Industry interest in producing thinner and smaller integrated circuit (IC) packages to match the performance of chip scale packages 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 (PCBs) 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 experience creep strain accumulation. Figure 2(a), shown below, represents board curvature at low temperature, while Figure 2(b) represents board curvature at high temperature, due to constraint by the solder attach consisting of a large thermal pad and perimeter solder joints. 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.
Figure 2. Direction of board and package expansion around corner joint in QFN at (a) low and (b) high temperatures.
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.
A Predictive Model 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. Figure 3 below illustrates model predictions for several QFN package sizes, ranging from 4 to 12 mm in size, with two different mold compound CTEs and two solder joint standoff heights. It shows that even a slight increase of 10 µm in solder height can contribute to a 1.5x increase in fatigue life. Likewise, mold compound CTE can influence cycles to failure more so than the distance to neutral or solder joint height. Even a 5 PPM/°C increase in the mold compound CTE can increase fatigue life by a factor of 3x by reducing the global CTE mismatch with the PCB. Packages in which the mold compound Tg falls within the temperature cycle will contribute to both local and global CTE mismatch. However, 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.
Figure 3. 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.
This model is applicable to both package-level and board-level reliability predictions and is proven to be capable in capturing the influence of extrinsic and intrinsic factors affecting fatigue life of solder interconnects in QFN packages. To improve efficiency, Physics of Failure (PoF)-based electronics design reliability analysis tools, like Sherlock Automated Design Analysis™ Software, can help to reduce model calculation time, along with producing a comprehensive prediction of product reliability before prototyping occurs.
To learn more about choosing the right mitigation for ball grid arrays (BGAs) and QFNs to prevent solder joint failure, watch our free webinar Select the Right Mitigation for BGAs and QFNs.