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Assembly of thinned die has become common practice to meet the demand for smaller and lighter electronic products. One way to achieve this goal is to embed the thinned die into two dielectric films, which results in a flexible ultrathin chip package (UTCP). This paper describes a new UTCP process flow with microvia formation by standard UV lithography through photosensitive polyimide (PSPI). Such microvia-formation method proved to be more reliable than laser drilling techniques and simpler than a dry etching process. Since the used PSPI is self-priming, a thin layer of potassium chloride was introduced as a release layer. In the end, the polyimide encapsulation of the thinned die can be released from the carrier substrate and becomes a flexible chip package with a total thickness of around 50 μm. Daisy-chain test dies were encapsulated inside spin-coated polyimide films. Excellent chip-to-package interconnection was demonstrated by electrical daisy chain and contact resistance measurements. Bending tests and thermal cycling tests were also performed on the daisy-chain test vehicles. Desired flexibility and reliability of UTCPs was observed.