Mike Bixneman, 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 four of a four-part series: 1.Identify the cleaning issues; 2. Mechanical driving forces; 3. Static driving forces; and 4. Cleaning process recommendations.

To correctly design a flip chip cleaning process, process engineers need a platform for understanding the influence of process variables. To optimize the process, cleaning fluids and impingement must be correctly matched. Sourcing a cleaning machine followed by the cleaning fluid is a common approach used by engineers. When testing a limited number of substrates to prove out a process, there are many hidden assumptions that fail to show up until the process is implemented.

Cause and effect relationships should be initiated by measuring process variables, designing experiments to improve the process and implementing statistical process controls for controlling the process. Best practice for designing the flip chip cleaning process requires a disciplined approach to understanding root cause and reducing process variations. This is done by mapping the process, defining key process input variables and key process output variables.

Measuring Process Variables

Many hidden variables create noise within the flip chip cleaning process. There are a variety of flux materials used to attach the die to the substrate. Each of these variables influences ease of cleaning. Heat excursions and peak reflow temperatures, and time at or above liquidus hardens the flux residue and increases the cleaning challenge. Cleaning chemistry and mechanical designs influence delivery, processing time and fluid dynamics. Once the cleaning process is designed, control procedures must be put into place to ensure an efficient and effective process window. Many hidden variables can upset the cleaning process.

Application testing labs, such as the one illustrated in the figure, offer engineers a valuable resource to understand both dynamic and static cleaning variables. Correctly used, the engineer can first match the right cleaning fluid for the soil. Once the cleaning fluid has been identified, transparent substrates allow the engineer to study impingement options. Integrating both the dynamic and static variables under one test lab provides raw data for specifying the cleaning machine.

Figure: Application Testing Lab

Analyze the Data

A factorial designed matrix allows for the determination of the flip chip cleaning process conditions needed to remove all flux residues under the die. Analyzing the data for visible flux residue removal provides insight on the process variables needed to open the process window. When designing flip chip cleaning processes, data findings infer a strong correlation to cleaning fluid selection, concentration of the cleaning fluid, time flip chip parts are exposed to the cleaning fluid, temperature of the cleaning fluid and the mechanical delivery design options.

Improve the Process

Process engineers measure yield to determine the value of the cleaning process design. Implementing quality tools such as Six Sigma reduces variations and the level of defective substrates. A number of factors should be considered, such as upstream processing conditions, cleaning fluid options, cleaning equipment options and downstream reliability. Flux selection and reflow parameters influence ease of flux removal since high temperatures cause flux residue to oxidize, which changes the residues physical property, stiffens and hardens the residue, and tends to cause weight loss and associated shrinkage. Post-cleaning effects such as surface properties of the laminate after the cleaning process, the influence of the cleaning fluid to the solder alloy, compatibility with materials of construction, post-cleaning electrical properties and underfill voiding must be considered and addressed before implementation.

Control the Process

Controlling the variables that influence the cleaning process is critically important to ensuring a repeatable process over time. Many variables influence the cleaning process window. Monitoring upstream processing conditions is needed to reduce variability of the parts entering the cleaning process. Within the cleaning process, variables such as wash bath control in the form of maintaining a tight concentration range, soil loading and temperature ensure the integrity of the cleaning fluid. Wash process variables such as air management, rinse water purity, throughput time, part fixtures and delivery of the fluid to the part influence process yields. Upsets in the cleaning process can occur from out of control variables within the process design. Using methodologies such as Six Sigma to control process variables ensures high yields and process control.