Die attach process boosts heat removal 15x
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UK startup QPT has developed a die attach process that can improve the heat transfer in power electronics by 15x.
As wide bandgap materials and higher frequency operation enables higher power levels, so the need to increase the heat removal through the packaging becomes increasingly more important.
The die attach process developed by QPT in Cambridge improves the thermal connection to heat spreaders or substrates which are typically Aluminium Nitride (AlN). The improved thermal transfer of the qAttach process also increases reliability as the assembly process places less stress on these substrates.
QPT developed the qAttach process to use with the gallium nitride (GaN) transistors that it uses in its high frequency and high voltage electric motor control topology. With GaN transistors now rated for higher voltages but with relatively small die sizes, there is less surface area to remove heat from. As a result, they are often down rated to enable them to function without overheating, which gave QGT issues in the development of its AI-developed control architectures for automotive, industrial motors. qAttach opens up GaN for the next generation, high power, high voltage applications.
The technology can also be used with silicon carbide (SiC) devices.
“The problem with the current attachment approach is that the sinter layer, which fixes the die to the substrate, is typically 30 to 60 microns thick and this forms a thermal barrier that impedes the transfer of heat away from the chip,” said Rob Gwynne, CTO at QPT. “We use reliable, well-established technologies from other fields in a novel way to enable us to create the qAttach attachment layer that is potentially down to a fraction of a micron thick. This major reduction in the thermal barrier thickness means that our solution is up to ten times better at transferring waste heat away from the chip. As we refine the process, we are expecting even better thermal transmission rates through this layer.”
With a conventional heat spreader, the heat from the die has to pass through the thick sinter layer to the substrate to be dissipated via the heat sink. The PCB is attached to the top and around the heat spreader in embedded packages so there is little heat dissipation that way.
QPT’s structure is a sandwich of heat sink, substrate, the qAttach layer, die, qAttach layer, substrate and heat sink with the PCB surrounding the structure at the sides. Because the qAttach layer is ultrathin, heat can be transferred through and away much quicker plus this can also now happen from the top of the die to increase the total rate of heat removal by up to 15x.
The technology has other improvements over the current sintering process. Firstly, the substrate can be much thinner as the application of the large force needed by sintering is not required. The thinner substrate significantly reduces thermal resistance to further help heat transfer away to the heat sink.
Secondly, the lower pressure required for this process means that the manufacturing stresses on the dies are less. This reduces the possibility of device failure which will be of particular interest to automotive companies where reliability is key.
The ultrathin qAttach layer is not a laminar sheet. It has a proprietary geometry which constrains expansion predominantly in the Z axis, which is perpendicular to the qAttach layer. This means there is no delamination of the attach layer from the die and substrate, addressing a major issue with current attachment methods. This is because the conventional, continuous sheet of the sintered approach has about seven times the thermal expansion of the die and about three times that of the AlN substrate.
These differing rates of expansion create considerable stresses over the length of a large power die that can result in the structure ripping itself apart when heated. This delamination is the largest cause of failures in power packages so this new approach further improves the reliability of the assembled device.
“qAttach is a universal solution to solving the growing problem of the removal of waste heat that would otherwise hold back the development of next generation, power electronics,” said Gwynne.. “The ability of qAttach to improve transfer heat away from the die by up to 15x can also be used to solve the removal of waste heat from almost any other type of transistors such as SiC to enable them to handle higher power loads than they can at present. We already have a couple of leading multinationals interested in licensing this process as they can see the strategic benefits that this innovation would bring to their product lines.”
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