Optimizing Performance in Reflow Soldering

Estimated reading time: 12 minutes

A new method of managing process gases in reflow ovens offers precise control over the thermal process, effective removal of flux and good solder joint quality.

Kerem Durdag

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 considerations such as operating costs, throughput and the environment. For reflow ovens, the challenges are achieving the correct thermal profile, optimum heat transfer and cooling, and effective removal of airborne flux - all of which impact yields and mandate total management of gas flow within the oven.

A process called "dynamic flow engineering" is an approach being taken to ensure the effective management of process gases in reflow ovens.

This process is both a concept and a process engineering approach to oven design for analyzing and managing gas flow 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. This article examines particular design features implemented to optimize the reflow process through the treatment and directing of gas flow in ovens featuring this system.

Certainly, other factors play an essential role in oven performance as well, including operating software and even the conveyor and board support systems. No aspect of oven design, however, is more critical to the forming of acceptable solder joints than the flow of oven gases that transfer energy to uniformly heat and cool the product. Effectively controlling the heat transfer through the temperature and flow of gas molecules allows the surfaces being joined to wet properly and form a strong metallurgical bond while, at the same time, the volatized flux is carried away and removed from the internal heating chamber.

Overview

The first step was to establish the desired performance specifications. Both air and nitrogen models were taken into consideration. Decisions were made as to how the process gas should flow through the individual heating and cooling zones to meet these specifications, including the minimization of flux residue and significantly reduced requirements for cleaning.

The process 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.

The process gas in the 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 (Figure 1). Such a design configuration offers a number of benefits that include even distribution of heat, controlled molecular flow through the oven, and movement of flux particulate from the heating to the cooling section. In addition, a thermal flux management system of a particular design can be used as an option for the heating section to remove contaminants before the process gas reaches the cooling section.

After exiting the heating section, the process gas flows through cooling modules. Whether for air or nitrogen, the cooling systems are effective in removing flux particulate. In fact, as much as 95 percent of the flux contaminants are removed.

In essence, this technology integrates all aspects of heating, cooling and flux management for ensuring solder joint quality and cost-effective performance, even for the demanding reflow requirements of high-density, high-component-mix applications.

Forced Convection Heating

Forced convection reflow is a mature technology, with most oven manufacturers incorporating this method of heat transfer in products offered on the market. It is characterized by the temperature of the heated printed circuit board (PCB), 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).*

Forced convection reflow ovens offer the benefits of 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 differences in forced convection ovens can be a problem. Fluid dynamics modeling can be used to analyze air flow and heat transfer for optimal convection heating processes and consistent board temperatures.

In ovens featuring this technology, a multi-port blower pushes gas through a heater core and a flow-balanced gas dispersion plate in each heating zone, producing "cones" of hot gas that impinge on the board and the components mounted on the pads (Figure 2). 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 board pads. This flow design optimizes the heat transfer process while negating the need for excessive blower speeds and power.

Serpentine Gas Flow

The flow of process gases through the heating zones is unique to these reflow ovens. As shown in Figure 1, 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 then passes through the plenum (blower, heater core and gas dispersion plate) and onto the boards in that zone, afterwards being drawn up and through the blower system of the next zone. With this system, approximately 80 percent of the gas recirculates in this manner, with the remaining 20 percent following the path of the boards from zone to zone.

Circulating the gas through the oven this way offers significant performance and maintenance advantages. The ovens direct the gas flow from chamber to chamber in the serpentine fashion to the cooling unit or to collection points of advanced thermal flux management systems. Therefore, flux buildup on the surfaces inside the heat tunnel is negligible, even after years of operation. Also, as proven in field studies, openings in the heater core are unaffected by flux, and the hole diameters remain constant.

Thermal Flux Management

A new type of thermal flux management system was recently designed for ovens featuring this technology. This system forms a loop outside the heating section of the oven, and is intended to collect flux contaminants before the process gas reaches the cooling section of the oven.

Simply stated, the system removes flux-laden 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 (Figure 3). The engineering concept behind thermal flux management is based on principles of particle deposition and a proven approach for separating flux particulate from process gas.

Between 50 and 80 percent of the flux contaminants are collected by the thermal flux management system, with another 15 to 45 percent being extracted in the cooling section, for a total of up to 95 percent of the flux contaminants. Ovens without an additional flux management device also remove up to 95 percent of the contaminants, but directly by the cooling units themselves. Ovens incorporating this type of thermal flux management, therefore, increase volumetric flux capacity and extend flux clean-out intervals. In addition to removing flux from process gas prior to cooling, recirculating process gas that would otherwise pass into the cooling section also conserves energy - and nitrogen, where appropriate.

Most of the particulate is generated as flux ingredients begin to activate and reduce the oxides on the solder surface. At this point, the contaminants that will later form on the cooler surfaces of the oven are essentially airborne and contained in the vapor. With the new process, the cooler surfaces (within the thermal flux management system and cooling units) can be strategically located to prevent flow restrictions and are easily cleaned. The purpose 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.

Operation is as follows: The ducting for the system is 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 across a set of baffles contained within a dual condensation system (two parallel ducts) and back into the upstream zone, passing through a dual condensation system (Figure 4).

The baffles are key to the success of thermal flux management. Angled into the flow of gas, they cause localized turbulent regions, forcing the gas stream to change direction and velocity. Hot gas molecules from the reflow chamber strike the surface of the baffles at an optimized velocity. The molecules then immediately lose their kinetic energy (as much as 40 to 60 percent) because of the change in direction and speed. The baffles are 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 is easily accomplished without shutting the oven down by unhooking the latches for the covers and lifting out the baffles mounted to the covers.

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 air and nitrogen cooling systems in these ovens are based on a filterless flux management design that incorporates large cooling surfaces. Filters or small fins, commonly used in other ovens, have an inherent problem because they can quickly become clogged with the flux-laden gas.

For air models, flux is collected by directing the process gas through a series of flow disrupters (baffles), based on similar principles as the thermal flux management system. Cooling is achieved by directing ambient air through a blower system into a plenum, and then injecting cool air via a gas dispersion plate. Cooling using a gas-to-gas heat exchanger on a nitrogen system 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. Air-model flux collection trays and nitrogen-model heat exchanger cores can be replaced with clean units in minutes, and permit contaminated units to be exchanged without cooling down the oven.

Nitrogen Reflow Ovens

In performing the analyses, the objective was to design a system that neutralizes the effects of outside air, such that the ovens are capable of reflowing components in inert atmospheres (less than 50 to 2,000 ppm oxygen) while consuming the smallest possible amount of nitrogen. To meet this objective, the oven extends the concept of serpentine flow, resulting in nonfluctuating static air pressure within the oven.

The direction of gas flow through the oven is controlled by "dams" located on the individual plenum systems. Gas movement consists of a dominant flow from entrance to exit. On air models without an additional thermal flux management system, this dominant flow is exhausted through the cooling units, and "made-up" by fresh air entering at the onload end of the oven. On models equipped with additional thermal flux management systems (including all dynamic flow engineering nitrogen models), the dominant flow is recirculated back to the onload end of the oven, reducing the need for makeup 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. The process gas can also be redirected from the end of the heating section through a thermal flux management system to an upstream zone. With the dominant flow from entrance to exit and the flow of process gas in the thermal flux management device moving in the opposite direction, the system stabilizes once the 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 of the gas into the oven at various points, via a diverse range of designs for specific flow characteristics. The serpentine flow through the oven with recirculation of the nitrogen gas in each heating zone, combined with the nonfluctuating static air pressure that provides a barrier to the atmosphere outside the oven, results in a reflow system that meets ppm objectives and is economical on the use of delivered nitrogen.

Conclusion

Dynamic flow engineering encompasses total analysis of gas flow through the oven and resulting heat transfer within both the heating and cooling sections. The result is precise control over the thermal process, even heating and cooling of boards, effective removal of flux, and solder joint quality for the most demanding reflow applications.

*In comparison, for infrared (IR) soldering systems, the increase in temperature of components and substrate are nonlinearly related with the temperature of the source, and hence to the absorption of IR energy delivered. In achieving a certain component temperature, the temperature of the source is always higher than what it would be for a convection oven. Differences in individual component temperatures are attributable to relative differences in mass.

KEREM DURDAG is a principal engineer with Conceptronic Inc., 6 Post Road, Portsmouth, NH 03801; (603) 431-6262; Fax: (603) 431-3303; E-mail: kerem@conceptronic.com.

Figure 1. Process gas is directed by the blower system through the heater plenum and onto the boards in the initial heating zone. Most of the gas is then drawn up and into the blower system of the next zone, and so on.

Figure 2. The heating plenums consist of the blower system, heating core and gas dispersion plate. Heated process gas impinges on the top and bottom surfaces of PCBs in overlapping "cones" of gas flow.

Figure 3. The flux-laden process gas at the end of the heating section is removed, and the "cleaned" gas is recycled back into the "soak" zone.

Figure 4. Flux is collected on baffles because of gas molecule bombardment, while the flux-laden process gas is cooled using ambient air drawn across the opposite side of a pleated surface.