18 POWER MODULES www.inseto.co.uk Issue 2 2023 Power Electronics Europe www.power-mag.com thermal mass, it will take longer to reach the sintering temperature, which is about 250˚C. The sintering process is relatively quick, at about 3 minutes. The top of the die is protected using a thin film of Teflon and the sinter tool head presses down, applying a pressure of between 15 and 30MPa. The thickness of the paste reduces by a further 50% as the silver particles bond to produce a bond line thickness (BLT) of between 30 and 40µm – see figure 1. It is advisable to allow the substrate and die to cool while still in the sinter press to avoid oxidisation, before going into test; followed by wire bonding (which might also include tests) and moulding (chip encapsulation). Figure 2 illustrates some of the equipment used in the packaging of sintered die. Results The main method of determining how well the die has attached to the substrate is to perform a mechanical shear test. A shear strength of more than 30MPa should be achievable at room temperature and more than 20MPa during a hot test (typically 100 to 120˚C). One way of increasing shear strength is to silver plate the DBC substrate in the areas where the sinter paste will be applied. This means the interface will be between the silver on the underside of the die, the silver sinter paste, and the silver plate on the substrate. In destructive tests performed by AMX Automatrix, shear strengths of up to 70MPa have been recorded. Without silver plating, dies were shearing at about 55MPa. Shear strength is also a measure of the presence of voids beneath the die, noting here that to see voids requires a scanning acoustic microscope (SAM). Voids can lead to delamination as a result of thermal cycling. Figure 3 shows a SAM image of voids and delamination. Voids are expressed as a percentage of die area. However, the figures regarding what are acceptable percentages have been inherited from industry standards for attaching die (usually Si) using solder. For SiC, far more power is being handled (to the extent there’s likely to be a bang in the event of delamination) and there’s a far greater need for efficient heat dissipation. Many believe there should be much tighter requirements where acceptable void percentages are concerned, and although sinter processes (for electronic component die attach) have not been around long enough for standards to be set, early results are looking good. Thermal cycling is an essential test, and AMX is aware of some OEMs subjecting sintered components to extremely aggressive tests. For example, some cycle between -55 and 250˚C, noting that -40˚C is the automotive industry’s current low temperature limit. And the jury is out on what should be an upper temperature test limit for SiC, though AMX is aware of one company that is testing to 300˚C and its power modules are performing well, i.e., no reduction in shear strength and therefore no indication of delaminating. Also, although 1,000 cycles are standard, many are subjecting their designs to 10,000 cycles. Summary Sintering technology has come a long way in a relatively short period of time, and the use of silver sinter paste has produced some great results and continues to do so. Copper sintering is on the horizon though, driven by the far lower cost of the metal. www.inseto.co.uk Above pressure sintering using an AMX Automatrix X-Sinter P54. Figure 3 – Left a SAM image of a number of sintered dies. The voids look like bubbles and the dark patches are a sign of delamination.
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