Liquid Compression Mold Underfill Optimization with Low Warpage and Narrow Gap Flow

In this report, we provide details on Liquid Compression Mold Underfill (LCMUF) for wafer level that can flow into a narrow
$(\mathbf{24}\ \times \mathbf{40}\ \boldsymbol{\mu} \mathbf{m})$
pitch and gap package while maintaining low warpage. LCMUF is a liquid resin that has the characteristics of both an underfill (UF) that can flow into the narrow gap flip chip package, and an overmold compound with low warpage at the wafer level using a compression molding equipment. First, the effect of the filler size on flowability was confirmed using a test vehicle with
$\mathbf{24} \boldsymbol{\mu} \mathbf{m}$
between the interposer and chip, and a bump pitch of
$\mathbf{40}\ \boldsymbol{\mu} \mathbf{m}$
. By using a filler with an average size of
$\mathbf{4}\ \boldsymbol{\mu} \mathbf{m}$
, uniform flowability was achieved without filler clogging. Second, we examined the high fine filler loading with an average size of
$\mathbf{2}\ \boldsymbol{\mu} \mathbf{m}$
. By optimizing the particle size distribution, we succeeded in reducing the viscosity by about 70%, and the filler amount was increased to 83.5%. Lastly, by combining these technologies with the previously developed low-warpage technology that uses flexible resin, we succeeded in developing a LCMUF with fine fillers, low CTE, and low modulus [1]. In the reliability test, MSL3 and the thermal cycle test of condition B confirmed that the results were positive without delamination or any cracks.