A CASE STUDY ON Pb-FREE PROCESS AND MATERIAL CHALLENGES IN THE ASSEMBLY OF LARGE FINE-PITCH BGA COMPONENTS

摘要

The objective of this paper is to present a case study to highlight the sensitivity of lead free (Pb-free) process and materials variation on quality and reliability of the large (> 20mm) fine pitch (sub 1mm) BGA components. A DOE was conducted to assess industry readiness towards building desktop mother boards using two commonly used PCB surface finishes (ImAg and OSP) in the Pb-free environment. The second level interconnect integrity and reliability was measured using a fully enabled desktop motherboard test vehicle configuration under different stress tests (thermal cycling, shock, and vibration) to simulate use condition environment. Material analysis was performed across a multitude of package types with varying ball pitch and body size. In addition, reliability performance was assessed by controlling material variations such as PCB surface finish, solder paste, and solder ball composition across multiple DOE legs using a reference assembly process window. The results from the case study revealed failure modes related to assembly process and material issues suggesting strong sensitivity to material and process interactions and narrower process window using SnAgCu (SAC) alloy. The failure modes observed on ImAg boards include nonwetting and open joints under potential un-optimized reflow-profile conditions, and BGA solder joint (SJ) cracks related to planar microvoids. The paper lists recommendations to reduce or eliminate the observed issues to produce robust Pb-free PCBA process.