Thursday, November 24, 2011

Power Semiconductor Devices for SMD Assembly

Essemtec’s Pick-and-Place machines are optimized for Power Packages from IXYS

The new ISOPLUS-SMPD package from IXYS allows the assembly of power and control electronics in one operation on the same SMD board. This increases efficiency and reduces the cost of power electronics. In a strong collaboration with Essemtec, IXYS has tested and optimized the automatic assembly of the new ISOPLUS-SMPD package.

Adrian Schärli, Azular GmbH, for Essemtec AG

Electric bicycles, electric cars, solar panels: in all these products power must be switched. Until recently, power switches such as IGBTs were available only as through-hole components or modules that had to be mounted, soldered and connected to heat sinks in traditional, mostly manual operations. IXYS, a leading manufacturer of power devices, however, has introduced a new package type that simplifies design, assembly and cooling concepts of power devices.

Power Semiconductors in a SMD Package with Integrated Insulation
Under the brand name ISOPLUS, IXYS offers packages with similar dimensions compared to standard TO-220 or TO-247 power packages with internal insulation. Therefore, multiple power switches can be directly attached to a common heat sink without additional insulation foils (Fig. 1). As an extension to these products, IXYS now introduces the ISOPLUS-SMDP, which is closing the gap between traditional discrete packages and modules.

The integrated insulation is a result of the implementation of a direct copper bond (DCB, Fig. 2) on which the silicon die is bonded. The DCB features several advantages in addition to its electrical insulation. Its thermal conductivity is excellent and it has a thermal expansion coefficient close to silicon. Therefore, temperature fluctuations do not induce mechanical tensions, which enables soldering in standard reflow soldering ovens.

Using the flexibility of the DCB patterning and different pin-out options, various circuit configurations can be formed within the ISOPLUS-SMPD and then multiple such devices can be used within one application. By default, power and control pins are on different sides of the package, which is ideal for circuit design. The package also is optimized regarding creep clearance and air distances.

Requirements for Pick-and-Place Systems
During development of the ISOPLUS SMPD package, IXYS collaborated with Essemtec, the Swiss manufacturer of production systems for electronics. For many years, IXYS has used specialized pick-and-place machines from Essemtec for the assembly of semiconductor dies and other components. Essemtec has now tested the automatic assembly of SMPDs and has analyzed what the new package type requires from automatic SMD placement machines.

The components are available in blister trays or in 44 and 56 mm tapes, and both can be processed by nearly any SMD pick-and-place machine. For prototyping and small series manufacturing, however, components typically are handled in nonstandard trays or in short tape strips. Automatic picking from such containers is possible only with highly flexible pick-and-place machines such as the FLX, Paraquda or Cobra Series from Essemtec.

The size of ISOPLUS-SMPD packages is in the range of QFPs but they have a higher mass. This mass may influence the placement machine’s regulation systems, especially to the rotation axis. Additionally, the higher mass can reduce acceleration values and the speed-optimization of collect-and-place placement heads. Pick-and-place machines from Essemtec are built for even heavier components and, therefore, offer ideal conditions for the placement of ISOPLUS-SMPD.

SMD placement machines generally transport a component using a vacuum tool. Some machine manufacturers offer tools with soft suction cups as a solution for large or heavy components. For ISOPLUS-SMPD this is not an option because it might cause alignment and placement errors due to the heavy weight. Essemtec’s pick-and-place tests were done using standard tools for large components. These tools are specially coated and feature a good grip to prevent from surface damage.

Controlled Placement Process
Placement height and parallelism is more important for power components than for other SMDs. For example, if multiple ISOPLUS-SMPDs will be attached to a common heat sink (Fig. 2), all components must be aligned exactly parallel to the PCB and the top surface must have exactly the same height.

Precise component alignment also is important because correct creeping and air distances must be maintained. ISOPLUS-SMPD can be well aligned using CyberOptics Laser Centering Systems and vision-based systems.

Essemtec’s test series have shown that the rectangular package body of the package is a good reference for laser alignment. For vision systems, pin alignment is the better choice. The pins provide a good contrast and can be detected easily by image processing systems. Additionally, pin arrangement is asymmetric, making it easy to align the package correctly (Fig. 3).

New Way of Building Power Devices
Essemtec and IXYS have intensively tested the processability of these new SMD packages. Due to these tests, several optimizations were implemented on trays, tapes and on the pick-and-place machines. Today, all SMD pick-and-place machines from Essemtec can assemble ISOPLUS-SMPD reliably, quickly and accurately.

ISOPLUS-SMPDs from IXYS are changing the way power electronics are designed and manufactured. The new package type reduces assembly effort, simplify circuit layout and allow fully automatic production of complete power electronic devices.