Solder ceramic substrate materials

The contact between the aluminium layers and the ceramic substrate requires a joining material which will wet both metal and ceramic, and solders such as the conventional Pb-Sn alloy have been used which are molten during the annealing process. The contact between the solder and the aluminium layer is frequently unsatisfactoty because of the intervention of the AI2O3 layer, and a practical solution appears to be to place drree layers of metal clrromium in contact widr the aluminium, copper in contact with the clrromium, and gold between the copper layer and the solder. 

Interconnect failures due to thermal fatigue of solder joints can be reduced by closely matching the x-y plane thermal expansion properties of the substrate to at-risk components. Large leadless ceramic components that are used because of their hermeticity pose a particular risk. Possible approaches include altering the laminate reinforcement material, adding constraining metal cores or planes, and switching to a ceramic substrate. The first two approaches are discussed here. A more extensive discussion of these options can be found in Ref. 33. 

Package Substrate Material. The choice of package substrate material can have a large impact on solder joint rehability. Some substrates require employing solder ball alternatives (such as the ceramic solder column carriers shown in Fig. 58.4) or even the elimination of a soldered solution due to interconnect reliability considerations. A wealth of information regarding ceramic and plastic substrates can be found in the literature. This section focuses on the impact of those materials on solder joint reliability. 

Materials used for substrates can be broadly classified into ceramics and metals. Gommonly used ceramics, ie, alumina, aluminum nitride, and beryUia, can be easily incorporated into a hermetic package, ie, a package permanently sealed by fusion or soldering to prevent the transmission of moisture, air, and other gases. 

In thin-film metallization by evaporation or sputtering of thin metal films onto a ceramic surface (Chapter 28), it has been demonstrated that a sequence of layers of different metals is required for optimum film properties. The first layer is usually a refractory metal such as Ti, Cr, or NiCr this layer provides adhesion to the ceramic. These elements are reactive and bond through redox reactions with the substrate. The second layer acts as a diffusion barrier. The barrier material will usually be a noble metal, preferably Pt or Pd. The top layer will be the metal of choice for the particular application, for example, Au for wire-bonding applications and Ni or Ag-Pd for solderability. 

Figures 1.1.9 and 1.1.10 show cross-sectional SEM photographs of solder joints at a corner of CSP after 1000 cycles, for 13 and 11.5 ppm/°C substrates, respectively. In the case of the 13ppm/°C material, there is no crack. In the other case of the 11.5ppm/°C ceramic material, failed solder joints are observed. Figure 1.1.11 shows TCT (—40 to 125°C) results for CSPs made of the 11.5 and the 13 ppm/°C material. The package failure was defined as 50%...

We have developed new high TCE ceramic material of 13ppm/°C. This material and 11.5 ppm/° C material have almost same properties with the exception of TCE. By using the 13 ppm/°C material, the TCE mismatch between the substrate and the potting compound decreases, and the improvement of solder joint reliability is obtained as shown in Figure 1.1.11. 

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