SMTA International Conference Proceedings


Authors: Harjinder Ladhar and Sundar Sethuraman
Company: Solectron Corporation
Date Published: 9/21/2003   Conference: SMTA International

Abstract: Microvia technologies have gained importance due to the growing need for low-cost, high density PCBs. Form factor requirements have led to shrinking component pitch, resulting in pads with blind microvias. While blind vias enhances efficient routing, it creates numerous manufacturing challenges for both fab suppliers and board assemblers. One of the main assembly issues with conventional Via-in-Pad (VIP) is the formation of voids in the solder joints. Voids could result when gases trapped within the microvia do not escape during reflow.

This paper primarily focuses on different microvia structures and their influence on the assembly process. A Design of Experiments (DOE) was conducted to understand the impact of via sizes, via locations, via types and other process parameters. Additionally, the influence of other critical factors such as Printed Circuit Board (PCB) surface finish, solder paste composition were also examined. Reflow parameters were optimized for reducing voids.

For assessing the results quantitatively, voiding data was recorded using an Automated X-ray Inspection (AXI) system. Cross-sections were performed to provide confirmation of the X-ray inspection results. The data from traditional dog-bone and plugged microvias were used as a baseline for evaluating new microvia designs. In addition to conventional microvia structures, assembly results from filled and inverted microvias are also presented.

Key words: via-in-pad, VIP, voiding, microvia, CSP.

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