Compensation of die warpage by capillary action

The stacking of dies for 3D-Packaging using TSVs is one of the most important developments in electronic packaging today. Dies for stacking are often very thin and therefore flexible. Hence, the mechanical stress in the deposited layers of the devices can cause a significant die warpage, which must be reduced during assembly. Currently an external homogeneous pressure is applied on the chip until soldering or bonding is finished to compensate the warpage. That requires a contact between chip and bonding tool over the entire time of bonding. This paper presents a possibility to compensate the die warpage without an external pressure by using capillary action. The gap between substrate and die forms a capillary, which is filled by a well wetting liquid. Thus, the flexible die is planarized by the capillary pressure. Experiments were done using 50 μm thin dies with an edge length of 10 mm. The initial die warpage of approximately 28 μm along the diagonal could be reduced by about 30% in case of negative warpage orientation and about 60% in case of positive warpage.