Aug 31, 2018
Who cares about flatness? Process and application engineers do! These are not flattering words as they truly know how critical it is to understand and control the shape of one’s substrate, base plate and heat sink in order to achieve the best possible production yield and module performance. In this blog, I want to share with you some information about flatness that you may wish to consider as you design or use power modules.
Perfectly flat metallized ceramic substrates do not exist
Metallized ceramic substrates usually are not perfectly flat and their shape is likely to change as they move along the production line. From a theoretical point of view, substrates are not perfectly flat due to the mismatch of Coefficient of Thermal Expansion (CTE) between metal (e.g. Cu =16.5ppm/K) and ceramic (e.g., Al2O3 =6.8 ppm/K). This effect is also determined by the ratio of copper to ceramic thickness and generally speaking by the design of the circuit and the arrangement of single pieces on the master card. Also the manufacturing process affects the flatness of substrates. While copper and ceramic sheets are joined under vacuum during the active metal brazing process, they just lay on each other during the direct copper bonding (DBC) process. This means that, other than gravity effect (which can be controlled with specific manufacturing tools), no pressure is applied on the stack of material. In both cases, copper and ceramic sheets are joined together at very high temperatures and residual stress is inevitably caught in the stack of material after cooling down due to the difference in CTE. When heated up again - for example during die attach - substrates may change their shape as temperature treatment relieves the stress. Besides, after joining copper and ceramic and long before module manufacturers can attach the dies, substrates will be exposed to pressure and temperature variations during the remaining production steps and later again during packaging and shipment. Also the techniques applied to singulate substrates may affect their flatness.
So it should be clear that perfectly flat metallized ceramic substrates do not exist. All of them will have either a convex or concave shape. To some extent, this can be accepted by module manufacturers. Metallized ceramic substrates are not completely rigid and they can be bent without breaking when some pressure is applied during loading and unloading of substrates in the manufacturers’ equipment, with or without vacuum clamping technology, printing solder or sinter material, die pick and place, die attach and wire bonding. But the machine parameters at each step of the process flow are set for only one given shape of the substrates. A significant variation of their shape may cause a local pressure increase and ultimately lead to ceramic breaking. Luckily, for more than 30 years now, Rogers Power Electronics Solutions (PES) experts keep on improving production processes to avoid variations and guarantee a good and consistent product quality.
Power modules are not perfectly flat either
Once populated with semiconductor devices, substrates are usually soldered on a 3mm thick copper base plate. Here again, an assembly cannot be perfectly flat due to the differences in CTE and the thickness of joined materials. As a consequence, when the module is mounted on the heat sink, the shape of the base plate is not likely to match the shape of the heat sink. At a minimum, it is certain that the shape of the base plate cannot match the shape of the heat sink over the entire contact surface across all operating temperatures. Air inclusions in the gaps between base plate and heat sink significantly impact the thermal resistance of the system. Therefore thermal interface materials (TIMs) are usually applied on the back side of the modules to fill such gaps. However, even though TIMs exhibit much better thermal conductivity than air, they still are considered a thermal barrier as they represent the biggest share of the thermal resistance from chip-to-ambient. As a consequence, it is crucial to control and reduce TIM thickness. Conventional modules using a base plate are pressed onto the heat sink with screws at the corners or at the edges. For these standard modules, TIM thickness is typically 100µm. In the case of modules without a base plate, pressure is not only applied at the corners or at the edges of the module but at many points between the chips in order to achieve a much better pressure distribution over the entire substrate surface and hence a lower TIM thickness.
Modules are designed to improve the contact with the heat sink and thereby reduce the thermal resistance from chip-to-ambient. Flatness definitely plays an important role in this regard. Due to the specificity of each design, die location, die size and distance between the dies, some cavities can occur in the profile of the baseplate. These cavities could have a significant impact on module performance if the end users would not follow the mounting instructions from module manufacturers. In order to avoid such issues, power modules now come along with pre-applied thermal interface materials. Moving forward, module manufacturers are developing new packaging technologies to overcome such issues. As an example, the Direct-Pressed-Die technology has been developed by Semikron: a defined pressure is applied on the top of the chips in order to eliminate the cavities underneath. Substrates no longer have to be soldered on the base plate but simply are pressed against the base plate and TIM thickness can be further reduced to typically 10µm.
Another consequence of the differences in CTE between different materials is that the shape of the module will change as temperature variations occur. With any switching operation, a relative motion between the module and the heat sink tends to squeeze the TIM out of the interface gap. This phenomenon is referred to as “pump-out” and results in increased thermal resistance due to loss of TIM from the interface.
In a nutshell: flatness is much more important than it appears at first glance. 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.