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Method for fatigue failure recovery through grain recrystallization

IP.com Disclosure Number: IPCOM000009504D
Publication Date: 2002-Aug-28
Document File: 4 page(s) / 497K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for fatigue failure recovery through grain recrystallization. Benefits include improved reliability.

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Method for fatigue failure recovery through grain recrystallization

Disclosed is a method for fatigue failure recovery through grain recrystallization. Benefits include improved reliability.

Background

        � � � � � Fatigue failure is one of the most significant threats to the integrity of solder joints. The source of fatigue load is the thermal cycle the joints experience as the device is turned on and off. Most of the strain cycle is shear, which tends to be concentrated in well-defined shear bands.� Eutectic lead-tin � (Pb-Sn) plating’s fine grain structure gradually coarsens as fatigue progresses Cyclic strain is concentrated within this recrystallized material, and the joint eventually fails along the shear bands (see Figure 1 and 2).

        � � � � � Studies on 1-mm flip-chip ball-grid array (FCBGA) packages reveal that having an optimized package solder resist opening-to-board pad size ratio can help to strengthen the fatigue life of the BGA package. Fatigue cracking occurs close to the package with low ratios, and cracking occurs close to the board with high ratios. The optimized ratio is between the package and board level crack (see Figure 3).

        � � � � � The fatigue failure is conventionally resolved by keeping the package solder resist opening-to-board pad size ratio at the optimum range. Optimization is achieved by tightening the substrate and the printed circuit board (PCB) pad size specification, which used to be a noncritical-to-function parameter. For example, the board pad size had a tolerance of +/‑20% of the target. However, not all suppliers can achieve that tolerance.

General description

        � � � � � The disclosed method is heating and rapid solidification for recrystallization of the eutectic microstructure. The bands of coarsened grains observed in eutectic Pb-Sn joints tend to fail in service or under simulated service conditions. The disclosed method uses an integrated heater embedded on board, removing coarsened grains that are responsible for fatigue failure.

        � � � � � The key elements of the method include:

•        � � � � BGA package

•        � � � � Embedded, on board heater, covering all the thermal balls (solder joints under die) of BGA packages (Figure 4)

Advantages

        � � � � � The disclosed method provides advantages, including:

·        Improved reliability due to the recrystallization of the solder grain structure, producing a fine grain microstructure that exhibits intergranular creep

·        Improved reliability due to the cyclic deformation by intergranular creep leads to little or no m...