By Dr. Phil Garrou, Contributing Editor

Continuing our look at the 2017 ECTC.

Tohoku Univ – Low CTE Underfill

Kino and coworkers at Tohoku Univ presented their data on the “Remarkable suppression of local stress in 3DIC by MnN based filler with large negative CTE”.

Generally, CTE of the underfill material is larger than that of metal microbumps. This CTE mismatch induces local bending stress in thinned IC chips, as shown in Fig. 2 below. Such local bending stress would affect transistor performance in thinned IC chips. Kino found that they could suppress the local bending stress by decreasing the CTE difference between the underfill material and the microbumps.

In general, silica, is usually used in underfill material to reduce the CTE of underfill material. A high concentration of filler is required to reduce CTE as low as metal microbumps. However, it is difficult to use the conventional filler for 3D IC with fine pitch microbumps since a high concentration of filler in underfill material increases the viscosity. They propose to use negative-CTE material as the underfill filler to suppress the local bending stress. They used manganese nitride-based material which has large negative-CTE of -45 ppm/K at the temperature from 65 to 100°C. Results indicate that negative-CTE filler can suppress the thinned Si chip bending more than 50% compared with SiO₂ filler. 

Hitachi Chemical – Expanding Film for WLP Sidewall Protection

Honda and co-workers from Hitachi Chemical discussed “Expanding film and process for high efficiency 5 sides protection and FO-WLP fabrication.”

WLP is well suited to mobile devices which require small, thin and light bodies. Fan in WLP (FIWLP) is fabricated by building up redistribution dielectric and metal layer on device wafer and attaching ball, and then it is diced to singulated packages. Device semiconductor die sides are exposed in such a FIWLP. The FIWLP fabrication process needs a wide die gap between die for molding compound and to dice, while leaving the molding compound on the die side wall for the protection.

To get the greater productivity and enhance the usage of the device area in the wafer, an expandable film and a novel process have been developedas shown below in fig 2. The film / process can also be applied to a die first type FO-WLP fabrication. Elimination of the die re-placement step can make the FO-WLP fabrication process simpler and less costly.

The 5 sides protection fabrication process is composed of 7 steps as illustrated in Fig. 2. The

expanding film with diced-wafer was put on the expander and the film expanded. After that the film is fixed to the grip ring , the film is cut out along the outer rim of the ring. After the singulated dice were transferred to the carrier with keeping the expanded die gap, the grip ring was removed. Then the expanding film was removed from the carrier. After over-molding, the molded wafer was singulated by dicing and 5 side protected packages were obtained.

The stress-strain curve of the film was optimized so that the die gap becomes large. Moreover, the die gap was able to be controlled from 0.5 mm to 3.5 mm. In the case of 1.5 mm die gap after expansion, the standard deviation was about 0.05 mm. Furthermore, the film was applicable to die sizes 1 mm × 1 mm, 5 mm × 5 mm and 10 mm × 10 mm.

Samsung – Compression Molding Encapsulants for FOWLP

Kwon and co-workers discussed “Compression molding encapsulants for wafer-level embedded active devices”. Challenges that FOWLP packaging technology is confronted with include wafer warpage, die shift/protrusion, and board level reliability. A solution to wafer warpage is considered crucial for successful subsequent wafer processing.

They propose to use a bilayer test structure with silicon wafer and epoxy molding compound as a standardized evaluation vehicle. Each layer is 300 μm thick. To further standardize testing, the molding conditions are fixed at 135 °C x 600 sec with a post mold cure of 150°C x 2 hrs. By standardizing the test vehicle and processing conditions, warpage behavior between mold compounds can be directly compared, and any observed differences are solely caused by the EMC.

Various parameters influencing wafer warpage were screened by the simulated calculation. Among all these parameters, Young’s modulus, CTE, and Tg have a significant effect on the controlling warpage. Generally, wafer warpage is reduced by lowering the Young’s modulus and CTE, and increasing the Tg. Although concurrent optimization of Young’s modulus, CTE, and Tg of a mold compound’s properties is very difficult because of tradeoffs for modifying each component, they developed new compression molding compounds with both low Young’s modulus and CTE, with relatively high Tg.

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