Silicon interposers, CoWoS and microbumps

At the recent ECTC conference, various presentations addressed silicon interposers for 2.5D (Shinko), CoWoS reliability (TSMC) and microbumping (imec).

Dr. Phil Garrou, Contributing Editor

Shinko and CEA Leti detailed their presentation entitled “Warpage Control of Silicon Interposer for 2.5D Package Applications.”

Large silicon-interposers when attached to an organic substrate can cause significant warpage problems. Shinko/Leti examined several warpage control techniques including:

  • Using a “chip first process” where chips are mounted on the interposer first vs “chip last process” where the silicon-interposer is mounted on the organic substrate first and chips are mounted onto the interposer last.
  • Using various underfill resins.
  • Using Sn-57Bi solder and thus lowering peak temperature 45-90 degree C. This reduced warpage after reflow to 75% of that using SAC305.

Warpage of silicon-interposer using three types of underfills for 0 level assembly (micro bumps) were investigated. Maximum warpage using U.F. A1, A2 and A3 were 108, 123 and 132mm, respectively. The lowest warpage was obtained at using U.F. A1. With U.F.A3, solder bump open failures were observed. The authors conclude that “using underfill material with low Tg and high storage modulus for 0 level leads to high reliability.”

TSMC and customer Xilinx presented “Reliability Evaluation of a CoWoS-enabled 3D IC Package” which used FEA to study the thermo-mechanical response of the interposer-based package during thermal cycle reliability stressing. Focus was especially on the fatigue failures of the C4 and BGA joints. Experimental data collected on CoWoS test vehicles were used to validate the FEM models. Parametric study of key package material and geometric parameters was performed to analyze their effects on C4 bump thermal cycle reliability. Package materials of interest include UF (underfill), lid and substrate, and the geometric parameters include lid thickness and C4 bump scheme.

Results showed that the CoWoS package using AlSiC lid has better C4 bump life than the CoWoS package using Cu lid. While a thicker lid has the higher stiffness and better co-planarity, the higher constraint from the thicker lid induces higher stress inside the package which negatively impacts C4 bump fatigue and the micro-bump Ti/Al delamination.

C4 bump layer underfill with Tg of 70°C or 120°C, were studied. The underfill with lower Tg has higher driving force to C4 bump fatigue. When temperature is above Tg, the underfill has much lower Young’s Modulus which has much lower capability to protect C4 bump; and therefore the underfill with lower Tg has higher driving force to C4 bump fatigue. On the contrary, the underfill with lower Tg has lower driving force to Ti/Al delamination in the micro-bump structure. The C4 underfill with lower stiffness can play as a buffer layer and results in lower driving force to Ti/Al delamination in microbump.

imec reported on “Key Elements for Sub-50μm Pitch Micro Bump Processes.” Scaling the microbump pitch from hundreds to a few tens of microns is not straightforward. Several process parameters need to be taken into account to allow a reliable Cu(Ni)Sn ubumping process. One of the challenges for fine pitch Cu(Ni)Sn stacking is to obtain a high bump uniformity. The non-uniformity prevents Cu and Sn from having good contact and subsequent intermetallic formation and increases the risk of underfill entrapment.

A bump scheme that offers better margin for alignment error is better based on a scheme where the size of top die bumps is smaller than the size of the bottom pads. For example it is better to achieve 20μm pitch with 7.5μm bump on 12.5μm pad than with 10μm bump and pad because equal bump and pad diameter can tolerate only 2μm misalignment whereas the 7.5μm/12.5μm bump/pad can tolerate 5μm. This is a significant difference when working close to the stacking tool’s limit of alignment accuracy.

Details on the plasma treatments necessary when attempting to plate into these fine featured plating resists are also discussed.

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