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STEP 2: Process Control
December 31, 1969 |Estimated reading time: 6 minutes
By Denis Barbini, Ph.D., Vitronics Soltec
The development of a controlled and robust reflow process exerts a significant effect on joint formation, and ultimately, board quality. A robust and reliable soldering process can be achieved only by a calculated and well-executed strategy of studying the effect of critical process parameters on defect formation and yield, as well as an understanding of the interrelation between components, paste, flux, and board material. The technology forecast continues to provide consistent data on the nature of the soldering landscape in electronics assembly.
The issues surrounding electronics manufacturing focus on the conversion to lead-free processing. Logistics, cost, material selection, and equipment functionality are some aspects that require planning and organization. However, soldering is at the core of an assembly process. Challenges that exist today are led by the enduring changes in materials and their maturity, combined with a significant effort to minimize cost. Expensive materials, such as the use of silver-bearing materials, affect cost adversely - pressuring assembly houses to find alternative, less expensive, and functional materials. Another pressure is complexity in board technology. Current high-end board designs are migrating to increased layer count, thickness, and complexity, exacerbating the challenges already associated with developing and maintaining a robust and reliable reflow, wave, or selective soldering process. This results in a situation where materials are pushed to their specification limits in terms of exposure to elevated temperatures for extended times. As lead-free implementation progresses, along with increased material complexity, critical gaps in assembly technology arise. Innovative solutions that incorporate process controls, allowing end users to develop a robust, optimized soldering process are needed. Moreover, the end user requires a process that can identify and mitigate process inefficiencies, so that first pass yields are improved.
Figure 1. Impact of cooling rate on joint performance for a chip-scale package (CSP).
Traditional tin/lead assemblies allow for a wide solder processing window for most applications. Focus on process control is warranted for three aspects:
- Identifying issues surrounding the implementation of a reliable, robust lead-free reflow soldering process;
- Characterizing the impact various parameters have in achieving an optimized solder joint;
- Developing process controls to achieve the desired quality and yield.
The critical parameters in any reflow process can be broken into several categories, including thermal performance, traceability, inert-atmosphere flexibility, cooling-rate optimization, flux management, and equipment operation. The metrics by which these parameters are characterized include thermal uniformity across and along the heating zone; thermal efficiency; repeatability of process under idle and loaded conditions at six sigma levels; oxygen parts per million (ppm) level and stability; and process traceability. This article identifies specific process parameters and attempts to quantify the relationship between the process set point and consequent joint quality.
While current forced-convection reflow ovens are designed to control the heating process with great stability and uniformity, two complementary reflow parameters influence an end user’s capability for process flexibility. These are cooling rate and soldering atmosphere.
When characterizing process controls to achieve higher yields, a focus on cooling rate is needed. The impact of controlled cooling on joint quality and yield has only been of recent focus, which illustrate a significant relationship between solder-joint formation and performance. Traditionally, electronic assembly cooling emphasized the board’s exit temperature and handling after reflow. For lead-free applications, higher processing temperatures may require a more negative cooling rate to compensate for increased exit temperatures. Besides norms, the benefits for increased (negative slope) cooling rates include decreased exit temperatures, minimized time at elevated board temperatures, board finish, heat-sensitive components, flux/paste, and minimized formation of bulk solder intermetallics. However, slower cooling rates (positive cooling slopes) minimize internal stresses due to differences in the various materials’ coefficient of thermal expansion (CTE), or thermal capacity. To address optimized cooling rates, the IPC/JEDEC standard 020C was referenced, which lists a range of -3° to -6°C/sec. as the cooling-rate limit. Based on this, an experiment was undertaken to characterize joint formation and performance. This study selected various cooling rates below, at, and above the limits defined in the IPC/JEDEC standard. The results of this work provided the following observations:
- No visual defects were found across all boards, regardless of cooling rate.
- Good solder joint formation was observed on all components.
- A change in the formation of bulk intermetallics was observed.
- Near-parabolic behavior of joint performance as a function of thermal cycling was noted.
Figure 1 illustrates joint performance as a function of cooling rate as measured by joint resistance taken in-situ. This result provides evidence that cooling rate affects joint performance. While faster cooling rates provide improved performance, this trend peaks at approximately -4.5°C/sec., the point at which joint performance trends downward. Boards of varying thickness and complexity that contain a mix of components are characterized by cooling rates that exceed the specified limit. The result is that while the end user will not visually characterize the defect, potential in-field issues may occur. Process-control parameters must include characterizing and maintaining the cooling rate within a defined window.
Soldering atmosphere has the potential to affect the soldering quality of a given electronic assembly. Variable atmosphere settings, combined with specific materials used, can either widen or narrow the effective process window. A nitrogen-soldering atmosphere is used to protect metal surfaces from oxidizing, and minimizes an adverse reaction with the flux chemistry. For these factors, nitrogen enhances an effective process window. However, the optimized nitrogen atmosphere condition is not clearly defined. Its cost considerations and availability limit desired use and consumption. As a result, end users have attempted to specify impurity levels as measured by oxygen ppms that are in excess of pure-nitrogen levels. The delicate balance of cost vs. enhanced joint formation is a factor that lends itself to a procss-control solution.
Figure 2a. QFP joint formed under a 25-ppm oxygen level at full tunnel.
Research shows that varying atmosphere by increasing oxygen ppm affects joint formation, as characterized by visual means and mechanical strength. Figures 2a/b illustrate typical lead-free QFP joints formed using the same reflow process with varying oxygen ppm levels. The difference in joint appearance is primarily on solder coalescence and residue formation. Under air-soldering conditions, metal surfaces are continuously oxidized to the point where the solder paste becomes less effective in producing a fully wetted joint. While QFP joints shown in Figures 2a/b are acceptable, the air process reduces process flexibility. In cases where board complexity increases, this observation is more significant for the potential in forming unacceptable joints.
Figure 2b. QFP joint formed under air atmosphere.
Figure 3 illustrates the mechanical force needed to remove a bump from its pad as a function of oxygen-ppm levels. In this analysis, a trend is observed between near-pure nitrogen-soldering atmosphere and air conditions. The use of 250-2,500 oxygen-ppm levels in the reflow oven provides similar results, which can assist an end user in optimizing the process as a function of cost.
ConclusionDefining the critical parameters for a soldering process is the first step in developing necessary process controls. This is followed by characterizing the effect that these parameters have on joint formation and performance. Once information is collected, an end user can set boundary limits for which the process must fit. The benefits of a controllable reflow process are high first pass yield and product performance. For a complete list of references, contact the author.
Denis Barbini, Ph.D., advanced technologies manager, Vitronics Soltec, may be contacted at (603) 772-7778; e-mail: dbarbini@vsww.com.