WLCSP and BGA Reworkable Underfill Evaluation and Reliability

Fei Xie, Ph.D., Han Wu, Daniel F. Baldwin, Ph.D., Swapan Bhattacharya, Ph.D., and Kelley Hodge
Engent, Inc.
Norcross, GA, USA

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

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 continues our recent published work. Prior work presented the reworkable underfill evaluation process and several criteria were investigated which included underfill flow rate, flux compatibility, reworkability, solder extrusion, material properties such as viscosity, Tg, modulus and cure time. This paper conducts a deeper and wider study on the underfill/flux compatibility issues. It investigates the flux compatibility between four most popular commercial solder pastes and over six popular commercial CSP and BGA underfills. Two most compatible solder pastes were then selected to assemble the WLCSP and BGA rigid and flex circuits. Ten reworkable underfills from various venders were applied to the devices on these boards. The thermal cycle, temperature humidity aging, autoclave, and drop reliability tests were performed. The underfill and solder paste combination with the best performance for both rigid and flex boards were selected and applied to the production process.

This work continues our previous work on the evaluation of the BGA and CSP reworkable underfills. Our prior work presented the reworkable underfill evaluation process and several criteria were investigated which included underfill flow rate, flux compatibility, reworkability, solder extrusion, material properties such as viscosity, Tg, modulus and cure time. This paper conducts a deeper and wider study on the underfill/flux compatibility issues. It investigates the flux compatibility between four most popular commercial solder pastes and over six popular commercial CSP and BGA underfills.

Two most compatible solder pastes were then selected to assemble the WLCSP and BGA rigid circuits. Ten reworkable underfills from various venders were applied to the devices on these boards. The thermal cycle, temperature humidity, high temperature storage aging, reliability tests were performed. The results show that the underfill C performed better than underfill B & E, which have a similar reliability performance. Underfill A is the next one and Underfill D performed the worst. All underfills passed the thermal humidity test and high temperature storage test.