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Evolving Profile Requirements
December 31, 1969 |Estimated reading time: 10 minutes
Reflow soldering process developments for lead-free assembly will be more challenging and more time consuming than that for tin/lead.
By W. James Hall
Reflow profile requirements for lead-free continue to evolve, but process windows at peak temperatures will be tighter than that of tin/lead. Tighter time/temperature requirements also may be required for other sections of the profile time at liquidus (TAL) and soak and cooling. This results from higher melting temperatures, reduced wetting and inherent temperature limits of materials within electronics assemblies. Critical material concerns include moisture-sensitive devices (MSD) and other potential damage to IC packages, deformation of components and time above the glass transition temperature (Tg) of PCB laminates. Profiles also will need to match the requirements of new flux systems in lead-free paste formulations. To deal with these issues, different reflow profile shapes are being developed. Therefore, lead-free process development will necessitate creating and optimizing multiple recipes for ovens currently in use. Smaller process windows require more profiling of individual assemblies, rather than a single, generic oven recipe for a family of similar products. This potentially will result in multiple and significantly diverse reflow profiles. The specification limits will be tighter, requiring more time and effort in profiling activities.
Wide Process Windows for Tin/Lead Reflow
Historically, electronics incorporating organic substrates and plastic components have been reflowed using tin/lead alloys to form the interconnections. High yields, low costs and product reliability have been achieved consistently with this process. One reason for this success has been a wide specification range or “process window,” particularly for peak temperature. Most components can sustain brief temperature excursions of 235°C or higher. Also, two solder alloys are used almost exclusively - tin/lead eutectic (63/37) and 2% silver (62/36/2). Both melt at about 183°C, a low temperature compared to material limits of 235°C. Lastly, tin/lead alloys exhibit excellent wetting properties on common surface finishes - copper, tin and nickel/gold.
The favorable interaction of these parameters was successful with reflow soldering. For example, typical peak temperature specification ranges from 210° to 230°C. This is conservative, as temperatures falling outside either end of this range probably will not cause damage or defects. Above the 230°C upper limit, there is at least a 5°C margin before material damage occurs. Successful reflow can be accomplished below 210°C, although an exact specification is difficult to quantify, an estimate can be performed. It is impossible to solder correctly at the liquidus temperature of an alloy, however, experts claim that with ideal conditions, adequate wetting can be achieved with an increase of as little as 8°C above liquidus. “Ideal” conditions seldom occur in production, so 191°C cannot be considered realistic. Experience shows that desired results have been obtained at 200°C, and in some cases 195°C. Therefore, the realistic process window may be as wide as 35° to 40°C. Because defects seldom result when peak temperatures are outside the stated specification, there has been little motivation to fine tune reflow recipes (Figure 1).
Generic Oven Recipes
The result of the wide process windows is that the setup of the reflow process has been simplified, requiring only a few trial profiles. This has reduced the time required for process development, product changeover and new product introduction ( NPI).
Figure 1. T-peak process window
In some facilities, one oven recipe is used to reflow all products, regardless of size or complexity. Another alternative is to use a few “generic” recipes (Recipe A for small assemblies; Recipe B for medium size). The advantage of these strategies is to reduce or eliminate process development or confirmation of reflow profiles for individual products. The complicated and time-consuming task of attaching thermocouples to a sample assembly, recording profile data, analyzing this data and making oven speed and temperature adjustments also is reduced.
Figure 2. Tin/lead 3-3-1 profile
If new recipes are created for products that cannot achieve specifications using a generic recipe, simple modifications are used. The most common recipe change is conveyor speed adjustment - faster for smaller boards and slower for larger ones. A more comprehensive change would maintain conveyor speed but change heating zone temperatures to adjust slopes and other parameters to within the specified ranges. The shape of the profile remains the same and no reconfiguration of zone/sections is needed. For example, a ramp-soak-spike shaped profile on a seven-zone oven might always use three zones for preheat, three zones for soak and one zone for reflow. This improves control of all of the specified time/temperature parameters. While it requires more time, it minimizes profiling skill requirements. All these simplifications of the profiling/oven setup procedure may not be possible for lead-free process development (Figure 2).
High-temperature Material Concerns
The specified temperature limits of materials and components on an assembled PCB impact the reflow specification limits. The most basic objective of reflow soldering is to solder all joints without thermally damaging the assembly. The peak temperatures required for lead-free alloys will test material and component compatibility severely. Many material/component parameters are of concern:
MSD - The moisture sensitivity levels (MSL) of plastic components may increase with higher peak temperatures. Damage occurs in MSDs when water absorbed by the component boils, generating steam within the component body during reflow. The pressure created by the steam can cause body fractures, and can be fatal to the component. Steam pressure increases exponentially with temperature.
A temperature under consideration for qualifying MSLs is 250°C. The tendency for a given amount of absorbed water to distort, crack or damage a component will be higher for lead-free reflow.
Material Deformation - A variety of plastic materials are included in electronics assemblies. No material can be allowed to reach its melting temperature during reflow. Most materials have been designed to withstand 230° to 250°C. However, without the adoption of lead-free alloys, some components already cause concern. For example, an FPC connector with a specification of 10 seconds exposure at 240°C may become the most critical temperature-sensitive component in a lead-free assembly.1 Often, increasing maximum temperature limits of a component to accommodate lead-free will increase its cost significantly.
Warping is caused by non-uniform heating or differential expansion or contraction of composite structures such as BGAs. It can occur below the melting point at any temperature because of rapid heating or cooling. For composites, heating over a wide temperature range aggravates this. Because temperature ranges increase for lead-free soldering, warpage may become a concern.
The Tg of FR-4, a common PCB material, is between 140°-175°C. Above this range, the resin softens, losing it structural rigidity and allowing warping and sagging between oven-edge conveyor supports. These problems increase at higher temperatures and longer times spent above Tg.
Smaller Process Windows
Several factors inherent to lead-free soldering will cause some, if not all, ranges to shrink.
The liquidus temperature for the lead-free, SAC alloy is 217°C, or 34°C higher than eutectic tin/lead. The rule of thumb suggests that the minimum peak reflow temperature should be about 20°C above liquidus. This would dictate 237°C as a minimum for lead-free soldering. Naturally, it is desirable to maintain this value as low as possible. However, this may prove difficult because it has been demonstrated that SAC does not wet as well as tin/lead. By all current knowledge, peak temperatures are going to be much higher than with tin/lead.
Even if minimum peak temperatures can be limited to 229°C, this is close to the limit of some current components and materials. If the traditional 20°C window for Delta T (ΔT) and process variation is applied, the top-end of the peak range becomes 249°C, well above many current material limits. Increasing the temperature capability of materials and components almost always increases costs. Therefore, there will be continuing efforts to minimize the maximum peak temperature by shrinking the process window.
Figure 3. Low soak 2-3-2: This profile meets the suggested LF specs.
Regardless of the specific shape, lead-free reflow profiles will require longer times at elevated temperatures. Because of potential damage or degradation to materials, specification limits for soak, TAL and other parameters probably will be tighter to ensure quality soldering while protecting critical materials. For example, TAL typically has a wide window of 30 to 90 seconds for tin/lead. Reduced wetting for SAC might increase the minimum to 45 seconds, while faster intermetallic growth at the higher temperatures might limit maximum to 75 seconds. This would reduce the size of the process window from 60 to 30 seconds (30-90 vs. 45-75) (Figure 3).
Impact on Oven Profiling
Smaller process windows will require more accurate oven profiling. To create a recipe that brings all locations on the assembly within the narrower specification ranges requires the evaluation of more combinations of oven settings (temperatures and speed). There is less room for error or approximation. Because oven variability must be accounted for within the process window, an oven with good repeatability will be an asset. The entire process development effort also may be magnified by the need to evaluate and implement multiple profile shapes. Changing to a different shape requires redefining the heating sections relative to the individual zones. This means that with the exception of the first few common preheat zones, the profiling procedure must be started from the beginning, rather than modifying previous data and experience. It has been observed that even simple changes, such as increasing heating in the reflow section, require significant zone reconfiguration and multiple trial-and-confirmation profiles (Figure 4).
Figure 4. High soak 3-3-1: This profile does not meet suggested LF specs.
Many of the profile shapes suggested for lead-free reflow require changes in heating slope between sections, especially soak-reflow. The ability to implement these slope changes depends on the zone definition capability of the individual oven. Time can be lost if these oven characteristics are not understood and applied to the selection of zone temperature setpoints during the profiling process (Figure 5).
Simulation Tools
Developing a completely new profile shape on a specific oven is complex because there are an infinite number of possible zone temperature-conveyor speed combinations (recipes). There also is the time required to compare the results of each trial profile against reflow specifications.
Figure 5. Flat-head 2-2-3: An unusual LF profile shape.
To reduce profiling time, a reflow simulation tool using output from the analysis to predict the correct recipe parameters can be used. Robust simulation packages use an actual thermal model of the heat transfer in the individual oven zones based on previous measured data. With such a tool, large numbers of oven recipes can be evaluated instantly. Simulation packages, if implemented properly, can eliminate time and costs of evaluating alternative profile shapes.
Robust oven simulation tools can quality by easing optimizations that could not be justified using manual methods.
Smaller lead-free process windows make any drift in an operating process more significant. Compared to tin/lead assembly, any variation in temperature calibration, fan performance or conveyor speed can exceed specification limits - increasing defects and reducing quality. Although it has been vital to monitor reflow and use statistical process control (SPC) techniques to identify variations, this becomes critical to long-term quality manufacturing.
Figure 6. Straight ramp profile
In an evaluation of lead-free reflow parameters, a comparison of manual vs. simulation profile development was presented.2 Results show reduced development time and better optimization for simulation (Figure 6).
Conclusion
Reflow soldering process development for lead-free assembly will be more challenging and time consuming than that for tin/lead. Process windows also will be smaller. This is significant mostly with peak reflow temperatures because of material temperature limits and reduced wetting characteristics of lead-free alloys.
Several profile shapes have been proposed for lead-free, and more will evolve. Evaluating and implementing these profiles will require numerous oven adjustments. These adjustments will require complete reconfiguration of zone temperature relationships.
Even when a specific profile shape has been selected, tighter specification ranges will require profiling for individual products, more restricted groups of products. Even with the best oven available, a generic recipe for all products is unrealistic.
Because multiple, interactive changes are required and a number of recipes exist, developing a new profile shape on a specific oven is complex. The problem is compounded by the fact that process engineers and reflow technicians have never changed the basic shape of a profile. There also is the time required to compare the results of each trial profile against all reflow specifications. If attempted manually the time could be lengthy. Robust profile simulation software can aid significantly in these activities by eliminating trial profiling, reducing time and allowing more process optimization.
References
- Omron, 0.5-mm pitch vertical FPC connector, XF2G.
- Houston, et al., “Taking the Pain Out of Pb-free Reflow,” Technical Proceedings, APEX 2003, Anaheim, CA, pp 5-7.
W. James Hall, principal SMT consultant, IMT Consulting, may be contacted at (603) 868-1754; e-mail: itm@itmconsulting.org.