Abstract
Abstract In this paper, we reveal the development of a novel two-layer Solder Resist (SR) film with low young’s modulus which consists of low young’s modulus layer that possesses excellent adhesion to substrate and thermal resistance layer which is composed of resins with high thermal resistance and great toughness. This novel two-layer SR film exhibits superior resolution and crack resistance. Furthermore, the amount of warpage is extremely low. In general, the material with low young’s modulus and high elongation is caused from weak cross-link density, resulting in poor thermal resistance and delamination which occurs when mounting at high temperature. Herein, we can inhibit delamination successfully by this new two-layer structure film with superior thermal resistance. The Coefficient of Thermal Expansion (CTE) of conventional SR for FC-BGA is 35 ppm, and modulus is 4.5 GPa, whereas, this advanced two-layer SR film exhibits CTE of 66 ppm, and modulus of 2.3 GPa. In order to compare the crack resistance between conventional SR and newly developed two-layer SR film, the film was laminated on BGA substrate (substrate size is 50 mm × 50 mm), patterned by photolithography and cured, and then, 25 mm × 25 mm chip was mounted on a BGA substrate by flip-chip bonder. Conducting thermal cycle test (TCT) and observing the number of cracks after 1000 cycles of TCT. The crack occurrence frequency of the conventional SR is 65 %, whereas that of the new two-layer SR film is 4 %. We proved clearly that high CTE and low young’s modulus demonstrate overwhelmingly high crack resistance. Besides, high resolution of this newly developed two-layer film enabled the formation of SR opening (SRO) as small as 40 μm. From the above results, the newly developed two-layer SR film with low young’s modulus is beneficial for the next generation high-density package, especially for the outermost layer of FC-BGA packages and interposers that require higher reliability.
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