This paper presents the design and implementation of an External Memory Built-In Self-Test (BIST) in Systemon-Chip (SoC) designed for System-in-Package (SiP). We implemented the BIST handshaking with the internal bus in the microcontroller core for the purpose of enabling the BIST to access the CPU address space. This implementation allows to reduce the area overhead of the BIST and vary the test conditions flexibly according to each phase of debugging, reliability evaluation and massproduction test. For testing SDRAM and Flash, we also designed a microcode based algorithmic pattern generator with enough loop-counters, an infinite looping function and a multiple command sequence generator. This BIST method was applied to consumer products with the IEEE 1149.1 JTAG TAP controller, and enabled multitest for mass-production on a burn-in tester.