test Jedec Level I and more than 3000 h at 85 degrees C / 85 % relative humidity without any electrical failures and delaminations. Temperature cycling from 55 degrees C up to 125 degrees C revealed an extremely high reliability. Molded Flip Chips passed more than 14.000 cycles without electrical failures. This is more than five times higher than standard Flip Chips assemblies on FR4 may withstand. Nondestructive and destructive analysis showed that the main failure mechanisms were starting cracks in the moldcap propagating into the die and causing there an electrical failure. The experimental reliability investigations were backed by thermo-mechanical simulations. For that purpose a Finite Element model was created including the measured material data for the used materials. As failure criterion the inelastic accumulated creep strain in the solder bumps was used within a Coffin-Manson approach. It was found, that the simulations could account for the extreme lifetime of the Flip Chip assemblies and a thermo-mechanical explanation could be given based on the favorable stress distribution generated by the encapsulation. However, there is a shift in failure mechanism away from solder fatigue to encapsulation cracking. This is also underpinned by simulation. In summary the reliability potential and limits as well as typical failure mechanisms of molded Flip Chips are presented.