Authored By:
Mina Yaghmazadeh, Ph.D., Youngjin Kim, Ph.D. Ningbo SJ Electronics Co., Ltd.
Zhejiang, China
Summary
Advanced high-power electronic chips can generate substantial heat during operation. Thermal interface materials (TIM) dissipate the generated heat from the die to the lid and facilitate the cooling of the device. Gaps and voids along the joint (die/TIM/lid) cause local discontinuity in the thermal path, decrease thermal conductivity, and reduce TIM effectiveness in thermal management. Indium has been widely used as an excellent TIM material in LGA and PGA packaging. However, as the industry moves towards BGA packaging, utilizing indium as a TIM layer has been challenging. So far, attempts to use indium in BGA packaging have failed due to excessive void formation during inevitable multiple reflow process steps in BGA packaging.
In this research, a new generation of indium TIM is introduced. This class of indium TIM has a protective layer to suppress surface oxidation. Therefore, a fluxless reflow process without the assistance of a reduction atmosphere is possible. Study shows the main advantage of this product is significant void reduction, able to reach >95% of the joint coverage, even after multiple high temperature reflow processes. Intermetallic compounds (IMC) formation at the interface of TIM-substrate is also examined and discussed in this work.
Conclusions
This work examines the performance of indium as TIM for BGA packaging. When a liquid flux is applied to activate the indium surface, excessive voids form during subsequential high temperature reflows, which are necessary for BGA packaging. A new generation of indium TIM with a protective layer is introduced in this research. This category of indium TIM does not require liquid flux to activate and wet the substrate. Moreover, even after subsequential high temperature reflows, it shows a significant reduction in voids. IMCs will form at the interface of In-Cu/NiAu during multiple reflow steps. While the IMC bond is developing, Au and Ni are consumed. Depending on reflow process parameters, a sufficient Ni layer thickness is required to maintain a uniform defect-free bond at the joint.
Initially Published in the SMTA Proceedings
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