Mike Bixenman, CTO, Kyzen Corporation

As the number of bumps under the die (I/O) increase, reliability concerns move design engineers to study the beneficial properties of removing flux residue before underfill. Cleaning flux residue under advanced packages requires process design considerations in the form of mechanical impingement and the cleaning fluid. The problem is that as the I/O increases, assemblers report the difficulty of removing flux residues. This edition is section two of a four-part series: 1.Identify the cleaning issues; 2. Mechanical driving forces; 3. Static driving forces; 4. Cleaning process recommendations.

Figure 1: Bump Pitch and Standoff

Flip chip assemblies are difficult to clean due to size, spacing and standoff height of the components. The capillary action and surface tension of flux residue at peak reflow fills the underside of the component with flux residues. The average spacing under a flip chip is approximately 2-6 mils, dependent on the I/O count and pitch (Figure 1). To clean under flip chip die, the static and dynamic cleaning rates must break the flux barrier, create flow under the part and dissolve all flux residues.

Drivers for removing all flux residues under flip chip include time in the wash section, nozzle type, pressure, cleaning fluid and temperature. The limiting factor is time. Data findings from designed experiments indicate 5-15 minutes is required in the wash section to remove all flux residue from high I/O die. The time required to clean all flux residues under flip chip die may create timing issues within the manufacturing process. To open the process window, process engineers study the problem with the objective of improving manufacturing efficiencies.

Stach and Bixenman published several research papers on the topic of optimizing the cleaning process. The process cleaning rate theory, developed from the research infers that the static cleaning rate plus the dynamic cleaning rate equals the process cleaning rate. The cleaning fluid (static) and energy (dynamic) applied to the uncleaned flip chip must be optimized to open the process window.

Flip chip assemblies processed in standard high impingement spray-in-air cleaning machine designs contact the surface with the deflecting force penetrating under the die. Energy losses upon surface contact narrows the process window and decreases cleaning efficacy under large I/O die. To address this deficiency, mechanical spray-in-air cleaning system engineers select dynamic designs that deliver pressure, flow, and directional forces. In the absence of one of these design criterions, cleaning performance will be compromised. Figure 2 illustrates cleaning variances from two different dynamic cleaning designs when cleaning perpendicular to the die using a spray-in-air cleaning process.

Figure 2: Dynamic cleaning design variances

In summary, to optimize the process, three factors must be considered — upstream processing conditions, dynamic cleaning rate and static cleaning rate. Properly engineered energy sources improve the dynamic cleaning rate. Reducing the time to clean flux residues under flip chip die opens the process window. Part three of the four-part series will focus on the static cleaning rate, which discusses the importance of designing the cleaning fluid to the soil.