Ensuring reliability at the system level is of utmost importance for home appliance developers and manufacturers, as it is a critical factor in gaining confidence among customers and fidelity to a brand. Induction cooktops, in particular, are complex devices integrating many components assembled in a limited space and subjected to varying and in some cases extreme conditions. Thus, the occurrence of any failure at the component level must be analyzed and causes must be found, in order to avoid further damage on other components and at the system level as well as to enable lifetime prediction of components -in line with iRel40 objective number 5 “Reduce the failure rates by 30% and enable lifetime prediction with connected and new test concepts along the ECS value chain”-.

In the context of the iRel40 project, BSH Electrodomésticos España (BSHE) has studied the failure mechanism of the photodiodes that are used as part of the pot temperature censoring system. The failure mechanism of this component consists of the breakage of the wire bonding/chip and is a consequence of the applied thermal gradient. In order to assess in-depth this failure mechanism, BSHE has defined a use case modelling a “95% user”, who performs about 10,000 cycles of cooking over the 10 years lifetime of the product.

To verify whether this use case was covered by the reliability testing provided by suppliers for “standard” use conditions or not, BSHE performed an accelerated test consisting of the application of thermal cycling on the wire bonding/chip to test breakage. Such test used “Coffin Manson equation”: AF = (ΔT_Test/ΔT_Field)^m, where 𝐴𝐹 is the acceleration factor, Δ𝑇_{𝑇𝑒𝑠𝑡} is the test gradient temperature, Δ𝑇_{𝐹𝑖𝑒𝑙𝑑} is the use case gradient temperature and 𝑚 is a parameter that depends on the failure mechanism and material. For this analysis, m=3,7 was considered taking into account other studies of similar components. The following thermal cycling was applied to the photodiode:

Figure 1. Test thermal cycle

The failure criteria for this test were dark current above 8nA and loss of electrical contact. The units under test (UUTs) were 40 photodiodes, and at the end of the test, 8 UUTs failed losing the electrical contact between the following cycles (1 between 500 and 1,000; 2 between 1,500 and 2,000; 5 between 2000 and 2500). It was concluded that the reliability cycle test carried out by the manufacturer does not cover the time specification required for BHSE (10,000 cycles, associated with the “95% user”).

This is why BSHE needed to analyze the physics of failure behind. Among other analyses, this required performing X-ray spectroscopy, metallographic cuts, microscope image filter, etc.) on the samples. Reviewing the survival UUTs dark current dispersion, it seemed the dark currents are within specification (<8nA). The failed samples were analyzed using X-ray:

Figure 2. Defective sample nr 31

Figure 3. Wire neck breakage images

Figure 3 shows a wire bonding breakage located in the upper wire neck. It has been concluded that the failure is caused by the mismatch of coefficients of thermal expansion between the wire, the mold compound, and the silicon causing a stress that the component is not capable of withstanding.

This conclusion has allowed BSHE to better define the factors that participate in the Coffin-Manson equation used to obtain a more accurate model for the estimation of the remaining useful life, in line with iRel40’s objective 5. In addition, this has provided insights for the component’s supplier in order to develop a more robust and reliable component design and manufacturing process.

Authors

Sergio Llorente Gil (BSH Electrodomésticos España); Alejandro del Cueto (BSH Electrodomésticos España), Eduardo Imaz (BSH Electrodomésticos España), Jorge Hidalgo-Saavedra Torres (BSH Electrodomésticos España).

Keywords

Reliability, wire bonding breakage, photodiode, Coffin-Manson, thermal cycling, remaining useful life, induction cooktops.

Reference

[1] Bing, Z., Tao, G. (2014). An improved Coffin-Manson model for mid-power LED wire-bonding reliability. DOI: 10.1109
[2] Vishay Semiconductors Reliability. Document Number: 80116
[3] Manoharan, S., Patel, C., Man Li, N., Hunter, S. (2018). Mechanics of Copper Wire Bond Failure Due to Thermal Fatigue