The field of power electronics covers a wide power range stretching from a few watts up to the megawatt range. Currents reach values between 1 A up to more than 6000 A, and voltage can range between 80 V and 10.5 kV. This results in a wide variety of different types of power devices, for which a complete systematic list of available options is virtually impossible.
A large part of the market for power semiconductors consists of discrete semiconductors, that is, with components that contain only one power semiconductor chip in the case.
A variety of housing methods are used. The most prevalant classes of discrete components, in terms of housing methods, include:
- Leaded Components - Two or more straight or bent connecting wires are soldered into holes of the printed circuit board, with or without a cooling surface
- Surface Mounted Devices (SMD) - include many cylindrical or rectangular shapes
Other types include press fit, screw shells, and disc cells.
Leaded Components
The wires of the leaded components are inserted through holes in the circuit board and soldered on the back. The distance between the PCB and component can be defined by bending the connecting wires. This method also helps save space.
Figure 1. Leaded component on circuit board
Surface Mounted Devices
For SMD components, very dense configurations can be accomodated and components on both sides of the circuit board can be done.
PCBs for SMD components are usually printed with solder paste and soldered after placement in a reflow process. SMD components on the underside of a printed circuit board are first glued to the bottom, and then soldered together with the conventionally assembled components by wave soldering.
Figure 2. Connection of surface mounted device to PCB
Passive components such as resistors and capacitors are available as a wired or SMD components.
SMD installation on a PCB procedure
The procedure for installing surface mounted devices on a printed circuit board involves the following:
- Printing the wire plate with solder paste and / or cement
- Assemble the PCB
- Reflow the soldering paste
- Soldering
With progressive integration density, especially for processors due to their many ports, advances were sought in connection technology. This led to the development of the grid array connection system. Here, the solder connections act as small metalized pads under the housing of the circuit.
For a conventional ball grid array, there are solder balls already present on the contact surfaces, which are melted during the brazing process.
Figure 3. Microprocesor with ball grid array (BGA) contact system
For a Land Grid Array (LGA), solder must be brought to be fitted on the board, so LGA components are rarely used for soldering. But they can however, in contrast to BGAs, be operated on a mating pin array. For appropriate designs, LGA components can be easily replaced. This feature of LGAs is often taken advantage of, for example, in microprocessor designs.
Figure 4. Microprocessor with Land Grid Array (LGA) contact system
Leaded and SMD components: problems and new developments
The ratio of chip size to package size is increasing. There is a gradual approach towards the Chip Size Package (CSP ), meaning that the chip and the housing are almost the same size.
This increases the heat produced in the housing and it is no longer possible to dissipate this loss through the leads to the circuit board and from the circuit board to the ambient air. The encapsulation of the chip with a plastic barrier also inhibits the release of heat over the surface of the component. Another problem is that not only due to larger chips, but also on account of the progress of chip technology, current levels in power electronics components are always increasing (up to 300 A / cm ² chip area).
One approach to better management of heat and power in power electronics components is the replacement of conventional bonding wires with massive copper parts. Recent developments include production of the IR copper strap shown in Figure 5 below.
Figure 5. IR Copper Strap
Of course, the Impact of all new developments on the reliability of componets must be considered.
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