Automotive-Qualified DFN Packages

­DFN packages offer high power densities and excellent thermal properties thanks to an exposed heat sink and optimized heat path. Side-wettable flanks also enable the automated optical inspection required in the automotive industry.

Dual flat no-lead (DFN) packages are designed for harsh environments, offering resistance to temperature fluctuations, vibrations, and other stresses. Due to their reduced installation height, high reliability and optimized thermal properties, they are often employed in the automotive industry. Their compact design helps save space on the printed circuit board, which is absolutely essential in vehicles.

By eliminating protruding leads, the package is able to establish shorter signal paths, thereby minimizing signal delay and enhancing signal integrity – a critical factor in high-speed scenarios where precision is also paramount.

Despite the numerous advantages of DFN packages, they also present certain challenges that must be considered during design and manufacturing. These challenges primarily revolve around assembly, soldering, inspection, and testing.

The challenges with DFN packages

Various processes are generally used for soldering DFN packages, depending on the specific requirements of the design and the materials, components, and printed circuit boards used. Reflow soldering is the standard method when it comes to soldering SMD components. The components are initially positioned on the printed circuit board, which then passes through a reflow oven that maintains a precise temperature-time profile. The entire assembly is heated to above the melting temperature of the solder paste, which melts to form the solder joints. The result is uniform and reliable solder joints. The process is well suited for large-scale production.

Furthermore, soldering DFN packages demands a high level of precision. In particular, the amount of solder paste on the bottom terminal pads must be carefully checked to ensure reliable electrical connections. Inadequate solder paste may result in weak or absent connections, while excessive paste can lead to solder bridging or short circuits between adjacent pads. Appropriate solder paste stencils and stencil designs, precise placement equipment, and optimized reflow soldering profiles must be selected and used to guarantee the integrity and reliability of the solder joints. Additionally, considerations such as pad design, solder mask application, and side-wettable flanks must be integrated into the assembly workflow.

Two types of solder masks are commonly used for surface-mounted leadless packages: solder mask defined (SMD) and non-solder mask defined (NSMD) (Fig. 1). With the SMD type, the solder mask partially covers the pads and prevents the solder from flowing over the edge of the SMD pad. With the NSMD pad type, the opening of the solder mask is larger than the solder pad. A gap forms between the solder pad and the solder mask, into which the solder flows and strengthens the joint. The choice between SMD and NSMD depends on several factors, including the design requirements, component pitch size, thermal considerations, and the desired reliability of the solder joint.

Ensuring the quality and reliability of printed circuit boards (PCBs) is of great importance. One of the key contributing technologies is automated optical inspection (AOI). This technology-driven method enables the automatic inspection and analysis of printed circuit boards, allowing for the detection of various issues, such as absent or misaligned components and soldering defects. Other anomalies that can negatively impact the functionality or reliability of a printed circuit board are also identified.

The X-ray inspection is also known as automatic X-ray inspection (AXI). AXI is particularly suitable for inspecting concealed solder joints and internal structures. However, it is not as effective as AOI in terms of detecting surface defects.

Solder joint inspection for DFN

DFN packages feature an exposed pad and connectors on the underside, eliminating visible solder joints from the package design. It is therefore difficult to visually determine whether the package has been soldered effectively. Electrical testing is often the only practical way to verify the quality of the electrical connections of the solder joints. However, for applications that require a high level of reliability, it may be necessary to visually check the integrity of the solder joints. For instance, the automotive industry demands that original equipment manufacturers perform a 100 percent automatic visual inspection after assembly.

In order to be able to inspect the typically invisible solder joints underneath DFN packages, these packages can be designed with side-wettable flanks (SWF) (Fig. 2). This allows a solder meniscus to form on the sides of the DFN package, which can be inspected using AOI (Fig. 3). This results in a further advantage: the mechanical strength and stability of the joint to the printed circuit board is better than with components without side-wettable flanks. SWFs thus reduce the risk of failures due to shear forces and printed circuit board bending.

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Figure 2: Side-wettable flanks of DFN packages

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Figure 3: Sketch of the solder meniscus. This can be inspected with an optical camera system to assess the solder result

Space-saving solutions with a high level of efficiency

Vishay Intertechnology offers four new series of ultra-fast 200 V FRED-Pt rectifiers in the flat DFN3820A package with side-wettable flanks. They guarantee space-saving, highly efficient solutions for cutting-edge power applications.

These 1 A to 5 A, 200 V FRED (fast recovery epitaxial diode) Pt (platinum doped technology) rectifiers ensure reduced space requirements, enhanced thermal performance and greater efficiency.

Compared to components in SMP packages (DO-220AA) with the same footprint, the VS-1EAH02xM3, VS-2EAH02xM3, VS-3EAH02xM3, and VS-5EAH02xM3 models feature a 12 percent lower installation height and more than double the current carrying capacity. Each model is also available in Vishay Automotive Grade AEC-Q101 qualified versions.

Typical applications in the automotive industry include dual-voltage injector drivers, piezo drivers, and engine control units (ECU), advanced driver assistance systems (ADAS), LiDAR, cameras, and anti-lock braking systems (ABS), as well as 48 V electrical systems, chargers, and battery management systems (BMS) in electric and hybrid vehicles (HEC).

Conclusion

DFN packages offer many advantages to automotive manufacturers and are ideal for the harsh environments found there. They also save space and increase signal integrity. Following a few easy steps when positioning the devices will ensure all of these benefits can be achieved.

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