Published by Olivier Mathieu, Market Development Manager
With almost 40 years of expertise in the field of metal to ceramic joining technologies, Rogers is one of the leading manufacturers and suppliers of Direct Bonded Copper (DBC) and Active Metal Brazing (AMB) substrates under the curamik® brand. Our manufacturing technology is highly automated from end to end to deliver the best quality and cost. Our technical sales and application engineers provide best in class and local design support for selecting the right substrate. Substrates are not only available in multiple copper and ceramic thicknesses, but also various design options and product features exist to fit the specific customers’ needs. In this blog, we take a closer look at these features.
Our curamik® DBC and AMB substrates usually come with a bare copper surface. Even though bare copper is prone to oxidation, semiconductor devices can be easily soldered onto bare copper surfaces when substrates are packaged, stored and handled according to Rogers’ instructions. Soldering processes are all set for bare copper surfaces. Pastes and preforms with or without flux can be used in combination with reducing atmospheres, such as forming gas or formic acid. Cleaning after soldering is often recommended to remove any residue that may impact the following processes, such as wire bonding.
As an alternative to bare copper, the surface can be plated with nickel and gold. This is an electroless plating process which is often referred to as ENIG (Electroless Nickel Immersion Gold). The thickness of the nickel layer is typically in the range of three to seven µm, but it can be increased on request such as in for applications in a harsh environment. The thickness of the gold layer is typically in the range of 10 to 130nm. In the case of power modules without baseplate, substrates with an exposed backside are very often plated with nickel and gold to protect them from oxidation. In addition, surface wettability and electrical conductivity are improved.
With the emergence of silver sintering process as an alternative to soldering for die attach, silver plating has become a standard and preferred surface finish to achieve the desired adhesion and reliability of the sinter joints. The thickness of the silver layer is typically in the range of 100 to 600nm, but it can be increased on request. Also, either the whole surface or only the chip area can be plated with silver depending on the technologies used for electrical connections between the top side of the chips and the substrate.
Another important property of curamik® DBC and AMB substrates is the surface roughness. While the standard roughness is good enough for state-of-the-art applications, special care is required when advanced assembly and interconnection technologies are used. Substrates with an exposed backside are often specified with a lower roughness to reduce and control the thickness of the thermal interface material between the module and the heat sink. Bonding thin wires on the front side of the substrates also require a lower roughness. Finally, a lower roughness helps achieving very thin bond lines with silver sintering pastes. On the contrary, a rough surface can be advantageous for the adhesion of mold compound. Therefore, additional mechanical and chemical surface treatments have been introduced to optimize the surface roughness depending on the very specific customers’ needs.
The substrates' identification and traceability is becoming increasingly important for many industries and is required for automotive applications. Of course, power module manufacturers can identify each substrate on receipt, but information about the production of the substrates is missing in this case. Instead, an individual data matrix code can be added, prior to the shipment, on the curamik® DBC and AMB substrates which include up to 16 alphanumeric characters in a size of typically 3 x 3mm. The code content can be defined according to customers’ requirements.
The front side of the substrate is divided into multiple functional areas, including for the die attach, wire bonding and other electrical interconnections. Printing a solder mask on curamik® DBC and AMB substrates is the most common way to prevent solder material used for die attach from spreading into the adjacent areas. As an alternative, grooves in the copper surface can do the job and come at lower cost, as their production does not require any additional etching step. However, this very much depends on the selected assembly and interconnection technologies, and this option must be evaluated case by case.
Substrates in a rectangular shape are cost effective and hence very popular. But holes through the ceramic may be necessary for mechanical fixation onto the heat sink. In addition, holes may also be required for electrical interconnections between the dies or copper tracks on the front side and the heat sink on the back side. These holes are typically laser drilled and their diameter can be as small as 1mm. The copper free perimeter around the holes can be adjusted to fulfill the requirements regarding electrical isolation between the metal screw and the copper tracks.
The different materials inside power modules are exposed to frequent and numerous temperature cycles, as semiconductor devices are repeatedly switched on and off. Wear out limits the lifetime of these materials and typical failure modes are solder fatigue and wire lift off. While curamik® DBC and AMB substrates are very robust and not likely to fail first, some applications are very demanding and stress relief features, such as dimples on the periphery of the copper tracks, help improve the resistance of substrates to temperature cycles. Unfortunately, dimples consume some precious area that can no longer be used otherwise. As an alternative, curamik® DBC substrates are now also available with a proprietary design without dimples. They can be smaller and achieve a superior endurance compared to the same substrates with dimples. This recent development is another example of Rogers' strong commitment to support the power electronics industry and push the limits regarding the price to performance ratio.
Are you missing an important feature? What are you looking for to make our substrates the perfect fit for your application? Do you have any questions or require some information about our substrates? Please contact us, if you need assistance.
Published on Jun 11, 2021