Introduction
Automotive electronics engineers need to meet stringent automotive reliability standards for high-stress applications such as engine controls and battery-management systems. Automotive surface-mount fuses, located under the dashboard in a fuse box, are the go-to choice fault protection. Of these fuses, two types are most commonly used: solid body (chip) fuses or wire-in-air fuses.
However, not only are surface-mount fuses different, those that are commonly used can have major drawbacks. For instance, the non-uniform performance of printed-circuit style ceramic fuses can lead to internal connection failure caused by mechanical/thermal stress (vibration or bending), or by common soldering defects (cold joints or poor wetting). This type of failure can lead to damage to the circuit board and surrounding components.
This article describes the trends in automotive electronics driving the need for more reliable surface-mount fuses, and explains why using conventional fuses to protect automotive applications can fall short. For a solution, it presents advancements in ceramic and wire-in-air surface-mount fuses designed to specifically address the AEC-Q200 automotive standard. It also shows results from simulation tests that illustrate the significant advantages of using advanced technology over the traditional approaches.
Evolution in Surface-Mount Fuse Technology
As cars get “smart” and “connected,” more and more embedded and distributed electronics require pc-board-mounted circuit protection. And with the rapid emergence of electric (EV) and hybrid electric (HEV) vehicles – most with high energy lithium battery systems – the demand for reliable circuit protection devices to protect against catastrophic failures is critical.
While most automobiles (and commercial vehicles, for that matter) provide passengers with a comfortable environment, automotive applications for electronics are subjected to among the harshest of environments – wide temperature variations, shock and vibration, exposure to humidity, water and salt. These applications have placed emphasis on the need for improving surface-mount fuse technology. Fortunately, advancements in both chip and wire-in-air fuse technology is offering dramatically better reliability results over traditional solutions.
Chip Fuses
Traditional Approach: Printed-Circuit Style Chip Fuse
The structure of a conventional printed-circuit style of solid body fuse has a single-layer fuse element. The photomicrograph in Figure 1 shows how the printed-circuit structure is mainly composed of the epoxy substrate and glass. The fuse element is bonded to the surface of the pc board and coated with a protective polymer.
Figure 2 shows the result of a printed-circuit style chip fuse that was placed under high-current overload test conditions. As can happen with this type of chip fuse, the fuse element vaporized, causing prolonged arcing that led to surface melting, cracking and compromised mechanical integrity.
New Approach: Multi-layer Ceramic Chip Fuse
The other most common type of chip fuse is the multi-layer ceramic fuse. Figure 3 shows how the ceramic fuse’s co-fired monolithic structure has two layers of fusible material embedded in the structure.
The ceramic type of fuse offers several distinct advantages for automotive-grade applications since its monolithic structure it is capable of higher current ratings in a smaller package, plus it has a wider operating temperature and can maintain stable operating characteristics in extreme conditions.
A new advancement in ceramic fuse technology is the SolidMatrix ceramic fuse (Figure 4). This solid-body ceramic fuse utilizes a proprietary, solderless end cap construction and provides excellent mechanical and thermal stability over a wide temperature range (-55°C to +150°C).
An important comparison between the two types of chip fuses is how they react mechanically to a fault condition. Where the printed-circuit style fuse can experience arcing and damage during exposure to fault conditions, the multi-layer structure ceramic fuses are able to sustain their integrity.
For example, Figure 5 shows a SolidMatrix ceramic fuse after it was subjected to the same high-current stress conditions as the printed-circuit style fuse described in Figure 2. Unlike the damage sustained by the printed-circuit fuse’s single-layer fuse element, the SolidMatrix’s multi-layered fuse element is diffused into the ceramic body and the device’s appearance shows no external damage
Wire-in-Air Fuses
Traditional Approach: Square Ceramic Tube Fuse
The conventional wire-in-air fuse is known as the square ceramic tube fuse, or square nano fuse. The image Figure 6 shows the common construction for this type of fuse wherein the fusible wire element is housed inside a ceramic tube and connected to the endcaps with solder beads.
There are several disadvantages associated with this conventional wire-in-air fuse. Endcap detachment is a common failure mode in its construction. Also, there is a lack of uniformity in performance due to the variability in the placement of the wire element inside the ceramic tube.
In a worse-case scenario, high-current/heat causes the solder to vaporize, pressure builds up and the fuse erupts/opens. Once this occurs, the temperature goes down, the solder condenses and is redeposited across the circuit where it presents a potential short-circuit condition.
Figure 7 shows two conventional wire-in-air fuses subjected to an EV short circuit condition. Sample A at 250V/250A (left image) and Sample B at 450V/450A exhibited significant damage to the fuse and collateral damage to the surrounding circuitry. In the waveforms, the current flow (yellow trace) through the fuses each display secondary current flow that ultimately resulted in pc board damage.
New Approach: AirMatrix Fuse
A new performance wire-in-air fuse, the AirMatrix, uses a proprietary, hermetically-sealed wire-in-air structure that assures consistent electrical performance (Figure 8).
The fuse element in the AirMatrix fuse is uniformly straight across the cavity and externally bonded to the endcap (Figure 9). Unlike the conventional square nano type fuse, with its ceramic body and solder connect design, the AirMatrix fuse uses a fiberglass-enforced body and solderless direct connect construction.
When subjected to the same EV battery short circuit as the square nano tube fuses described earlier in Figure 7, the AirMatrix fuse’s advanced construction withstood 450V/450A conditions without experiencing any external damage (Figure 10). Note how in the waveforms, the current flow (yellow trace) through the AirMatrix fuse drops to zero. The voltage (green trace) shows an open circuit for the AirMatrix fuse with no secondary conduction.
Summary
Automotive electronics engineers need to look past traditional chip and wire-and-air fuse solutions when setting out to qualify their devices for the AEC-Q200 standard. As shown in by testing results covered in this article, advancements in ceramic and wire-in-air fuses offer significant advantages over conventional approaches. Representing a new approach with significant advantages, the SolidMatrix multi-layer ceramic chip fuse and the AirMatrix wire-in-air fuse are designed in a TS16949-certified facility and are specifically designed for reliable operation in high-stress automotive applications.
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