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Fine Pitch Flip Chip Assembly Process Underfill, Evaluation and Reliability
Fine Pitch Flip Chip Assembly Process Underfill, Evaluation and Reliability
The paper compares the best flux and underfill combination in reliability performance was selected and applied to the production process.
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Authored By:
Fei Xie, Ph.D., Daniel F. Baldwin, Ph.D., Han Wu, Swapon Bhattacharya, Ph.D., Kelley Hodge
Engent, Inc.
Norcross, GA, USA

Qing Ji, Ph.D., Ben Bo
H. B. Fuller
Yantai, Shandong, China

Summary
Interest in using fine pitch SMT components has increased greatly in recent years due to the growth of portable, hand held electronics and due to miniaturization trends in consumer and industrial electronics markets. The reliability of those fine-pitch portable electronics products is a great concern particularly in the areas of impact and shock performance. For very fine pitch SMT components such as WLCSPs and BTCs without ground planes (0.5mm pitch or lower), underfills can be used to improve the impact and thermal cycle reliability. Historically, the target properties of underfills can be generally summarized as high glass transition temperature (Tg), high modulus (E) and matched coefficient of thermal expansion (CTE) to solder. However, the underfill selection and evaluation process has become increasingly complex, time consuming and cost prohibitive due to increasing product design constraints, introduction of new package materials, and ever changing from factor of semiconductor packages. With every new generation of package technology, one must factor into the underfill selection process, new solder alloys and soldermasks, thinner substrate core materials, finer pitches, and increasing package dimensions.

This paper focused on the extra fine pitch Flip Chip assembly process, Flip Chip underfill dispensing process, evaluation and reliability issues. A 0.2mm pitch Flip Chip PB8 was used for this study. Four types of flux, seven types of Flip Chip commercial underfills from various venders were applied. The details of the assembly process, underfill dispensing issues are discussed in this paper. The air-to-air thermal cycle and thermal-humidity reliability testing data are included. The comparison and evaluation of these fluxes and underfills for Flip Chip application are presented as well. The best flux and underfill combination in reliability performance was selected and applied to the production process.

Conclusions
A very fine pitch Flip Chip PB8 application on the test vehicle was conducted. The assembly process was developed and material reliability was evaluated. The devices used in this application was 0.2mm pitch daisy chain PB8. Four flux and 6 underfill materials were used for the assembly and reliability in this study. The details of the assembly process including dip flux, component placement, reflow and underfill dispensing have been discussed in this paper. JEDEC standard air-to-air thermal cycling and thermal humidity tests are performed for the reliability evaluation purpose.

All the cross section analysis and shear test results show that the assembly process with the Senju and indium flux was very robust. And the underfill material dramatically improved the reliability performance of the assembled devices. For the thermal cycle test, underfill C had the best performance, and underfill D & F are followed.

Initially Published in the SMTA Proceedings

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