Surface Mount re-flow soldering is the most widely accepted method of attaching electronic components to a printed circuit board. Applications vary depending on the components, atmosphere and solder type, all of which dictate the process profile. Finer pitches and challenging materials, particularly lead-free solders, continue to reduce the process window and require optimum thermal re-flow performance. The resulting solder joint forms a mechanical and electrical connection to the surface of a conductive land pattern that does not utilize through-holes or terminals.

The EMPF is utilizing the BTU Pyramax 98 oven (Figure 4-1) in a wide variety of applications such as SMT Boot Camp training (helping SMT students develop solder re-flow profiles), testing lead-free processes, wafer bump re-flow for lead-based or lead-free solders, nitrogen process development, high temperature applications for military projects and lead-free process development to assist manufacturers with their specific process challenges. Most recently, the EMPF utilized the BTU Pyramax 98 for a lead-free workshop.
SMT re-flow is typically performed using forced air, convection dominant ovens utilizing either air or nitrogen as the process gas. The typical lead-free process profiles consist of four phases: preheat, soak, reflow and cool down. For thermal profiles, the following parameters for air temperature settings must be considered for each zone to achieve satisfactory results: Nitrogen flow rate (to achieve the appropriate reduction of the oxygen levels required by the paste manufacturer), conveyor speed, cooling rate and fan speed (optional on some ovens). Lead-free processing presents additional challenges. For example, since re-flow temperatures are higher than that of Eutectic solder, the difference between re-flow temperature and maximum component temperature ratings is smaller. This results in a significantly reduced process window.
The central component of any solder re-flow system is its heat transfer technology. The BTU oven uses jet impingement convection, which is an efficient means of transferring heat to a relatively flat surface such as a printed circuit board (PCB). A small, singular jet of high-speed gas can transfer its temperature to a surface very effectively at the spot where it impinges on the surface. An array of such jets is able to extend that high heat transfer coefficient across the surface, so long as the impinging gases have clearance to escape between the jets. In the BTU furnace design, the size and velocity of the jets, and the spacing of the arrays have been optimized using Computational Fluid Dynamics (CFD) computer modeling.

The benefits of the very high heat transfer coefficients available with jet impingement are that both small and heavy components heat up quickly, and the oven zone temperature set points can be closer to the desired product temperatures. As a result, small components do not overheat while large components are heating up. All of the components on a board go through the same narrow process window, which is a necessity for a Lead-free PCB assembly. In less effective, convection dominant and radiantly heated ovens, the zone temperature set points are higher in order to achieve the same profile for the heavy components. This leads to larger temperature differences across the PCB assembly and a less controlled process.

The Pyramax 98 has the process capability and flexibility to accommodate all thermal profile recipes developed at the EMPF. If you would like to see a demonstration of this oven at the EMPF, please call Jeff Stong at 610-362-1200 x-224.