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Novel Approaches for Minimizing Pad Cratering
Novel Approaches for Minimizing Pad Cratering
Pad cratering is laminate fracturing under the copper pads of a surface mount component. Two novel approaches to minimize pad cratering are presented.
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Authored By:
Chen Xu
Alcatel-Lucent, Murray Hill, NJ 07974, USA

Yuan Zeng and Pericles A. Kondos
Unovis-Solutions, Binghamton, NY 13902

Yunhu Lin
Alcatel-Lucent Shanghai Bell, Shanghai, PR China

Summary
Pad cratering is defined as laminate fracturing that may occur under the Cu pads of a surface mount component. Typically, the fracture initiates within the laminate during a dynamic mechanical event. The initial crack propagates in the laminate under external stress and eventually into adjacent Cu conducting lines, resulting in an electrical open circuit.

The term "pad cratering" has been widely adopted for this defect since the extent of fracturing may be sufficient in severe cases for the pad to be pulled from the laminate during normal processing leaving a crater.)

Though not a common issue, pad cratering is more often experienced in lead-free assemblies than in those using SnPb solder due to the use of different laminate materials and has been extensively studied in the electronic industry. Identified mitigation approaches are based upon either reducing the stress on the laminate or using a stronger and more pad cratering resistant material.

Several methods have been used to reduce stress on the laminate, including increasing the effective pad size through use of solder mask defined pads, corner glue on the components, and strict limitations on board flexure during circuit board assembly operations such as ICT testing.

idea is that, with reduced stress on the laminate, pad cratering will be minimized or its onset significantly delayed. The second approach is to use laminates with improved pad cratering resistance. Increasing the strength and pad cratering resistance of the laminate material has proved difficult and very little progress has been achieved in this regard with the most popular epoxy based lead-free laminates.

Polyimide has higher pad cratering resistance than epoxy based laminates due to its higher intrinsic strength and can be used for mitigating pad cratering. However, polyimide is more expensive and difficult to process compared to the widely used epoxy based laminates.

As a compromise, polyimide has been used as the external layer (in the form of ZetaTM Cap) for minimizing pad cratering in epoxy-based laminates [9]. The hybrid structure formed by using epoxy laminate as inner layers and polyimide as cap layers improves the pad cratering resistance of the board compared to boards using epoxy laminates only.

In this paper, we propose two approaches for mitigating pad cratering issues. The first approach uses a pad cratering resistant material for the external layers in a hybrid structure and falls into the second category of mitigation strategies (see above).

The second approach is based on minimizing defects (the initiation site for pad cratering) and forms a new and third category of mitigation strategies. Some preliminary testing result will be presented to demonstrate the viability of these two approaches.

Conclusions
Two novel approaches for mitigating pad cratering are presented. The first approach, based on the realization of distinctly different locations of pad cratering (in the external layer) and delamination (in the inner layer), uses pad cratering resistant material in the external layers and lead-free compatible material for the inner layers.

The second approach is based on the importance of defect sites in brittle fracture mechanics and attempts to minimize pad cratering by reducing the initiation sites. Future work is planned to determine the lead-free compatibility of the hybrid structure and the optimal Cu surface roughness for minimizing pad cratering.

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