Contact Pressure and Load Measurement Techniques for Applications in Semiconductor Packaging

Semiconductor packages undergo mechanical loading at various stages of the manufacturing, assembly and test process. Also, during the assembly of the thermal enabling solution in a system environment, mechanical loads are applied to ensure contact of the thermal solution. These loads can be static and/or dynamic in nature, applied over a short or long periods of time. It is important to characterize these applied loads and the resulting contact pressure. However, the measurement of the contact pressure at the interface is complex and requires special techniques without altering the mechanical boundary conditions or the state of stress. In this paper, some of the existing methods used to measure the in-situ load and pressure distribution will be discussed with special attention provided to methods including: pressure-sensitive paper, piezo-resistive based pressure films, and load cells. We will also discuss the applications of these approaches in semiconductor packaging. In the pressure paper technique, a thin film containing microbeads is placed between the interfaces of interest. These microbeads contain colored ink and rupture when a threshold pressure is applied. The intensity and location of the ink provides the magnitude of pressure and its distribution. While these are inexpensive and easy to use, they are single-use and fail to provide real time data. Another method is to use a thin piezo-resistive sensor. While this technique can provide real-time pressure measurements, it is expensive and comes with additional challenges such as hysteresis, aging, and history-dependent response due to the inherent nature of materials used in the sensor. As a result, obtaining accurate and reproducible measurements is quite challenging. In this paper, we describe an experimental method to quantify the challenges and present an approach to mitigate them to achieve highly accurate and repeatable measurements. Lastly, embedded button load cells or dynamic load cells are also used to study mechanical loads a package is experiencing, but they are bulky and hence hard to use especially when space is limited.