DFE T ECHNOLOGY As components become smaller, component mixes become more diverse, and board densities continue to increase, the processes of packaging, interconnection, and assembly are challenging the capabilities of existing production equipment. Contributing to the difficulties for design and development engineers are such considerations as operating costs, throughput, and the environment. For reflow soldering ovens, the particular challenges are to achieve the correct thermal profile, optimum heat transfer and cooling, and effective removal of airborne flux all of which impact yields. Dynamic Flow Engineering (DFE) is both a concept and a process engineering approach for analyzing and managing the flow of gases through the oven. The result is precise control over the thermal process, removal of flux in both the heating and cooling sections, and outstanding quality and reliability during reflow. At Conceptronic, the design and management process for reflow soldering systems is called Dynamic Flow Engineering, or DFE. In essence, DFE is a technology that integrates all aspects of heating, cooling, and flux management. It ensures solder joint quality and cost-effective performance even for the most demanding reflow requirements of high-density, high component mix applications. The basis for this discussion is Conceptronic s third generation of forced convection reflow ovens, the new PROFILE Series. S E R I E S
Dynamic Flow Engineering Features of PROFILE Reflow Soldering Ovens Forced impingement convection with serpentine gas flow Filterless flux management Exclusive multi-port blower design Ambient air or gas-to-gas heat exchange cooling modules Optional automated underside board support with low mass cable for single- and dual-lane versions Windows NT operating system Closed-loop process control software Virtually no maintenance required Optional Aurora recirculation system to extend maintenance intervals in the cooling section Optional edge rail and dual-lane conveyors 2 In designing PROFILE ovens and implementing Dynamic Flow Engineering, Conceptronic initially established the desired performance specifications. Both air and nitrogen models were taken into consideration. Decisions were then made as to how the process gas should flow through the individual heating and cooling zones in order to meet these specifications, not the least of which was the minimization of flux residue and significantly reduced requirements for cleaning. DFE begins with the heating plenum (blower, heating core, and gas dispersion plate), which produces optimized, overlapping cones of turbulent gas flow designed-in to achieve maximum heat transfer and higher velocities without component movement on the boards. As will be discussed in detail, the process gas in PROFILE ovens is directed to flow in a serpentine fashion through the heating chambers, impinging on boards in one chamber and then being directed through the blower and gas delivery system of the next chamber. This design configuration offers a number of benefits, including: even distribution of heat, controlled molecular flow through the oven, and movement of flux particulate from the heating section to the cooling section. An advanced flux management system, called Aurora, is also available as an option for the heating section. Aurora removes Dual-Lane Configuration delivers twice most of the contaminants before the throughput. the process gas reaches the cooling section of the oven. After exiting the heating section, the process gas flows through cooling modules which have been proven through extensive field operation and testing. The system incorporated in air models is known as Polaris, while the patented Genesis is used in ovens using nitrogen as the process gas. Both types of cooling modules are particularly effective in removing flux particulate. In fact, with PROFILE ovens, as much as 95% of the flux contaminants are removed outside of the heat tunnel.
Forced Convection Heating Heating Forced convection reflow is a mature technology and has been incorporated into most ovens offered on the market. Forced convection is characterized by the fact that the temperatures of the heated circuit board and the components on the board are close to equilibrium with the temperature of the heat source (the hot air being blown onto the board).* The multi-port blower design provides uniform gas flow and temperature across the entire length and width of the heater panel. Profile Uniformity Jet Temperature Jet Velocity 6000 5000 4000 3000 2000 1000 0 360 ( C) Forced convection reflow ovens offer superior control over board and component temperatures, as well as smaller temperature deltas across the board. In practice, however, edge-to-edge and edge-to-middle temperature variations in forced convection ovens can be a problem. At Conceptronic, consistent board temperature has been addressed successfully. Design of the heating system in PROFILE ovens is based on more than 10 years of engineering experience in forced convection heating and on mathematical equations developed during extensive analysis of air flow and heat transfer using fluid dynamics modeling. As can be seen above, each heating zone has a patent-pending, multi-port blower which pushes gas through a heater core and a flow-balanced gas dispersion plate. This produces cones of hot gas that impinge on the board and (ft./min.) The patent-pending multi-port blower (right), as opposed to the single-port blower (left), provides more uniform pressure distribution in the heater plenum. the components mounted on the pads. The orifices in the plate are of the exact diameter and pattern to produce the overlap and flow velocity required to maximize heat transfer and evenly heat the assembly without disturbing the components loosely placed on the pads of the board. This optimized flow design out-performs other convection ovens with heating systems that depend on excessive blower speed that may move components off the pads or higher wattage to achieve the desired heat transfer. Conceptronic ovens optimize the process at lower re-circulation volumes and power. 310 260 210 160 1 2 3 4 5 6 7 8 9 10 Profile Multi-Port Technology Control Thermcouple 1 2 3 4 5 6 7 8 9 10 Single-Port Technology The multi-port blower provides more consistent jet velocity and temperature uniformity than the single-port blower. *In comparison, infrared (IR) soldering systems, the next most popular type of heating in reflow ovens, are systems in which temperature increases of components and substrate are non-linearly related with the temperature of the source, and hence to the absorption of IR energy delivered by the source. In achieving a certain component temperature with IR, the temperature of the source is always higher than what it would be for a convection oven. Differences in individual component temperatures (greater in IR than convection), moreover, are largely attributable to relative differences in mass.
Serpentine Gas Flow The unique flow of process gases through the heating zones differentiates Conceptronic reflow ovens from competitors products. As shown below, heated gas blown through the gas dispersion plate and onto boards being conveyed through one zone is then recirculated through the blower input of the following zone. As with the previous zone, the gas passes through the plenum (blower, heater core, and gas dispersion plate) and onto the boards in that zone. The gas is then drawn up and through the blower system of the next zone. Conceptronic has determined that approximately 80% of the gas is recirculated in this manner, with the remaining 20% following the path of the boards from zone to zone. Circulating the gas in this manner offers significant performance and maintenance advantages. Some ovens are designed to circulate and re-circulate the process gas within the individual zones. As each board passes through the uncleaned, recirculating gas within a particular zone, additional flux volatiles are added to the gas. This causes flux to saturate in the zone and collect on the surfaces inside the heat tunnel. With Conceptronic ovens, gas flow is directed from chamber to chamber in the serpentine fashion to the cooling unit or to Aurora collection points. Flux buildup is negligible, even after years of operation. Field studies have proven that openings in the heater core are unaffected by flux; the hole diameters remain constant. Because of the internal design of PROFILE and other Conceptronic ovens, consistent thermal profiles are guaranteed for 5 years without any required cleaning of the heating zones. In addition, blowers and heaters are covered with a lifetime warranty. Aurora Aurora is a patent-pending system that forms a loop outside the heating section of the oven, and is designed to collect flux contaminants before the process gas reaches the cooling section of the oven. The engineering theory behind Aurora is based on principles of particle deposition. Aurora removes fluxladen gas at the end of the heat chamber and directs the gas through an external duct. Here, molecules of gas are bombarded against a series of baffles. As a result, flux particles collect on the baffles and the cleaned gas is then directed back into an upstream heating zone. The design is the culmination of field-proven technology developed by Conceptronic in separating flux particulate from process gas. In PROFILE ovens with Aurora, between 50 and 80% of the flux contaminants are removed by Aurora, with 15 to 45% extracted in the cooling section, either in the heat exchanger of Genesis (nitrogen models) or the flux trays in Polaris (air models). In total, as much as 95% of flux contaminants are removed. In ovens without Aurora, up to 95% are removed by the cooling units themselves. Therefore, the addition of Aurora increases volumetric flux capacity and extends flux clean-out intervals. In addition to its ability to remove flux from process gas prior to cooling, Aurora also conserves energy (and nitrogen, where appropriate) by recirculating process gas which would otherwise pass into the cooling section. The serpentine flow path of the gas, along with the optional Aurora flux removal system, guarantees that the heaters will never clog with flux. 4
Flux particles collide with and congeal upon the baffles of the Aurora flux removal system. As shown, the Aurora system is located above the heat tunnel. The flux collectors are easily removed and can be cleaned while the oven is at operating temperature. Operation of Aurora is seen in the figure on page 4, which shows the ducting for the system installed at an upstream heating zone and at the end of the last heating zone. Most of the process gas flowing from chamber to chamber is extracted at the end of the reflow zone. It is then directed through Aurora and back into the upstream zone, passing through a dual condensation system, with each leg housing the series of baffles previously mentioned. The baffles are key to the success of Aurora. Angled into the flow of gas, they cause localized turbulent regions, forcing the gas stream to change direction and velocity. Molecules of hot gas from the reflow chamber strike the surface of the baffles at an optimized velocity. The molecules immediately lose their kinetic energy (as much as 40 to 60%) because of the change in direction and speed. The baffles are also designed to extend over the center line of the ducting to ensure turbulence and impingement of the molecules. The length and location of the duct outside the heat tunnel cause the gas to cool, thereby enhancing flux collection. As the airborne particles lose temperature, the contaminants in the gas congeal on the face of the baffles. Cleaning of the Aurora is easily accomplished without shutting down the oven. Operators simply unhook the cover latches and lift out the baffles mounted to the covers. Flux Management Most of the particulate in process gas is generated as flux ingredients begin to activate and reduce the oxides on the solder surfaces. At this point, the contaminants that will later collect on the cooler surfaces of the oven are airborne and contained in the vapor. With DFE, the cooler surfaces (within Aurora and cooling units) can be strategically located to prevent flow restrictions and are easily cleaned. The purpose of Aurora is to remove as much of this particulate from the oven as is possible before the process gas exits the heating section. By doing this, required cleaning intervals for the cooling section are significantly extended.
Nitrogen Reflow Ovens In designing the Profile oven, the objective was to neutralize the effects of outside air, so the ovens can reflow components in inert atmospheres (<50 to 2000 ppm oxygen) while consuming the smallest possible amount of nitrogen. To meet this objective, Conceptronic engineers developed a unique design, based on the company s serpentine flow concept, which results in non-fluctuating static air pressure within the oven. The direction of the flow of gases through the oven is controlled by dams located on the individual plenum systems. Movement of gases consists of a dominant flow from entrance to exit. On non-aurora air models, this dominant flow is exhausted through Polaris cooling units, and made-up by fresh air entering at the onload end of the oven or air returning through a duct. On Aurora equipped models (including all nitrogen models), the dominant flow is recirculated back to the onload end of the oven, dramatically reducing the need for make-up gas, while still supporting serpentine flow in the heat tunnel. The gas above and below the edge rail conveyor system flows from zone to zone in the serpentine fashion previously described. With Aurora, the process gas is also redirected from the end of the heating section to an upstream zone. With the dominant flow from entrance to exit and the Aurora flow in the opposite direction, the system stabilizes once the PROFILE oven has been running for a few minutes, and air intrusion into the oven is sharply reduced. The system has stabilized once the relative high and low pressure areas are fully developed for any given operating temperature. Nitrogen is supplied to the oven by direct injection into the oven at various points. The serpentine flow through the oven, combined with the non-fluctuating static air pressure, results in a reflow system that not only meets ppm objectives, but is also economical on the use of delivered nitrogen. Dynamic Flow Engineering encompasses the entire PROFILE system, from heating to cleaning to cooling. 6
The Genesis gas-to-gas heat exchanger cools the PCB and provides filterless flux management. Gas-to-Gas Heat Exchange Flux residue in process gas reaching the cooling section is removed with systems that rely on easy-to-clean baffles or gas-to-gas heat exchangers, rather than recirculating water or refrigerant heat exchangers and closed-loop chillers. Both the Polaris and Genesis cooling systems are based on a filterless flux management design that incorporates large cooling surfaces instead of filters or small-finned liquid heat exchangers. Filters or small fins, commonly used in other ovens, have an inherent problem, in that they can quickly become clogged by the flux-laden gas. For Polaris, cooling is achieved by particles bombarding a series of flow disrupters (baffles). Much like with Aurora, the flux collects on the baffles, which can be easily accessed through hinged doors. Cooling using the patented Genesis system (shown above) occurs by drawing the process gas across one side of the pleated surface of the heat exchanger, while ambient air is drawn across the other side, thereby exchanging energy. (The two gas flows remain separated.) The flux collects on the process side of the surface or drips to the collection area at the bottom of the chamber. Polaris flux collection trays and Genesis heat exchanger cores can be replaced with clean units in minutes, and allow contaminated units to be exchanged without cooling down the oven. Cooling
Conclusion Dynamic Flow Engineering is more than a concept. It ensures optimum heat transfer and cooling, as well as effective removal of airborne flux. While elements of DFE can be found in other reflow ovens manufactured by Conceptronic, the integrated engineering approach to oven design is optimized in the company s PROFILE Series of ovens. The result is precise control over the thermal process, even heating and cooling of boards, filterless flux management, and the assurance of product quality for the most demanding thermal processing applications. www.conceptronic.com Performance, Reliability, and Innovation in Thermal Processing Six Post Road, Portsmouth, NH 03801 Tel: 603-431-6262 Fax: 603-431-3303 Windows NT is a registered trademark of Microsoft Corporation. Profile and DFE Technology are trademarks of Conceptronic, Inc. 1999 Conceptronic, Inc. 11/99/10K