Avoiding the Flux-free Zone
BY HORATIO QUINONES AND WESLEY W. WALTERS, Asymtek
Surface preparation prior to metallurgical bonding ensures a strong intermetallic formation of soft solder and various surfaces. Traditionally, a solder paste mixed with some cleaning agent was used. This process is well suited for ball grid arrays (BGAs) and large chip-scale packages (CSPs), ceramic ball grid arrays (CBGAs), and ceramic columns grid arrays (CCGAs), where the pad pitch is large enough to accommodate solder printing or solder deposition on the corresponding bump pads. Denser packages that use flip chip technology - where bump dimensions and pitch distances do not permit the implementation of discrete solder deposition on the corresponding pads - require new technologies. An alternate process is to dispense a flux cleaning agent onto the substrate and in areas around pad metallization.
Fluxing reduces or eliminates oxidation, and confines the products of oxidation that promote solder ball wetting to package interconnect sites during the reflow process. Only a small amount of flux is needed to create a well-cleaned surface for solder joining preparation, as tin oxidation layers are extremely thin. Flux also prevents component displacement as packages go through moving conveyors and other environments prior to complete solidification of the solder bumps. Flux requirements include full-pad area coverage, consistent and even layer thickness, and no contamination of the package or undesired surfaces, which requires a process with good edge definition capability.
Flux Application Methods
Methods for depositing flux include pin transfer, stencil printing, ultrasonic spraying, dipping, and flux jetting, with and without conical air assist. The pin transfer process involves a simplistic method using special tooling that is dipped into a flux reservoir and transferred onto the package substrate or PCB. However, because the process lacks consistency, it is only used with low input/output (I/O) count applications, where cleaning the flux residue is less challenging. Additionally, touching down to each component site to transfer flux to each solder pad makes this a slow process, and separate tooling for each package type makes cleaning difficult.
Stencil printing flux onto contact pads is another method for assembly processes where the substrate surface is free of components, such as resistors and capacitors, and where the pad pitch is spaced far enough apart to be compatible with current stencil technology. One disadvantage of this method is that many flip chip on board (FCOB) and flip chip in package (FCIP) components are applied after other components have already been attached to the substrate surface. Due to the miniaturization of many electronic assemblies, the ball pitch or spacing for current FCOB and FCIP components either challenges the current stencil printing capability or the balls are so close that stencil printing is not an option.
Ultrasonic spraying with a controlled atomized spray, which develops when the flux is excited to its resonant frequency using a piezo-ceramic actuator, is another method of flux application.1 In this process, air pressure plays an important role in attaining consistent volume deposition. However, ultrasonic spraying is limited to low-viscosity fluids of 5-50 cps.2
Flux dipping is an easy method used to apply a rather tacky flux or low-viscosity fluid onto solder bumps. The depth of bump array immersion into the flux fluid container needs to be determined to accommodate bump geometry tolerances, die tilt, etc. However, if not properly set up and monitored, the flux dipping process can result in excess flux on many of the bumps or even on portions of the die.
Figure 1. Flux dipping process.
Figure 1 shows the flux dipping process. A doctor blade process uses a continuous blade motion to keep the fluid flux homogeneous and even, and prepares the flux layer that will contact the bump array. Most of the time, the dipping process is carried out in the placement machine. Estimates show a throughput decrease of up to 25% by using component placement equipment to apply flux.
As a result of reduced throughput, fluid viscosity limitations, and limited accessibility to die-placement locations, an alternate approach has been developed. A new-generation flux jetting system* combines the benefits of non-contact fluid application with software-driven high precision X-Y placement systems. This high-capacity production process include full area coverage, thinner flux layers, and an increase in overall process throughput.
From a position above the package substrate or PCB, the system jets a micro-droplet of fluid followed by a pulse of air to form a thin film of flux on the substrate surface. This flux layer conforms to the substrate topology and can provide a homogeneous thin layer of full coverage flux that coats the package substrate or PCB on the bump array. This ensures that flux is available to all pad sites. When the package is reflowed, surface tension will pull the die down and the pads with smaller or misshaped bumps will make contact and form a reliable solder joint.2 Figure 2 shows a cone-in-seat jet arrangement, where flux is jetted from the nozzle using a combination of fluid and air pressure. The cone provides excellent control of the flux volume and the coaxial air assist is used to help overcome the surface tension of the substrate and lower the overall flux film thickness.2 This jet system can work with tacky flux or high-solvent flux materials.
Figure 2. Cross-section coaxial flux jetting.
Very low viscosity fluids can be jetted with the combination of fluid pressure and valve-on-time, using the piston mechanism to complete a positive shutoff in the valve seat. As the viscosity increases, the fluid pressure alone is not sufficient to cause a thicker flux to break away from the nozzle. Thus, a different approach leverages the momentum transfer of the piston to generate an energy transfer caused by the piston’s force impacting the set, which will cause the droplet of flux to be released onto the substrate below. Both high- and low-viscosity fluid droplets are dispersed into a controlled film build-up with the use of the coaxial air burst.
Figure 3. Coaxial air flux jetting.
Figure 3 shows the dot being jetted from a momentum-transfer mechanism followed by coaxial air in the shape of a hollow cylinder to flatten the flux drop for even thicknesses independent of the surface wettability characteristics.
Flux jetting systems operate by selectively firing a high-speed series of micro-droplets onto the substrate, enabling the consistent delivery of a wide variety of flux patterns while maintaining ultra-precise edge definition. The jetting head moves in an X-Y plane to dispense pre-programmed patterns without the need for Z-axis motion or complex height-sensing requirements. Rapid cycle times of only a few milliseconds per micro-droplet, combined with fast and precise X-Y motion systems and the complete elimination of Z-axis movements, produce high processing speeds which keep pace with the rest of the production line. In addition, the complete elimination of physical contact with the substrate further speeds the process while simultaneously avoiding contamination risks. By augmenting the jet fluxing process with the use of pulsed-air assist techniques, even greater precision control can be achieved. In essence, the pulsed-air assist process emits a quick pulse of air after each micro-droplet or line is jetted, thereby helping to break the natural surface tension and smoothly spread the material onto the substrate. Unlike other spraying processes in which the droplets are randomly atomized in the air, pulsed air-assist jetting is precisely controlled and deposits uniform lines or dots of flux in exact locations and then uniformly controls their flow out.
There are essentially two modes of fluid jetting. The first is fluid spelling by applying relatively high pressure to a fluid and controlling opening of this state to a lower pressure state. This method is especially effective for low kinematic viscosity fluids. The second mode consists of fluid jetted from a momentum-transfer mechanism. A piston is accelerated and then comes to a stop or final zero velocity state, thereby imparting momentum onto the fluid and eventually causing a fluid separation as a way of dissipating some of the energy in addition to the kinetic energy component that the droplet carries. This process is suitable for high- and medium-viscosity fluids (1E3 to 1E5 cps).
Several factors are present during flux jetting that can determine its quality and consistency. In a manufacturing environment the robustness of the process and the control of the dispensed volume are of extreme importance.
Fluid Pressure: Viscosity of the flux is one factor in determining the pressure required to jet a fluid. High kinematic viscosity fluids, for example, require higher fluid pressures. For very low viscosity fluxes, the fluid pressure strongly determines the flux volume dispensed. This correlation is less for higher viscosity fluids. There, the role is simply that of feeding the fluid into the jet chamber.
Valve-on-time: These same trends are also observed for this parameter. The longer the valve is open, the more fluid will exit. For high-viscosity fluxes, long valve-on-times may cause nozzle tip contamination.
Dispensing gap: This is the distance of the nozzle tip to the substrate surface. This gap determines the width of the flux reaching the surface, higher gaps leading to wider area coverage.
Coaxial air assist: This parameter’s main purpose is to atomize the fluid exiting the nozzle. The fluid-flow rate will determine the degree of atomization and the spread of the flux.
The x-y head velocity: This is essentially the motion of the head relative to the substrate surface and is responsible for determining the flux layer thickness. Faster speed will generate thinner flux layers, while slower speeds produce thick flux layers.
Flux jetting is indeed a viable alternative to other fluxing processes such as pin transfer, stencil printing, ultrasonic spraying, and die dipping. It is a true non-contact process, fast, and reliable. As flip chip technology usage increases and denser package-on-package (PoP) and package-in-package (PiP) configurations become commonplace, jetting fluxes may become the standard technique for all flux applications.
The authors would like to thank Robert Ciardella for his influence in the direction of this research, as well as George Vastola, Tom Chang, Brad Perkins, and Richard Zakrajsek for their experimental work and data generation.
- Stefan Behler and Dominik Hartman “Comparison of Flux Application Methods for Flip chip Die Bonding” proceedings of Semicon Singapore, 2001
- 2. Steven J. Adamson and Stephen Heveron-Smith “Jetting Dispensing of Fluxes for Flip Chip Attachment and Measurement Methods for Ensuring Consistent Flux Coatings”, proceedings of IMAPS Flip Chip Workshop in Austin Texas, 2004.
HORATIO QUINONES, chief scientist, and WESLEY W. WALTERS, business development manager, Win3 Relationships, may be contacted at Asymtek Headquarters, 2762 Loker Avenue West, Carlsbad CA; 760/930-3374, 760/930-7468; E-mail: email@example.com, firstname.lastname@example.org.