Meeting performance and reliability standards
BY GARY CAPLINGER
The push in electronics toward greater connection density is a primary driving force in circuit package design today. It is challenging to reach and clean the contaminated flux in the center of flip chip packages, because the interconnect areas are only accessible from the periphery.
Manufacturers have three pri-mary alternatives for cleaning post- reflow flux residues from flip chip devices: inline spray, ultrasonic bath and centrifugal cleaning. Table 1 summarizes the primary characteristics of each of these cleaning methods.
 Table 1. Flip chip post-reflow cleaning options. |
Inline spray cleaners are extend-ed conveyorized systems that direct cleaning solvents against parts to be cleaned using vertically oriented spray nozzles. Devices being cleaned typically pass through wash, pre-rinse, rinse and drying zones on a continuous con-veyor. Inline systems are good for removing flux residues and oils on exposed surfaces in a continuous production setting, but they are generally less effective in penetrat-ing the close tolerances and hidden gaps of flip chip assemblies.
The nozzle count and spray pressure may be adjusted for more difficult cleaning applica-tions. It may be necessary to incorporate additional cleaning stages to achieve desired results, but this can extend the footprint of the system.
Batch style ultrasonic cleaners place devices to be cleaned in a solvent immersion bath, where ultrasonic energy aids in flux removal by cavitation of the cleaning solution, which varies according to fluid density and the frequency and amplitude of the excitation. This process dis-places surface contamination effectively and is better than inline cleaning for complex parts. However, in the case of flip chip components, ultrasonic energy may not be able to fully penetrate the gap and reach hid-den contaminants. Additionally, mechanical resonance induced during ultrasonic cleaning can cause micro-fracture of delicate parts, thereby degrading long-term reliability.
 Figure 1. Centrifugal cleaning method. |
Products cleaned in a centrifugal cleaner, another batch process, are typically held in place and secured to the rotating robot arm using universal adjustable fixtures, standard fixtures or custom fixtures that are specific to a prod-uct. Fixtures are available for small circuit modules, wafers, sin-gulated packages, Auer boats, magazines, cassettes and JEDEC trays. A robot arm assembly containing fixtured flip chip com-ponents is lowered into a cleaning solvent bath in a sealed process chamber, and rotated alternately in clockwise and count-er-clockwise directions. This causes the solvent to be forced in a direction parallel to the plane of the substrate, driving it between and under components. The solubilized contaminants are sus-pended in solution and driven off the product (Figures 1 and 2).
 Figure 2. Centrifugal cleaning system, showing fixtures mounted on a robot arm. |
The centrifugal cleaning process combines immersion in an alcohol or terpene solvent with agitation and solvent flow resulting from the centrifugal force of rotation. The chamber is sealed and nitrogen inerted, so the solvent temperature can be safely raised to a level near the flash point; this reduces surface tension and lowers viscosity values to increase solvent action. The centrifugal system is compatible with semi-aqueous, aqueous, alcohol and traditional cleaning solvents.
A centrifugal cleaning system can incorporate an automatic, closed-loop and fully integrated waste-water treatment system. When using a solvent that will phase-separate from water, the system automatically extracts it from the rinse water and returns the solvent to the wash reservoir for reuse. Used rinse water is processed through a four-stage purification process that includes microbial control, micro-filtration, carbon adsorption and mixed-bed deionization to restore the water to its original level of purity. No drain or external water treatment is required, and the entire process takes place in the cleaning system housing.
Operating parameters for centrifugal cleaning may require adjustment for optimum performance depending on part complexity, the solvent in use and the nature of contaminants to be removed. These variables include centrifugal rotation speed, the radius of the rotating element, dwell times of the wash, rinse and dry cycles, and solvent temperature. Typical total cycle time is about 7 to 15 minutes.
A closed centrifugal cleaning process provides good odor containment because cleaning, rinsing and drying steps all take place in a sealed process chamber. With centrifugal clean-ing, there is no aerosol phase as with a spray system, no solvent drag-out, and no objectionable odors because the solvent is never exposed to the atmosphere.
The primary factors to be considered in choosing a solvent for flip chip package cleaning are solvency for the flux and con-taminants in question, as well as surface tension, vapor pres-sure, flash point, safety, toxicity and cost of the solvent. Because of the sealed nature of centrifugal cleaning, concerns related to flash point, safety, toxicity and waste disposal are minimized.
Experiments Using Glass Plates
In controlled tests, it has been shown that with centrifugal cleaning, average levels of contamination on cleaned parts can be reduced by one-half to one-third compared to the results achieved by the best-performing spray-type systems. These tests were conducted using a hydrocarbon cleaning compound in the centrifugal cleaning system, with processing 24 hours or less after reflow. These results were compared to spray cleaned parts that were processed within one hour of reflow, minimizing consolidation of contaminants.
An evaluation of centrifugal cleaning illustrates the cleaning performance that can be achieved with centrifugal energy. The experiment tested flux removal efficiency using square glass coupons sandwiched to glass plates, with clearances of 2, 5 and 10 mils. The test contaminant was a rosin mildly activated flux, dyed for visual inspection. This material was injected into the spaces between plate and coupon, and then heated to a temperature that simulated reflow levels typical of flip chip manu-facturing conditions.
 Figure 3. Glass test plates cleaned with inline (left) and centrifugal (right) cleaning systems. |
Spray cleaning was performed using an inline system with an HCFC-based solvent. Results from this process are shown in Figure 3. The inline spray test plate (left side in the photo) illustrates the amount of flux remaining beneath closely spaced components following this traditional cleaning method, which relies on capillary force for solvent penetration. These tests did not simulate flow impediments, such as the fine pitch leads and area array. The photographs show the minimal flux removal around the edges of all coupons, with results proportional to the clearance between glass surfaces. It may appear that there is more flux on the larger standoff coupons, because of the darker color, but this is simply the case of more flux being applied initially because there was more clearance area.
Comparable glass plates were prepared for testing in a cen-trifugal cleaning system, using a semi-aqueous solvent. The samples on the right hand side of the illustration show the amount of flux remaining following this cleaning method. The test sequence confirmed total elimination of visible flux residue upon inspection at 30x magnification.
Flux and Solvent Performance
One important factor in developing a cleaning process is finding the appropriate cleaning chemistry to solubilize the flux residue. Hydrocarbon solvents were the first materials found to be effective for flux cleaning, yet concerns for flammability and workplace safety led to the adoption of non-flammable chlorinated solvents. These materials were identified as potential carcinogens, however, and were replaced with chloroflourocarbons (CFCs), which were welcomed because of their solvency, non-flammability and favorable toxicity properties. CFCs in various forms become predominant in many industrial applications, with consumption levels reaching nearly 1 billion pounds annually in the United States. Scientists eventually discovered the stratospheric ozone depletion properties of these chemistries, and by the mid-1990s worldwide restrictions were placed on ozone-depleting compounds.
The challenge for solvent manufacturers today is to develop non-ozone depleting chemistries that balance sol-vent effectiveness, environ- mental properties, non-flammability and safety and toxicity properties. The primary solvent categories used in conjunc-tion with flip chip flux formulations are terpenes, long-chain alcohol blends and petroleum and hydrocarbon derivatives. The introduction of lead-free solders, which require higher reflow temperatures, has led to new flux and paste formulations that also call for special solvent properties.
Package manufacturers may find it useful to test the performance of various solvent options with specific assemblies, which make the flexibility of the cleaning system an important consideration. Factors to be considered are cleaning performance with selected solder and flux materials, solvent consumption, foaming potential, environmental impact, odor, toxicity, and water treatment and disposal issues.
For some low-cost or short-term applications, it may be appropriate to eliminate cleaning of certain devices. However, the cost of effective cleaning is incidental in terms of overall manufacturing investment, and the removal of flux residues and manufacturing contaminants yields substantial benefits in package quality and reliability.
Summary
Flip chip packages present some of the most difficult cleaning challenges in electronics manufacturing. Fortunately, with an appropriate cleaning solvent and a properly designed and programmed centrifugal cleaning system, it is possible to deliver contamination-free flip chip devices ready for subsequent assembly steps. AP
Acknowledgement
The author wishes to thank Texas Instruments for performing the cleaning tests.
Gary Caplinger, product engineer, can be contacted at Speedline ACCEL, 1825 East Plano Parkway, Suite 250, Plano, TX 75074; 972-424-3525; Fax: 972-424-7526; E-Mail: [email protected].