Improved packaging technologies and component miniaturization can often lead to new or unexpected design, manufacturing, and reliability issues. This has been the case with QFN packages, especially when it comes to adoption by new non-consumer electronic
OEMs.
Design and manufacturing Some key QFN design considerations are pad and stencil design. When it comes to bond pad design two approaches can be taken:
solder mask defined (SMD) or non-solder mask defined (NSMD). A NSMD approach typically leads to more reliable joints, since the
solder is able to bond to both the top and sides of the copper pad. The copper etching process also generally has tighter control than the solder masking process, resulting in more consistent joints. This does have the potential to affect the thermal and electrical performance of the joints, so it can be helpful to consult the package manufacturer for optimal performance parameters. SMD pads can be used to reduce the chances of
solder bridging, however this may affect overall reliability of the joints. Stencil design is another key parameter in QFN design process. Proper aperture design and stencil thickness can help produce more consistent joints (i.e. minimal voiding, outgassing, and floating parts) with proper thickness, leading to improved reliability. There are also issues on the manufacturing side. For larger QFN components, moisture absorption during
solder reflow can be a concern. If there is a large amount of moisture absorption into the package then heating during reflow can lead to excessive component warpage. This often results in the corners of the component lifting off the
printed circuit board, causing improper joint formation. To reduce the risk of warpage issues during reflow a
moisture sensitivity level of 3 or higher is recommended. Several other issues with QFN manufacturing include: part floating due to excessive solder paste under the center thermal pad, large solder voiding, poor reworkable characteristics, and optimization of the solder reflow profile.
Reliability Component packaging is often driven by the consumer electronics market with less consideration given to higher reliability industries such as automotive and aviation. It can therefore be challenging to integrate component package families, such as the QFN, into high reliability environments. QFN components are known to be susceptible to
solder fatigue issues, especially thermomechanical fatigue due to
thermal cycling. The significantly lower standoff in QFN packages can lead to higher thermomechanical strains due to
coefficient of thermal expansion (CTE) mismatch as compared to leaded packages. For example, under accelerated thermal cycling conditions between -40 °C to 125 °C, various
quad flat package (QFP) components can last over 10,000 thermal cycles whereas QFN components tend to fail at around 1,000-3,000 cycles. however this has primarily focused on die and 1st level interconnects.
IPC-9071A attempted to address this by focusing on 2nd level interconnects (i.e. package to PCB substrate). The challenge with this standard is that it has been more adopted by OEMs than component manufacturers, who tend to view it as an application-specific issue. As a result there has been much experimental testing and
finite element analysis across various QFN package variants to characterize their reliability and
solder fatigue behavior. Serebreni et al. proposed a semi-analytical model to assess the reliability QFN solder joints under thermal cycling. This model generates effective mechanical properties for the QFN package, and calculates the
shear stress and
strain using a model proposed by Chen and Nelson. The dissipated strain energy density is then determined from these values and used to predict characteristic cycles to failure using a 2-parameter
Weibull curve. ==Comparison to other packages==