Building Power Modules is not Magic but Requires High Quality Substrates

Jul 31, 2018

Reliability

Design engineers are selecting Direct Bonded Copper (DBC) and Active Metal Brazed (AMB) substrates as circuit material for bare semiconductor chips in their power modules as they efficiently dissipate the waste heat from the semiconductors and increase the lifetime of the modules. But process and production engineers first have to build these modules. They have to join and connect multiple components with care to provide the required electrical, thermal, chemical and mechanical functions. In this blog, we describe a typical production flow with a focus on the most important characteristics of the substrates at each step of this assembly process.

Die attach

First step is usually to attach multiple bare semiconductor chips to the substrate. Ideally the dies would be directly pressed against the substrate. This would significantly reduce the thermal resistance between the dies and the substrate and reliability could be improved as no additional material is needed for the joining process. However, even though some promising R&D activities are ongoing, this is not very popular yet because this requires spring contacts and special foams to apply sufficient and homogeneous pressure while not damaging the chips. Also the substrates would have to be almost flat, perfectly clean and without any defects like scratches, indents, bumps or alike. Instead, still today, soldering is the most popular die attach technology. This process varies as one can use different solder alloys, as paste or as preform, with or without flux, in different reducing atmospheres, with heat applied by means of a hot plate, light or vapor. But in any case, the process is performed under vacuum to avoid the formation of voids that would impact the thermal transfer. And the solder wettability on the surface of the substrate is critical. Irrespective of the bare copper, nickel or gold finish, contamination has to be avoided. This is achieved by proper cleaning, packaging, shipment, storage and handling of the substrates. Operators should be trained to increase their awareness regarding cleanliness as they will have to strictly follow instructions.

Silver sintering is an alternative die attach technology which increased in importance recently. In this process, silver nano or micro particles delivered as paste or thin films are sintered together under pressure and temperature around 250°C to form a thin silver bond layer. Such layers outperform traditional soft solder layers in thermal resistance and reliability thanks to their high melting point, high thermal conductivity and typically thin bond line. But the process is still pretty new and requires new equipment to apply the required amount of pressure. Along with the new sinter press and new sinter materials, new requirements for substrates have appeared with reduced surface roughness and silver finish being the most important ones. In addition, a selective plating process has been developed to provide silver finish only where required in the chip areas.

With or without base plate?

In the next step, the substrates populated with bare semiconductor chips are attached to a copper or AlSiC base plate. This is particularly true for high power modules where multiple substrates have to be mounted on one very large base plate. Here again, as for die attach, soldering is the most common technology and sintering is an interesting option, too. But this process step is much more critical than die attach as the surfaces to be joined are much larger. Next to cleanliness, surface finish, roughness and any kind of defects, now also the bow of substrates and base plate plays a very significant role and may heavily impact the production yield if not well under control. And as the yield loss increases, the costs associated are higher due to the higher value of populated substrates and base plate.

For low to medium power modules where only 1 substrate is required, module manufacturers avoid using a base plate. This is possible as the module housing includes screws and spring contacts which provide sufficient and homogeneous pressure over the whole surface of the substrate. In this way, a perfect contact with the heat sink can be achieved if the end users follow the mounting instructions and apply the right and sufficient amount of thermal paste between module and heat sink. Unfortunately, this was not always the case in the past and module manufacturers had to deal with many claims. In the meantime, many module manufacturers have launched a range of modules with pre applied thermal interface material to avoid such issues. Modules without base plate differ from their peers with base plate in a way that the backside of the substrate is exposed to the ambient environment. As a consequence, this surface can be easily oxidized or contaminated if protective measures all along the supply chain are not taken. As an example, some module manufacturers prefer to use a nickel or gold rather than bare copper finish because any slight oxidation of the latter can be easily observed. Also, minor defects that were acceptable for the joining process with the base plate are often a concern the end user. Even though such minor defects do not impact the performance of the module in the system, more investigation results are required to prove that they are just cosmetic failures and harmless for the application.

Electrical connections

In a power module, multiple devices have to be connected in parallel or in series and ultimately they have to be connected with the module terminals. However, most of the semiconductors are vertical devices with their electrical contacts not only at the bottom but also at the top side of the devices. As a circuit material, one substrate alone cannot connect all these contacts together. In the most traditional approach, Al wires are used to realize the missing connections between chip contacts on top and bottom (in serial connection) or on top and top (in parallel connection). The typical diameter of such wires is in the range between 100µm and 500µm. When such wires have to be connected with the surface of the substrate, it is clear that defects like bumps or holes would negatively impact the wire bonding process. Our operators and our equipment for automated optical inspection are paying particular attention to the corresponding wire bonding areas to deliver only substrates that are compliant with the agreed specification.

For many years already, module manufacturers have known that these Al wires are limiting the reliability of their modules. They have been developing new materials and technologies to replace them like Cu wires or ribbons and flexible circuits. Another approach is to use two substrates, one on each side of the chips, in a kind of sandwich package. This comes along with new challenges for the design of the module and for the substrates with increased requirements for flatness and parallelism. But this also offers another path for heat dissipation and this dual side cooling approach is becoming more and more popular.

Finally, the module terminals are either soldered or attached to the substrates in an ultrasonic welding process. The latter offers significant advantages regarding reliability as no additional material is used and a direct copper to copper contact can be realized. The force and mechanical vibrations which are applied to the substrates during this process may lead to damages in the ceramic if the process parameters and design of the substrate are not chosen with special care.

Encapsulation

In order to provide mechanical rigidity and protect the active devices from humidity and other chemical substances in the ambient environment, the populated substrates are encapsulated with silicone gel, epoxy resin or mold compound. These encapsulants are critical to guarantee the required isolation. A lack of adhesion on the surface of the substrates or any bubble in the bulk material may lead to isolation failures during operation. Special treatment or finish on the surface of the substrate may be necessary to optimize the adhesion of the encapsulant depending on its coefficient of thermal expansion (CTE), E-modulus and glass transition temperature. A lot of activities are ongoing in this regard as the chip junction temperature keeps on increasing and new encapsulants that can withstand higher temperatures have to be qualified.

All along the production flow, the substrates are critical to achieve the best possible yield and deliver the required module performance later during operation in the field. Module manufacturers can rely on the commitment, experience and expertise of the Rogers Power Electronics Solutions (PES) team to provide substrates with a good and consistent quality for existing and future applications.

Do you have any design questions or require some assistance with the selection of a suitable substrate or cooler for your application? Rogers PES’ experts are available to help. Please contact us if you need assistance.