Far more than constructive or material differences, it is the principle – or more specifically: the pattern – of laser trimming that makes the difference for a SMD chip resistor’s reliability and performance. Let us investigate, why:
Resistors - seemingly the most accessible of components in almost any application. However, the process of manufacturing surface mount chip resistors is rather complex. In a typical construction of a surface mount chip resistor, there is a block of alumina substrate, which could be manufactured from several kinds of ceramics. Then, there are the electrodes which conventionally have several layers including nickel and tin. The resistive element is printed on the ceramic substrate - and then trimmed using a laser. It is the laser trimming that ultimately defines the resistance value of the final chip resistor.
The laser trimming is a deciding factor in the quality and performance of the end product. There are several factors that play a decisive role here: The laser trimming must be stable in the long run and should not deteriorate. Taking almost any application into account, resistors go through thousands of cycles in their lifetime and the resistance value is expected to remain constant. Additionally, the laser trimming should be of utmost accuracy, so that the desired resistance value can be achieved within the range of the defined tolerance. Furthermore, the trimming speed itself is important while not “sacrificing” the performance of the resistor. In component manufacturing, there is usually a trade-off between speed and accuracy. Panasonic Industry factories pursue the highest efficiency and speed in manufacturing; however, it is always made sure that the quality of the products does not drop because of this. Last but not least, trimming geometry is the most crucial aspect of all. This geometry defines other characteristics of the resistor like power range and pulse withstanding capabilities.
The trimming pattern can have several shapes and geometries – in the following overview we’ll have a deeper look at two (important ones!) of them:
The L-pattern is a fast and well-known trimming pattern. This pattern includes two parts: the first part marked with 1 in image 1 defines the resistance value slightly below the targeted resistance value, while the second part marked with 2 accurately corrects and adjusts the resistance value within the allowed tolerance range. Analyses in the Panasonic Industry technical laboratories illustrate, however, for certain features like pulse-withstanding characteristics, this trimming method is not ideal, because, with the occurrence of pulse, certain areas around the trimming are exposed to higher temperatures and hot spots, see image 3. This is why this laser pattern is mostly used in the more conventional resistors, in which pulse withstanding characteristics are not that essential. The L-pattern is otherwise very fast and easier to apply.
Figure 1. The L pattern
Panasonic Industry’s solution for producing premium resistors with pulse withstanding capabilities is the symmetrical C pattern. As opposed to the L- pattern, where the pulse load and heat would concentrate on the corner and edge of the pattern and, in this pattern, the trimming shape is changed to an arc, similar to the letter C. The current pathway here is bi-directional, so two symmetrical trimming patterns are used, see image 2. Based on the Panasonic load and heat simulation study, the heat concentration is reduced by 64% using our unique technology. This enables the resistor to have a higher power range and better pulse characteristics.
Figure 2. The symmetrical C pattern
However, the C geometry is far more complex to realize with the laser, because it is much slower and has to be monitored meticulously to maintain the desired resistance value. But for applications requiring pulse-resisting chip resistors, the Symmetrical C-pattern would be a perfect solution.
Figure 3. Comparison of hot spot buildup using L pattern and symmetrical C pattern
Panasonic Industry is constantly seeking state-of-the-art solutions to further improve its resistor portfolio. The highly-trained engineering team at the Morita factory in Fukui, Japan, is continuously researching and testing new materials and methods to enhance the quality and reliability of the resistors.