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

Static Cleaning Forces

Cleaning fluids vary in their design based on solvency, saponification, wetting (surfactancy), inhibition, and defoaming characteristics. The best cleaning fluids optimize and build performance characteristics that effectively accomplish several tasks in combination. Critical cleaning fluid design features improve the static cleaning rate from superior wetting, low surface tension, low foam, and protection of the alloys.

The static cleaning rate requires materials that rapidly soften and solubilize the flux soils soon after contact. The challenge is to create an engineered material that runs at low concentrations. Low concentrations improve wetting and surface tension effects by reducing the water droplet size. To clean at low concentrations, the paste flux selection must dissolve rapidly. Flux characteristics that meet this process design dissolve readily in hot water and fit the description of a water soluble flux. Flux residues that support a high static cleaning rate are much easier to remove using wash temperatures in the range of 60°-80°C, targeted spray impingement, and low levels of cleaning fluids.

Surface tension measures the tendency of a liquid to “wet out” on a surface. Water has poor wetting properties with a typical surface tension of 72 dynes/cm. Cleaning fluids reduce the surface tension of water to an optimal range of 24-30 dynes/cm. A measurable reduction in surface tension at low concentrations in DI-water improves the ability to penetrate and wet the soil under flip chip die. Surface tension reduction opens the process window for larger flip chip assemblies.

Foam is a critical parameter when using high-impingement sources. Foam increases surface tension and allows millions of air bubbles to become entrained within the cleaning fluid. Foam causes pumps to cavitate, which reduces cleaning effectiveness under tight standoffs. Foam also can be concentration dependent. When cleaning under flip chip die, users must select a cleaning fluid that does not foam when processed at low concentrations and high-impingement energy. Cleaning fluids that generate very low to no foam at low or high concentrations are the best choice when cleaning under low standoffs.

Foam studies illustrate the effect concentration, temperature, and shear has on the foaming properties of the aqueous cleaner chemistry. Two hundred milliliters of a dilute solution in DI-water is heated to the prescribed temperature — high sheer is applied for 30 seconds and immediately transferred to a 1000 ml graduated cylinder. Figure 1 illustrates low foaming character at temperatures above 120°C (50°C) and higher. At temperature below 120ºF, many aqueous cleaning fluids generate foam, which compromise cleaning results.

Figure 1.: Foam Study

Saponification is a common method of cleaning flux residue. Alkaline materials react with the resin structure to form a water-soluble soap. Most aqueous cleaning fluids use some form of saponification. Saponification improves the static cleaning rate but also may interact with the bump metallization. Dull solder joints occur when the cleaning fluid removes the skin layer from the surface of the solder. Inhibitors designed within the cleaning fluid protect the alloy and other soft metals.

Properly designed cleaning fluids effectively remove all the flux from under the flip chip assemblies. Figure 2 illustrates before and after images of a flip chip die cleaned in an aqueous spray-in-air cleaning process using a 5 percent solution of cleaning fluid in the wash tank. The cleaning fluid improved the static cleaning rate and enhanced the removal of all flux residues under the die.

Figure 2: Flip Chip Before and After Cleaning

Author: Mike Bixenman is the CTO for Kyzen Corporation. Kyzen Corporation is a leading provider of engineered cleaning fluids for high technology manufacturing environments. For more information on aqueous cleaning fluids for cleaning Flip Chip flux residues, please email Dr. Bixenman at mikeb@kyzen.com.