Understanding Head-In-Pillow Defects – The Role Of Thermal Stability In Paste
Authors: Ranjit Pandher, Rahul Raut, and Mike Liberatore Company: Cookson Electronics Date Published: 10/16/2011
Abstract: The Head-in-pillow (HIP) defect is a growing concern in the electronics industry. This defect is usually believed to be the result of several factors, individually or in combination. Surface quality of the BGA spheres, activity of the paste flux, improper placement/misalignment of the components, a non-optimal reflow profile, and warpage of the components are some of the major contributing factors. To understand the role of each of these factors in producing the head-in-pillow defect and to find ways to mitigate the same, we designed an apparatus that simulates the reflow process and has an in-situ monitoring of the solder joint formation process. This apparatus allows each key contributing factor to vary individually, therefore experiments can be designed to understand the role of each factor separately rather than monitoring the cumulative effect of all of them. This apparatus has been used to conduct a detailed comparative study of a number of lead-free solder pastes. In this study we especially focused on the thermal stability of the paste flux and its role in producing/mitigating the HIP defect. Experiments used a set of reflow profiles designed to challenge the thermal stability of the paste in different ways such as increasing the peak temperature, increasing the time above liquidus or increasing the soak time and soak temperature. A quantitative comparison of performance of a number of pastes with known differences in thermal stability of their activator packages will be presented.
Head-in-Pillow defects, HIP test methods and HIP factors.