Asymmetric Hybrid Printed Circuit Board Design: Warpage Considerations

Estimated reading time: 2 minutes

Editor’s note: This excerpt is from a white paper presented at IPC APEX EXPO 2024, and is intended to give an overview and recommendation on the topic. To see a list of full papers and other conference proceedings, please click here.

Abstract

The printed circuit board (PCB) accounts for a significant portion of the PCBA’s (printed circuit board assembly) BOM (bill of material) cost. Designs with hybrid PCB stackup are adopted to reduce the cost of the PCB by using less expensive laminate materials in layers that do not have routings for high-speed signals (e.g., power and ground layers). The conventional hybrid PCB stackup design is symmetrical in the middle¹. This engineering technique has been employed in the data center industry for quite some time. To extend the idea and optimize the purpose of hybrid PCB stackup design, the feasibility of an asymmetric hybrid stackup is currently being studied.

Asymmetric hybrid stackup offers greater flexibility and potential cost savings in high-speed traces routing². However, the mechanical reliability risks in the asymmetric hybrid stackup are even more challenging than those in conventional hybrid PCB stackup. In this research, a team with diverse expertise, including design, material, and PCB manufacture process, was formed to study the effects of (1) different hybrid material combinations, and (2) various asymmetric hybrid stackup constructions on mechanical reliability performance using a qualitative approach. Unlike most other research that uses test coupons as the subject, a production-level data center mainboard design was used in this study, which provides more reliable test results and more accurate assessments.

Reliability test methods and results concerning warpage, delamination, via reliability, and brief signal Integrity comparison will be discussed in this paper. Recommendations on PCB manufacturing and design guidelines to mitigate the reliability risks associated with asymmetric hybrid PCB will also be provided.

Introduction

Reliability risks (warpage, delamination, conductive anodic filament, and via reliability) remain the major concerns in the usage of asymmetric hybrid PCB stackups. Conventional wisdom suggests that the manufacturing challenges and reliability risks of asymmetric hybrid PCB stackups are even higher than with symmetric hybrid PCB stackups. This research aimed to (1) understand the effects of different copper-clad laminate (CCL) material combinations and various constructions on the reliability performance of the asymmetric hybrid stackup, and (2) establish design and manufacturing guidelines to mitigate the reliability risks of the PCB, using a qualitative approach.

To read the entire article, which originally appeared in the August 2024 Design007 Magazine, click here.