Defluxing of Copper Pillar Bumped Flip Chips
Authors: Parthasarathy Ravi , Tosun Umut 
Published: February 2024

Flip-chip technology has become increasingly prevalent within the electronics industry due to its lower cost, increased package density, improved performance while maintaining or improving circuit reliability, and increased I/O density. This technology is a method for interconnecting semiconductor devices such as IC chips and MEMS to external circuitry with solder bumps that have been deposited onto the chip pads. However, solder bump technology has proven to be problematic below 125µm pitch as it is challenging to manufacture and assemble. As pitch is reduced, both standoff height and joint reliability decrease, and the risk of shorts are increased.

Given this, traditional solder bumps are being replaced by copper pillar technology. Used as first-level interconnect, copper pillar technology is increasing in popularity. Copper pillar technology has been documented to be efficient to pitches of 80 µm and appears to be promising down to pitches of 40µm. Along with reduced pitches, copper pillar brings several other benefits, including improved electrical performance. This technology is becoming popular since it allows for smaller devices, greater control of standoff height and reduces the number of package substrate layers which reduces cost.

Devices utilizing copper pillar technology have more interconnects per surface area resulting in tighter pitch and lower standoffs heights. As standoff height reduces, flux residues have less area to outgas during reflow. For that reason, there is a critical need to investigate the conditions that would be required to successfully remove flux residues to ensure the functionality and reliability of the final product.

Flux residues can affect reliability, especially with respect to underfill in two different ways. Firstly, if present on the solder bump, substrate or die, thin films of flux residue can significantly reduce interfacial adhesion between the underfill and the surfaces. Once the underfilled device is stressed by thermal shock, humidity or other factors, the underfill delaminates from the surface, and a gap can be detected using acoustic microscopy. Secondly, fluxes can affect reliability by physically impeding the flow of underfill material. Flux residue buildup in the gap between bumps or between the die and the substrate can narrow the gap to a point where the underfill cannot flow or the edges flow faster, encapsulating air and creating a void. To ensure a void-free underfill, homogenous wetting of the underfill must occur on all surfaces. If wetting is not homogeneous, voids in the uncured underfill may translate into reliability problems later.

The study involved using straight DI-water and novel low-concentration alkaline cleaning agent on copper pillar bumped flip chips. The challenge was to effectively clean flux residues underneath these components. The outcome of this study could provide a benchmark for conducting further studies involving bump pitch lower than 15 µm and denser packages including 2.5Ds and 3Ds.

The cleaning assessment methodologies employed analytical/functional testing including FTIR, Ion Chromatography, SEM/EDX, Thermal Cycling (TC) Test, Underfill Test, High Temp Storage Life (HTSL) Test and Moisture Sensitivity Level 3 (MSL-3) Testing.