We demonstrate flip-chip solder assembly of InP chips on Silicon-Photonic (Si-Ph) substrates aimed at high volume manufacturing using typical microelectronic lead-free solders. In our show-case application, an InP die is both a light source and a detector in an integrated optical methane gas sensor that operates near 1.6mm. For high-resolution laser absorption spectroscopy sensing, a single-mode tunable laser is desired. We create an external cavity laser with InP as optical gain, butt-coupled to a Si-Ph external cavity, which incorporates the laser's frequency selective elements. For minimal reflection at the InP-Si interface, waveguides are angled to the facet, an index-matching medium is applied between the mating surfaces, and an anti-reflection coating designed for the index-matching medium is applied to the optical coupling facet of InP chip. Sub-micron alignment accuracy is obtained without high-accuracy assembly tooling. Lithographically defined alignment features on both InP and Si components allow reproducible high-accuracy alignment. Interface throughput loss were measured to be as low as 1.4 dB, and interface reflections are more than 30dB smaller than main signal beams.