PCB Warpage and Solder Joint Fatigue

Dated:2017-08-09      Popularity:1387

Thinner and smaller integrated circuit has resulted in the wide application of quad flat no-lead (QFN) components. However, the small-form factor of QFN packages can place solder joints at risk of coefficient of thermal expansion (CTE) mismatch, which can potentially lead to PCB warping and failure. To help mitigate this risk and accurately assess the fatigue life of solder interconnects in QFN packages, a predictive model incorporating the material and geometric parameters that influence solder joint fatigue should be used.

The effect of temperature extremes

Due to the larger CTE of printed circuit boards compared with the effective expansion of the QFN package, one common issue QFN packages experience is the warping curvature of the PCB. Cold temperature extremes can induce plastic strains, while solder joints under high temperature extremes can creep strain accumulation. In order to account for the total damage contribution to reduction in fatigue life, both stress and strain magnitudes at each temperature extreme need to be considered.

Likewise, material properties and geometry equally influence solder joint fatigue during thermal cycling. Solder joint standoff height, distance-to-neutral, mold compound CTE and glass transition temperature (Tg), and die to package ratio all play significant roles in the fatigue life of solder interconnects.

An effective solution

To accurately assess the fatigue life of these factors, it is important to use a predictive model that incorporates the material and geometric parameters influencing solder joint fatigue. The exact increase or decrease in fatigue life due to changes in package design and material properties will also depend on solder alloy fatigue properties and fluctuations in temperature the package is exposed to.

Predicted cycles to failure for QFN packages with different package size, mold CTE and solder height for thermal cycle of 0°C to 100°C with 15-minute dwell at each temperatures extreme.

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