Process Innovations to Prevent Glass Substrate Fracture from RDL Stress and Singulation Defects

McCann, S. R., Sing, B., Smet, V., Sundaram, V., Tummala, R. R., and Sitaraman, S. K., “Process Innovations to Prevent Glass Substrate Fracture from RDL Stress and Singulation Defects,” IEEE Transactions On Device And Materials Reliability, Vol. 16, No. 4, December 2016, pp. 622-630.
Glass is an ideal material for package substrates due to the excellent electrical properties, tailorable coefficient of thermal expansion, high mechanical rigidity, availability in large and thin panel form, and smooth surface for fine line fabrication. Glass does have challenges, arising mainly from the brittle nature of glass, and glass substrates with copper and polymer re-distribution layers can suffer brittle fracture after dicing processes. This paper demonstrates three methods to prevent cracking induced by redistribution layer (RDL) stress and dicing defect in glass substrates for RDL build-up up to 90 μm polymer and 40 μm copper. These methods are edge coating, two-step dicing, and laser ablation dicing. Edge coating is a protective layer of polymer located on the diced edge of the glass substrate. After dicing, edge coating puts the glass in compression and prevents moisture from reaching the glass and, in turn, prevents the glass cracking from RDL stresses and dicing defects. In two-step dicing, the first step is used to ablate the RDL using a laser and the second step is used for blade dicing within the ablated region to singulate the glass. The resulting structure from such a dicing process reduces stress at the free glass edge, ensuring that the glass does not crack during dicing. Two-step dicing also allows for thicker RDL build-ups while ensuring no glass cracking occurs from dicing defects. Laser ablation dicing uses a CO2 laser to singulate the glass panel into individual substrates instead of blade dicing, which reduces the dicing defect size and creates a heat affected zone. Use of laser ablation dicing allows for thicker RDL build-ups with minimal process change when compared to blade dicing.
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