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What kind of experiment is there to perform thermal matching between the DPC ceramic substrate and the packaged outer casing?

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The DPC ceramic substrate of the domestic micro-circuit unit has higher and higher assembly density, and the heat generation is getting larger and larger, and the request for the package casing is also increasing. Compared with shell materials such as kovar, tungsten copper, aluminum silicon alloy, aluminum alloy materials have the advantages of small specific gravity, good electrical and thermal conductivity, easy processing and low cost, so they are now used as package casings for microcircuit modules.
Ceramic substrates are commonly used substrate materials in microcircuit modules. However, the thermal shortening coefficient of the ceramic substrate is very different from that of the aluminum alloy case, and the thermal stress caused thereby is sometimes sufficient to constitute cracking of the ceramic substrate. Unless it is packaged in a ceramic case together, how to improve the reliability of the ceramic substrate in the aluminum alloy package ensures that the ceramic substrate can withstand various environmental temperature changes without cracking becomes a serious problem. Slipton Xiaobian today will take you to discuss how to minimize the thermal stress of the ceramic substrate and maintain the substrate at the highest level.

When using a ceramic substrate in an aluminum alloy casing, it is generally necessary to participate in a transitional pad as a stress strain buffer between the casing and the ceramic substrate. The material is generally selected from molybdenum or kovar, and the finite element method is used to analyze such structures. The size and distribution of stress during the temperature cycling experiment. Temperature cycling conditions: -65 ° C ~ 150 ° C, the time of 30 min, the conversion time 1 min. The constitutive model of the solder was selected from the Anand model. In order to make the analysis more general, a total of three different combinations of the box, the pad and the ceramic substrate were analyzed.


DPC ceramic


The thermal shortening factor of ceramic and aluminum alloy materials is different. When the temperature changes, the two shorten or shorten the deformation and constrain each other (the thermal shortening coefficient of the pad is close to that of the ceramic, and can be approximated as one in the theoretical analysis). This mutually constrained thermal deformation produces thermal stress not only in the structural plane direction (i.e., the x direction and the y direction). More importantly, due to the different scales and mechanical properties of the materials, the stiffness of the two is different, so that the strain and the stress in each longitudinal section of the whole structure are different in size and direction, thereby generating additional bending moments. The entire structure is bent to the inside or outside of the casing. At low temperature, the aluminum alloy under the shortening is larger than the upper ceramic substrate, and the structure is bent to the inside of the casing. At high temperatures, the deformation is just the opposite. The ceramic substrate stress is the result of the combined effect of the thermal stress caused by in-plane restraint and the tortuous stress caused by the additional bending moment.
Based on the stress composition and deformation process described above, the following finite element analysis was stopped: the thickness of the ceramic substrate was 0.4 mm, the thickness of the backing plate was 0.2 mm, and the thickness of the bottom of the case was 1.0 mm. There are two cases: one case allows the whole structure to deform freely under the temperature regulation state, and the other case fixes the bottom of the aluminum alloy case to suppress its tortuous deformation. The most important cracking method for brittle materials such as ceramics is the type I open crack propagation crack under the tensile stress effect.
Therefore, the first principal stress (ie, the maximum tensile stress) of the DPC ceramic substrate is selected as the evaluation target of the substrate reliability. When the structure is free to deform, the bottom of the aluminum alloy bends to the inside, and the maximum stress of the ceramic substrate reaches 258 MPa. After fixing the bottom of the aluminum alloy case, the stress of the ceramic substrate is greatly reduced, and the stress peak is only 34.9 MPa. It can be seen from this that the stress on the ceramic substrate is mainly derived from the tortuous stress caused by the additional bending moment. Decreasing the stress of the ceramic substrate in such a package structure should mainly consider reducing the bending moment effect.
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