Journal of SMT Article

Lead-Free PTH Rework Process Challenges And Solutions For A Complex Server PCBA

Authors: Willie Davis, PK Pu, Wang Yong Kang, Alex Chen, Daniel Tan, Wai Mun Lee
Company: IBM Corporation and Celestica Inc.
Date Published: 7/29/2010   Volume: 23-3

Abstract: Although most sectors of the electronics industry have transitioned to lead free assembly, many high-end complex servers continue to be manufactured using SnPb solder under the EU RoHS lead in solder server exemption 7b. As the industry prepares for potential exemption expiry in July 2014, many studies are being conducted to understand and overcome the technical challenges of lead free assembly of these highly complex PCBAs. This added complexity includes thicker and higher layer-count PCBs with denser component placements consisting of components that are thermally sensitive to lead-free processing. Additionally, these assemblies will often have electrical requirements resulting in component-to-component DfM (Design for Manufacturing) violations. These added complexities result in increased challenges in both the primary build and during rework.

A particular challenge in lead free processing of products in the server space is the assembly of the PTH (Pin Through Hole) components. The card thickness of these complex servers can exceed 130 mils with layer counts exceeding 20 layers. Additionally multiple layers of one and two ounce copper planes further complicate the wave process and rework of the PTH components.

Achieving adequate hole-fill during initial assembly and rework is considerably more difficult in complex servers, especially in cases of ground or power planes that are connected to multiple layers. Rework is further challenging as the elevated lead free processing temperatures drive longer solder contact times which contribute to copper dissolution of the PTH barrel due to the Pb-free metallurgy. With these additional challenges, traditional methods of PTH rework may not give acceptable results.

This paper presents design and process considerations and a new methodology for PTH rework where traditional methods were not successful. The test vehicle used in this study was a highly complex customer server product which was under development.

Key words: Lead Free Server, High Complexity, PTH Rework, Product Vehicle, New Rework Methodology

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