Flange Materials - Materion Now and Next Generation
Materion manufactures ceramic packages designed to house high power transistors that amplify radio frequency (RF) signals. Our packages are used in amplifiers found in base stations for cell phones, broadcast TV and radio. The most popular type of RF power transistor for the commercial cell phone base stations is called Si LDMOS. These transistors are soldered directly onto a Ni and Au plated flange - an Air Cavity Ceramic package. Examples of Air Cavity Ceramic packages manufactured by Materion are shown in Figure 1.
Figure 1: Various ceramic air cavity packages manufactured by Materion. All packages have CuW flanges except for the one on the middle left, which is CPC.
When soldered into a package, the LDMOS transistor chip is surrounded by a metalized ceramic frame. Two or four plated leads are brazed onto the ceramic in order to conduct the input and output signals. The flange is bolted or soldered onto a heatsink which is electrically grounded.
In many respects, the performance of the Air Cavity Ceramic package is controlled by the properties of the flange material (Figure 2). The flange material must simultaneously exhibit this combination of properties:
- High thermal conductivity
- Constrained thermal expansion (ideally to match the CTE of the ceramic)
- High stiffness (to resist bending upon bolt-down)
- Compatibility with braze assembly at 820°C, followed by electroplating with Ni & Au
Figure 2: Flanges prior to brazing into air cavity packages.
The conventional flange material is called “copper tungsten” or CuW. CuW can be thought of as an open-celled, reticulated foam of tungsten that is infiltrated with a continuous phase of high purity copper. The tungsten skeleton constrains thermal expansion, while the copper phase provides good thermal conductivity. CuW is an isotropic composite available in a variety of Cu and W ratios. The composition most commonly used by Materion is 11 wt% Cu + 89 wt % W.
Since the historical trend in LDMOS industry is to further increase the power output of the transistor, the industry has demanded flange materials with greater thermal conductivity. About 12 years ago, a flange material called “CPC” was introduced. CPC can be thought of as a sandwich of two copper layers flanking a core of CuMo. CPC offers two advantages over CuW, namely a higher thermal conductivity and the ability to be stamped, thereby reducing the machining cost. CPC is made by hot rolling layers of Cu onto the CuMo core. When the flange is heated to 820°C during braze assembly, the Cu layers exhibit grain growth due to the stress applied to the copper during hot rolling (Figure 1).
The next generation of RF power transistors will be made with a new semiconductor material called gallium nitride (GaN). GaN power transistors can also be soldered onto an electrically grounded flange. However, GaN transitors have the potential to deliver power two to three times that of Si LDMOS transistors. Theoretically, one GaN transitor could replace two Si transistors, thereby reducing the size, weight and part count in an RF amplifier. However, before such a 2 for 1 substitution can be achieved, GaN transitor manufacturers are requesting a flange material with thermal conductivity higher than that of CPC, while still retaining the desired properties of constrained CTE, high stiffness and braze/plating compatibility. Unless such a superior flange material is invented and commercialized, GaN RF transistors will not realize their full potential for high power. Materion is working to develop a next generation flange material tailored for the demanding requirements of the new GaN industry.
If you have any questions about flanges or any of Materion’s full line of high performance RF and microwave packages, please contact: Richard.Koba@materion.com .