Key Takeaways & Executive Findings
- •• Copper foam (CF) matrix embedded in indium (In) effectively prevents leakage and pump-out of liquid In during multiple reflow cycles in FCBGA packaging, leveraging capillary forces. • The CF matrix promotes the formation of rod-like Cu11In9 at the CF-In interface and inhibits the growth of Ni3In7 IMC at the In-substrate interface, enhancing metallurgical bonding. • The embedded CF matrix improves the shear strength of solder joints by 22.9% and enhances thermal conductivity, while altering fracture behavior from ductile to ductile-brittle mixed type. • This novel TIM1 design eliminates the need for sealants or dams, reducing risks of In explosion and thermal degradation, and avoids additional keep-out zones, offering a more reliable solution for advanced FCBGA packaging.
Abstract
Indium (In) has been used as a thermal interface material (TIM1) in high-performance central processing unit (CPU) for better heat dissipation. However, leakage or pump-out of liquid indium during the multiple reflow cycles limits its application in advanced flip chip ball gray array (FCBGA) packaging. Former researchers place a seal or dam structure to prevent In leakage, leading to the risk of In explosion, thermal degradation, or require additional keep-out zones. In this work, a copper foam (CF) matrix was embedded in In to absorb the liquid In and eliminate the leakage of In TIM1 during the multiple reflow cycles, as the CF capillary force. Au/Ni/Cu-Au/Ni/Cu joint was fabricated by soldering with the composite solder at 190 ℃ for 2 min. After reflow cycles, good metallurgical bonding was formed at interfaces of joint. Rod-like Cu11In9 formed at the CF and In interface, due to the re-dissolved of Cu11In9 crystal. Small amount of Cu atoms from CF can reduce the activity of In, which inhibits the growth of Ni3In7 intermetallic compound (IMC) at the interface of In and Au/Ni/Cu substrate. The CF matrix also improved the shear strength (22.9%) and thermal conductivity of the solder joints. Besides, the fracture behavior of solder joints without CF matrix was classified to be ductile type while that with CF matrix was changed to be ductile-brittle mixed type.
1. Introduction
With the advancement of semiconductor technology, the characteristic dimension of chips has been miniaturized to several nanometers, tens of billions of transistors were integrated into a chip, thus the power density of chips has risen dramatically. Removal of heat from the active semiconductor die and lowering the operation temperature of chip have long been a critical issue to determine logic devices life and speed, especially for high-performance central processing unit (CPU) [1 −3]. The thermal interface material between the die and the lid is defined as TIM1, serving to minimize contact thermal resistance and accommodate the strain from the coefficients of thermal expansion (CTE) mismatch. To meet the heat dissipation requirement of high power CPU, Intel [4] used indium (In) TIM1 to remove heat from chip to lid in land grid array (LGA) packaging and AMD [1] used In TIM1 in pin grid array (PGA) packaging, as the low modulus (12 GPa) and high thermal conductivity (86 W/mK) of In [5]. Figure 1 shows the manufacture processes of flip chip ball gray array (FCBGA) packaging with In TIM1.
Compared with PGA and LGA packaging, ball grid array (BGA) packaging has many advantages, such as smaller size, better heat dissipation and electrical performance [6, 7]. However, the melting point of In (157 ℃) could be a weak point for FCBGA with In TIM1, which needs additional reflow cycles for solder ball (Figure 1(f)) after In TIM1 reflow cycles (Figure 1(d)). In TIM1 remelting could occur during the additional reflow cycles, cause leakage or pump-out of In TIM1 [8]. As the migration of liquid In TIM1 from the up surface to the side of the dies, voids would occur in In TIM1, which could decrease the thermal performance of packaging. And leakage of In TIM1 can also potentially contact adjacent die-side components, creating a short-circuit of components. MCCLURE et al [9] found that voids mainly formed near the center region of the In TIM1 in FCBGA packaging after reflow due to large volume of In extended beyond the die. According to the research reported by LEE et al [10]. In out from the bondline may create extrusion failures. AMD corp. [1] and Intel corp. [4] also revealed the leakage of In.
To prevent In leakage, some solutions proposed by Intel, TSMC and other corporations were used to contain thermal interface materials, such as placing a sealant (Figure 2(a)), a dam (Figure 2(b)) on the integrated circuit device, or incorporating fins (Figure 2(c)) into the integrated heat spreader [11, 12]. However, the sealant and fins on the integrated circuit device form a sealed structure, which could lead to the In explosion phenomenon during reflow. Fins integrated into the heat spreader may also increase costs and create new keep-out zones on the 3821
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WEN Jing, SUN Guo-liao, WANG He-xin, FAN Yi, YI Ming-ming, CHEN Zhuo, WANG Lian-cheng, ZHU Wen-hui (2025). Fabrication and performance of embedded matrix indium thermal interface materials for advanced FCBGA packaging. Journal of Central South University. https://doi.org/10.1007/s11771-025-6052-x
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Frequently Asked Questions
What is the main problem with indium TIM1 in FCBGA packaging?
Indium TIM1 can leak or pump out during multiple reflow cycles due to its low melting point (157°C), leading to voids, reduced thermal performance, and potential short-circuits.
How does the copper foam matrix prevent indium leakage?
The copper foam matrix is embedded in indium and uses capillary forces to absorb and hold the liquid indium, preventing it from leaking or pumping out during reflow cycles.
What are the benefits of using a copper foam matrix in indium TIM1?
The copper foam matrix improves shear strength by 22.9%, enhances thermal conductivity, and changes fracture behavior from ductile to ductile-brittle mixed, while eliminating the need for sealants or dams.
What is the role of Cu11In9 in the solder joint?
Rod-like Cu11In9 forms at the copper foam and indium interface due to re-dissolution, and the small amount of Cu atoms from the foam reduces indium activity, inhibiting the growth of Ni3In7 intermetallic compound at the substrate interface.
How does this new TIM1 design compare to traditional sealant or dam methods?
Unlike sealants or dams, the copper foam matrix avoids the risk of indium explosion, thermal degradation, and additional keep-out zones, offering a more reliable and cost-effective solution for advanced FCBGA packaging.
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